VET 223 Exam 2

PHYSIOLOGY OF CARDIAC MUSCLE

• The atrial and ventricular types of muscle contract in much the same way as skeletal muscle, except that the duration of contraction is much longer.

• Heart action potential conduction system

  • The heart’s AP conduction system consists of weakly contractile cardiac muscle cells called autorhythmic cells that are specialized for generation and distribution of AP throughout the myocardium. These cells are located in the Sinoatrial (SA) node, Atrioventricular node (AV) node and through the Purkinje fibers.

• Mechanism of the AP conduction system

  • The resting membrane potential of the SA fiber between discharges is about –55 to –60 millivolts, in comparison with –85 to –90 millivolts for the ventricular muscle fiber.

    • The cause of this lower negativity is that the cell membranes of the sinus fibers are naturally leaky to sodium ions (via Na+ “funny current” channels), therefore positive charges entering (sodium ions) progressively neutralize some of the intracellular negativity (until reaching a “threshold level of around - 40mV).

    • Cardiac muscle has three main types of membrane ion channels that play very important roles in causing the voltage changes of the action potential:

    • Sodium voltage gated Na+ channels; same as in neurons – mainly inactive in the AP conduction system because

      • The resting potential in the SA node is much less negative—only –55 millivolts in the nodal fiber instead of the –70 millivolts in neurons.

        • At this level of –55 millivolts, the fast sodium channels mainly have already become inactivated or blocked.

        • This is because any time the membrane potential remains less negative than about –55 millivolts for more than a few milliseconds, the inactivation gates on the inside of the cell membrane become closed and remain so (remember from neuronal AP generation?).

      • Therefore, only the voltage gated calcium channels (se bellow) can open (i.e., can become activated) and thereby cause the action potential.

      • As a result, the atrial nodal action potential is slower to develop than the action potential of the ventricular muscle (SA node about 80 – 120 depolarizations per minute – normal heart rate!!).

    • Voltage gated calcium channels (fast - new! For AP generation)

    • Potassium channels (same as in neurons – work in repolarization)

• Steps in creation and conduction of heart action potentials

  • Heart AP conduction starts at the sinoatrial (SA) node (cardiac pacemaker), located between top of right atrium and cranial vena cava.

    • SA node contains autorhythmic cells called pacemaker cells that spontaneously initiate action potentials at a rate of ≈100 times/min.

    • The pacemaker cells in the SA node are muscle cells, so the SA node is a myogenic pacemaker that does not require excitation by nervous system to create slow response APs.

    • SA node APs are initiated in pacemaker cells using If channels in their plasma membranes.

      •  If stands for “funny current”

    • If channels leak Na+ into the pacemaker cells until the membrane potential (Vm) reaches about -50 to – 40 mV, this triggers the opening of voltage-gated Ca+2 channels

    • Ca+2 will flow through these channels bringing + charge to inside of cell eventually reaching ~ +10 mV -> depolarization!

    • Repolarization of the pacemaker cells involves closing of the voltage-gated Ca+2 channels and opening of voltage-gated K+ channels (same as in the neuron!).

    • K+ flows out of the cell and brings the cells back to -55 to -60 mV.

    • After the membrane repolarizes back to-60 mV (VRest), the If channels open and leak Na+, thus starting the process of depolarization for the next heartbeat.

  • From the pacemaker cells of the SA node, AP spreads through gap junctions over both atria, which then contract (atrial systole).

  • Atria and ventricles are separated by a sheet of nonconductive connective tissue so atrial AP does NOT spread directly to ventricular myofibers through gap junctions.

    • To excite ventricular muscle cells, AP from SA node travels through the internodal pathway to the atrioventricular (AV) node, which is the only electrical connection between atria and ventricles.

    • The slow conduction in the transitional, nodal, and penetrating A-V bundle fibers is caused mainly by diminished numbers of gap junctions.

    • This slowing down allows both atria to complete their contractions before the ventricles begin to contract.

    • The AV node also has endogenous pacemaker activity (depolarizes ≈40 to 60 times/min), but it is slower than SA node (≈100/min).

  • From the AV node, APs move to the atrioventricular bundle.

  • AP spreads from AV bundle into the right and left bundle branches, which run through the interventricular septum, to the apex of the heart and up both sides of the heart.

  • Purkinje fibers branch off of the bundle branches and conduct fast response AP into the contractile muscle cells of the ventricular myocardium and papillary muscles.

    • Special Purkinje fibers lead from the A-V node through the A-V bundle into the ventricles.

    • The rapid transmission of APs by Purkinje fibers is believed to be caused by a very high level of permeability of the gap junctions at the intercalated discs between the successive cells.

    • Note: This heart action potential conduction system leads to a coordinated contraction of the ventricular myocardium (ventricular systole) from the apex (bottom, cone end of the heart) up to the base (atrio-ventricular connection), which facilitates the squeezing out of the blood from the ventricles into the large arteries.

Cardiac muscle anatomy

• The intercalated discs are cell membranes that separate individual cardiac muscle cells from one another.

  • Cardiac muscle fibers are made up of many individual cells connected in series and in parallel with one another.

  • At each intercalated disc, the cell membranes fuse with one another to form permeable communicating junctions (gap junctions) that allow rapid diffusion of ions.

  • Therefore, from a functional point of view, ions move with ease in the intracellular fluid along the longitudinal axes of the cardiac muscle fibers so that action potentials travel easily from one cardiac muscle cell to the next, past the intercalated discs.

  • Thus, cardiac muscle is a syncytium of many heart muscle cells in which the cardiac cells are so interconnected that when one cell becomes excited, the action potential rapidly spreads to all of them.

• The heart actually is composed of two syncytia.

  • The atrial syncytium, which constitutes the walls of the two atria; and

  • The ventricular syncytium, which constitutes the walls of the two ventricles.

  • The atria are separated from the ventricles by fibrous tissue that surrounds the atrioventricular (A-V) valvular openings between the atria and ventricles.

    • Normally, potentials are not conducted from the atrial syncytium into the ventricular syncytium directly through this fibrous tissue.

    • Instead, they are only conducted by way of a specialized conductive system called the A-V bundle, a bundle of conductive fibers several millimeters in diameter (you already know this!).

  • This division of the muscle of the heart into two functional syncytia allows the atria to contract a short time ahead of ventricular contraction, which is important for the effectiveness of heart pumping.

• Action potentials in cardiac muscle

  • During the action potential generation of a cardiac muscle fiber the intracellular potential rises from a very negative value between beats, about −85 millivolts, to a slightly positive value, about +20 millivolts, during each beat.

    • After the initial spike, the membrane remains depolarized for about 0.2 second, exhibiting a plateau, followed at the end of the plateau by abrupt repolarization.

    • The presence of this plateau in the action potential causes ventricular contraction to last as much as 15 times longer in cardiac muscle than in skeletal muscle.

• What Causes the Long Action Potential and Plateau in Cardiac Muscle?

  • In cardiac muscle, the action potential is caused by opening of two types of channels:

    • Similar voltage activated fast sodium channels as those in neurons and skeletal muscle; and

    • Voltage gated calcium channels (slower)

      • This second group of channels differs from the fast sodium channels in that they are slower to open and, even more importantly, remain open for several tenths of a second.

    • During cardiac muscle depolarization, a large quantity of both calcium and sodium ions flows through these channels to the interior of the cardiac muscle fiber, and this activity maintains a prolonged period of depolarization, causing the plateau in the action potential.

      • Furthermore, the calcium ions that enter during this plateau phase activate the muscle contractile process, this is different from the calcium ions that cause skeletal muscle contraction that are derived from the intracellular sarcoplasmic reticulum.

    • Another major functional difference between cardiac muscle and skeletal muscle is that the when the slow voltage calcium gated channels are open, the permeability of the cardiac muscle membrane for potassium ions decreases about fivefold, an effect that does not occur in skeletal muscle.

      • The decreased potassium permeability greatly decreases the efflux of positively charged potassium ions during the action potential plateau and thereby prevents early return of the action potential voltage to its resting level.

      • When the slow calcium channels do close at the end of 0.2 to 0.3 second, and the influx of calcium and sodium ions ceases, the membrane permeability for potassium ions also increases rapidly.

      • This rapid loss of potassium from the fiber immediately returns the membrane potential to its resting level, thus ending the action potential.

Phases of Cardiac Muscle Action Potential

Phase 0 (Depolarization):

• Fast Sodium Channels Open.

  • When the cardiac cell is stimulated and depolarizes, the membrane potential becomes more positive.

  • Voltage-gated sodium channels (fast sodium channels) open and permit sodium to rapidly flow into the cell and depolarize it.

  • The membrane potential reaches about +20 millivolts before the sodium channels close.

Phase 1 (Initial Repolarization):

• Fast Sodium Channels Close.

  • The sodium channels close, the cell begins to repolarize, and potassium ions leave the cell through open voltage gated potassium channels (similar to what we see in the neuron).

Phase 2 (Plateau):

• “Slower” voltage gated calcium Channels Open.

  • A brief initial repolarization occurs (because of the fast opening of the voltage gated potassium channels), however the action potential then plateaus as a result of increased calcium ion permeability through the “slower” voltage gated calcium channels.

  • The voltage-gated calcium ion channels open slowly during phases 1 and 0, and calcium enters the cell.

  • The continuous escape of potassium ions outside of the cell and of calcium ions inside of the cell causes the cardiac muscle plateau in the action potential.

  • During this phase it is also thought that the permeability for potassium ions is reduced with some researchers describing two different potassium channels (fast and slow) that work during this phase. For this course, we will focus on the interrelationship of the “slower” calcium channels and the voltage gated potassium channels.

Phase 3 (Rapid Repolarization):

• “Slower” Calcium Channels Close

  • The closure of calcium ion channels and increased potassium ion permeability (through the voltage gated potassium channels), permits potassium ions to exit the cell rapidly, ends the plateau and returns the cell membrane potential to its resting level.

Phase 4 (Resting Membrane Potential):

This averages about−80 to −90 millivolts.

Refractory Period of Cardiac Muscle

• Cardiac muscle, like all excitable tissue, is refractory to restimulation during the action potential. Therefore, the refractory period of the heart is the interval of time during which a normal cardiac impulse cannot re-excite an already excited area of cardiac muscle.

  • The normal refractory period of the ventricle is 0.25 to 0.30 second, which is about the duration of the prolonged plateau action potential.

  • There is an additional relative refractory period of about 0.05 second during which the muscle is more difficult to excite than normal but can be excited by a very strong excitatory signal, as demonstrated by the early premature contraction.

  • The refractory period of atrial muscle is much shorter than that for the ventricles (about 0.15 second for the atria compared with 0.25 to 0.30 second for the ventricles).

Excitation-contraction coupling— function of calcium ions and the transverse tubules

• As is true for skeletal muscle, when an action potential passes over the cardiac muscle membrane, the action potential spreads to the interior of the cardiac muscle fiber along the membranes of the transverse (T) tubules.

  • The T tubule action potentials then act on the membranes of the longitudinal sarcoplasmic tubules to cause release of calcium ions into the muscle sarcoplasm from the sarcoplasmic reticulum.

  • In another few thousandths of a second, these calcium ions diffuse into the myofibrils and catalyze the chemical reactions that promote sliding of the actin and myosin filaments along one another, which produces the muscle contraction.

This mechanism of excitation-contraction coupling is the same as that for skeletal muscle, but there is a second effect that is quite different.

• In addition to the calcium ions that are released into the sarcoplasm from the sarcoplasmic reticulum, calcium ions also diffuse into the sarcoplasm from the T tubules at the time of the action potential (Ca++ entering from the extra cellular fluid).

• Calcium entering the cell then activates calcium release channels, also called ryanodine receptor channels, in the sarcoplasmic reticulum membrane, triggering the release of more calcium into the sarcoplasm (calcium dependent calcium release).

• Calcium ions in the sarcoplasm then interact with troponin to initiate cross-bridge formation and contraction by the same basic mechanism as that described for skeletal muscle.

Without the calcium the extracellular fluid, the strength of cardiac muscle contraction would be reduced considerably because the sarcoplasmic reticulum of cardiac muscle is less well developed than that of skeletal muscle and does not store enough calcium to provide full contraction.

The strength of contraction of cardiac muscle depends to a great extent on the concentration of calcium ions in the extracellular fluids. In fact, a heart placed in a calcium free solution will quickly stop beating.

Note: The T tubules of cardiac muscle are much more developed than in skeletal muscle, so in reality extracellular calcium readily enters the T tubules and binds to mucopolysaccharides located inside the T Tubules “waiting” for an AP to reach the T tubules, thus activating special Ca++ voltage gated channels that will then release the Ca++ ions into the sarcoplasm. However, since the Ca++ concentration inside the T tubules is highly dependent of the concentration of Ca++ in the extracellular fluid in this course, we will just consider extracellular calcium as triggering the release of sarcoplasmic reticulum calcium and itself entering the cell to cause cardiac muscle contraction.

* Now that we have learned how the AP starts in the cardiac conduction system (SA node) and how it spreads through the cardiac muscle to then cause muscle contraction we need to review the AP conduction pathway and understand its importance.

Conduction of heart action potentials (Summary)

1. The impulse (AP) is generated in the SA node.

2. The impulse spreads at moderate velocity through the atria but is delayed more than 0.1 second in the A-V nodal region before appearing in the ventricular septal A-V bundle.

3. Once it has entered this bundle, it spreads very rapidly through the Purkinje fibers to the entire endocardial surfaces of the ventricles.

4. The impulse once again spreads slightly less rapidly through the ventricular muscle to the epicardial surfaces

Control of excitation and conduction in the heart

The impulse normally arises in the sinus node.

• In some abnormal conditions, this is not the case.

• Other parts of the heart can also exhibit intrinsic rhythmical excitation in the same way as the sinus nodal fibers; this is particularly true of the A-V nodal and Purkinje fibers.

• The A-V nodal fibers, when not stimulated from some outside source, discharge at an intrinsic rhythmical rate of 40 to 60 times per minute, and the Purkinje fibers discharge at a rate somewhere between 15 and 40 times per minute.

  • These rates are in contrast to the normal rate of the sinus node of 70 to 80 times per minute.

Why does the sinus node rather than the A-V node or the Purkinje fibers control the heart’s

rhythmicity?

• The answer derives from the fact that the discharge rate of the sinus node is considerably faster than the natural self-excitatory discharge rate of either the A-V node or the Purkinje fibers.

• Each time the sinus node discharges, its impulse is conducted into both the A-V node and Purkinje fibers, also discharging their excitable membranes.

• However, the sinus node discharges again before either the A-V node or Purkinje fibers can reach their own thresholds for self-excitation.

  • Therefore, the new impulse from the sinus node discharges both the A-V node and Purkinje fibers before self-excitation can occur in either of these sites

Why is the order and pathway of the cardiac AP so important?

Abnormal Pacemakers—Ectopic Pacemaker

Under rarer conditions, a place in the atrial or ventricular muscle develops excessive

excitability and becomes the pacemaker.

• A pacemaker elsewhere than the sinus node is called an ectopic pacemaker.

  • An ectopic pacemaker causes an abnormal sequence of contraction of the different parts of the heart and can cause significant weakening of heart pumping.

• When A-V block occurs (when the cardiac impulse fails to pass from the atria into the ventricles through the A-V nodal and bundle system) the atria continue to beat at the normal rate of rhythm of the sinus node while a new pacemaker usually develops in the Purkinje system of the ventricles and drives the ventricular muscle at a new rate, somewhere between 15 and 40 beats per minute.

• During these 5 to 20 seconds, the ventricles fail to pump blood, and the individual faints after the first 4 to 5 seconds because of lack of blood flow to the brain.

• This delayed pickup of the heartbeat is called Stokes - Adams syndrome.

  • If the delay period is too long, it can lead to death.

The control of the Autonomic Nervous System over cardiac contraction

Sympathetic and parasympathetic nerves control heart rhythmicity and impulse conduction by the cardiac nerves

• The parasympathetic nerves (the vagus nerve) are distributed mainly to the SA and AV nodes, to a lesser extent to the muscle of the two atria, and very little directly to the ventricular muscle.

• The sympathetic nerves, conversely, are distributed to all parts of the heart, with strong representation in the ventricular muscle, as well as in all the other areas.

Parasympathetic (Vagal) Stimulation Slows the Cardiac Rhythm and Conduction.

• Stimulation of the parasympathetic nerves to the heart (the vagus nerve) causes acetylcholine to be released at the vagal endings – you already know this!

• This neurotransmitter has two major effects on the heart.

  • First, it decreases the rate of rhythm of the sinus node, and second, it decreases the excitability of the A-V junctional fibers between the atrial musculature and the A-V node, thereby slowing transmission of the cardiac impulse into the ventricles.

• Weak to moderate vagal stimulation slows the rate of heart pumping, often to as little as one-half normal.

  • Furthermore, strong stimulation of the vagi can completely stop the rhythmical excitation by the sinus node or completely block transmission of the cardiac impulse from the atria into the ventricles through the A-V node.

  • In either case, rhythmical excitatory signals are no longer transmitted into the ventricles.

  • The ventricles may stop beating for 5 to 20 seconds, but then some small area in the Purkinje fibers, usually in the ventricular septal portion of the A-V bundle, develops a rhythm of its own and causes ventricular contraction at a rate of 15 to 40 beats per minute.

    • This phenomenon is called ventricular escape.

Mechanism of the Vagal Effects.

• The acetylcholine released at the vagal nerve endings greatly increases the permeability of the fiber membranes to potassium ions, which allows rapid leakage of potassium out of the conductive fibers.

• This process causes increased negativity inside the fibers, an effect called hyperpolarization, which makes this excitable tissue much less excitable.

• In the sinus node, the state of hyperpolarization makes the resting membrane potential of the sinus nodal fibers considerably more negative than usual—that is, −65 to −75 millivolts rather than the normal level of −55 to −60 millivolts.

• Therefore, the initial rise of the sinus nodal membrane potential caused by inward sodium and calcium leakage requires much longer to reach the threshold potential for excitation.

• This requirement greatly slows the rate of rhythmicity of these nodal fibers.

• If the vagal stimulation is strong enough, it is possible to stop the rhythmical self excitation of this node entirely.

Sympathetic Stimulation Increases the Cardiac Rhythm and Conduction

1. It increases the rate of sinus nodal discharge.

2. It increases the rate of conduction, as well as the level of excitability in all portions of the heart.

3. It increases greatly the force of contraction of all the cardiac musculature, both atrial and ventricular

In short, sympathetic stimulation increases the overall activity of the heart.

• Maximal stimulation can almost triple the heartbeat frequency and can increase the strength of heart contraction as much as twofold.

Mechanism of the Sympathetic Effect.

• Stimulation of the sympathetic nerves releases norepinephrine at the sympathetic nerve endings (you already know this!).

• Norepinephrine, in turn, stimulates beta-1 adrenergic receptors, which mediate the effects on heart rate

• This increases the permeability of the fiber membrane to sodium and calcium ions.

• In the A-V node and A-V bundles, increased sodium calcium permeability makes it easier for the action potential to excite each succeeding portion of the conducting fiber bundles, thereby decreasing the conduction time from the atria to the ventricles.

• The increase in permeability to calcium ions is at least partially responsible for the increase in contractile strength of the cardiac muscle under the influence of sympathetic stimulation

Heart Anatomy Review

Cone-shaped hollow muscular organ found in the mediastinum à space between the lungs in thoracic cavity.

In quadrupeds, the base of the heart (region where large blood vessels enter and exit the heart) is directed dorsally (superiorly) and the apex (pointed end) of the heart is directed ventrally (inferiorly).

Pericardium is the connective tissue sac that encloses the heart.

Consists of an outer fibrous pericardium and an inner serous pericardium

Functions of the fibrous pericardium include:

1. Preventing overstretching of the heart

2. Providing some protection for heart

3. Anchoring the heart into the mediastinum

• The heart fits into the serous pericardium like "a fist pushed into a water balloon".

• The serous pericardium is composed of two layers separated by a fluid-containing cavity.

• The visceral layer (also called the epicardium) adheres to the myocardium or muscle of the heart

  • Between the two layers of the serous pericardium is the pericardial cavity, a space containing pericardial fluid that lubricates and reduces friction between the two pericardial membranes as the heart contracts and relaxes.

The wall of the heart is composed of three layers:

1. Epicardium (visceral layer of serous pericardium) à connective tissue layer on the outer surface of heart.

2. Myocardium -> thick layer of cardiac (striated, involuntary) muscle that makes up the bulk of the mass of the heart.

   • Responsible for the heart's pumping ability.

3. Endocardium is the innermost layer

Heart chambers

• Two (Right and left) upper atria (entry hall) collect blood from the veins and then pump it into the ventricles.

  • Each atrium has an auricle which is a small earlike extension on the cranial (anterior) side of the heart that increases the volume of the chambers.

• Two (right and left) lower ventricles (little belly) that do most of the pumping of the blood out to the tissues of the body.

  • The two ventricles are separated by a thick muscular septum called the interventricular septum.

• Heart valves are composed of connective tissue covered by endothelium.

  • Valves prevent backflow of blood between various heart chambers and thus ensure that blood flows in single direction through the heart.

The heart contains 4 valves:

• Two atrioventricular (AV) valves are found between the atria and their corresponding ventricles.

  • Tricuspid valve or Right AV valve is found between right atrium and right ventricle

  • Bicuspid (mitral) valve or Left AV valve is found between left atrium and left ventricle. This valve has two cusps or flaps.

Both are attached to chordae tendineae that are attached to papillary muscles -> Keep the cusps of the valves pointing in the direction of the blood flow -> prevent blood from moving from the ventricles back into the atria.

• Two semilunar valves between the ventricles and large arteries exiting the heart. Both semilunar valves are tricuspid -> Prevent backflow of blood from the arteries back into the ventricles.

  • Pulmonary semilunar valve is found at junction of right ventricle and pulmonary artery.

  • Aortic semilunar valve is found at the junction of left ventricle and the aorta.

Cardiac cycle

Two main concepts: Systole and Diastole

Systole: When the cardiac muscle contracts, it can be atrial or ventricular. Functionally it is when either the atria or ventricles “expel” blood towards the ventricles, or lungs/body.

Diastole: When the cardiac muscle relaxes and dilates, it can be atrial or ventricular. Functionally it is when either the atria or ventricles receive blood.

When the ventricles are full of blood and ready to start contracting, the following events take place:

Period of Isovolumic (Isometric) Contraction.

• Immediately after ventricular contraction begins, the ventricular pressure rises abruptly, causing the atrioventricular (A-V) valves to close.

• An additional 0.02 to 0.03 second is required for the ventricle to build up sufficient pressure to push the semilunar (aortic and pulmonary) valves open against the pressures in the aorta and pulmonary artery.

• During this period, contraction is occurring in the ventricles, but no emptying occurs.

• This period is called the period of isovolumic or isometric contraction, meaning that cardiac muscle tension is increasing but little or no shortening of the muscle fibers is occurring

Period of Ejection.

• When the left ventricular pressure rises slightly above 80 mm Hg (and the right ventricular pressure rises slightly above 8 mm Hg), the ventricular pressures push the semilunar valves open.

• Immediately, blood is ejected out of the ventricles into the aorta and pulmonary artery.

• Approximately 60% of the blood in the ventricles at the end of diastole is ejected during systole

  • About 70% of this portion flows out during the first third of the ejection period, with the remaining 30% emptying during the next two thirds.

  • The first third is called the period of rapid ejection, and the last two thirds are called the period of slow ejection.

Period of Isovolumic (Isometric) Relaxation.

• At the end of systole, ventricular relaxation begins suddenly, allowing both the right and left intraventricular pressures to decrease rapidly.

• The elevated pressures in the distended large arteries that have just been filled with blood from the contracted ventricles immediately push blood back toward the ventricles, which snaps the aortic and pulmonary valves closed.

• For another 0.03 to 0.06 second, the ventricular muscle continues to relax, even though the ventricular volume does not change, giving rise to the period of isovolumic or isometric relaxation.

• During this period, the intraventricular pressures rapidly decrease back to their low diastolic levels.

• Then, the A-V valves open to begin a new cycle of ventricular pumping

During ventricular systole, large amounts of blood accumulate in the right and left atria

because of the closed A-V valves. Therefore, as soon as systole is over, and the ventricular pressures fall to their low diastolic values, the moderately increased pressures that have developed in the atria during ventricular systole immediately push the A-V valves open and allow blood to flow rapidly into the ventricles.

End-Diastolic Volume, End-Systolic Volume, and Stroke Volume Output

During diastole, in humans, normal filling of the ventricles increases the volume of each

ventricle to about 110 to 120 ml. This volume is called the end diastolic volume.

• As the ventricles empty during systole, the volume decreases by about 70 ml,  which is called the stroke volume output.

• The remaining volume in each ventricle, about 40 to 50 ml, is called the end-systolic volume.

• The fraction of the end-diastolic volume that is ejected is called the ejection fraction, usually equal to about 0.6 (or 60%).

• The ejection fraction percentage is often used clinically to assess cardiac systolic (pumping) capability.

  • When the heart contracts strongly, the end-systolic volume may decrease to as little as 10 to 20 ml.

  • Conversely, when large amounts of blood flow into the ventricles during diastole, the ventricular end-diastolic volumes can become as much as 150 to 180 ml in the healthy heart.

  • By both increasing the end-diastolic volume and decreasing the end-systolic volume, the stroke volume output can be increased to more than double that which is normal.

Graphic analysis of ventricular pumping

Phase I: Period of Filling.

• Phase I begins at a ventricular volume of about 50 ml and a diastolic pressure of 2 to 3 mm Hg.

• The amount of blood that remains in the ventricle after the previous heartbeat, 50 ml, is called the end-systolic volume.

• As venous blood flows into the ventricle from the left atrium, the ventricular volume normally increases to about 120 ml, called the end-diastolic volume, an increase of 70 ml.

Phase II: Period of Isovolumic Contraction.

• During isovolumic contraction, the volume of the ventricle does not change because all valves are closed.

• However, the pressure inside the ventricle increases to equal the pressure in the aorta, at a pressure value of about 80 mm Hg.

Phase III: Period of Ejection.

• During ejection, the systolic pressure rises even higher because of still more contraction of the ventricle.

• At the same time, the volume of the ventricle decreases because the aortic valve has now opened, and blood flows out of the ventricle into the aorta.

Phase IV: Period of Isovolumic Relaxation.

• At the end of the period of ejection the aortic valve closes, and the ventricular pressure falls back to the diastolic pressure level.

• The ventricle returns to its starting point, with about 50 ml of blood left in the ventricle at an atrial pressure of 2 to 3 mm Hg

Heart sounds

The act of listening to sounds within the body is called auscultation, which is performed

using a stethoscope. The sound of a heartbeat comes primarily from turbulence in

blood flow caused by the closure of the valves, not from contraction of the heart

muscle.

1. First heart sound (LUBB) represents the closing of the atrioventricular valves soon after ventricular systole (contraction) begins.

2. Second sound (DUPP) represents the closing of the semilunar valves slightly after ventricular diastole (relaxation) begins.

Heart murmur - An abnormal heart sound usually consisting of a flow noise that is heard before or after the LUBB-DUPP or that may mask the normal sounds entirely.

Two potential causes of heart murmurs are:

1. Valvular insufficiency - the incomplete closing of the valves.

2. Valvular stenosis - the narrowing of the valves.

Not all murmurs are abnormal or symptomatic of disease

 Circulatory routes

Two basic circulatory routes found in a mammal are:

1. Systemic circulation à system of vessels that carry oxygenated blood from left ventricle to the tissues (where it becomes deoxygenated) and then back to the right atrium.

2. Pulmonary circulation à system of blood vessels that carries deoxygenated blood from right ventricle to lungs (where it is oxygenated) and then back to the left atrium.

Systemic circulation

Cranial and caudal vena cava (Deoxygenated blood from tissues of the body – large systemic veins and coronary sinus) à right atrium of the heart à tricuspid valve à right ventricle à pulmonary semilunar valve à pulmonary artery à lungs where it is oxygenated.

Pulmonary circulation:

Lungs (Oxygenated blood flow) through pulmonary veins à left atrium à bicuspid valve à left ventricle à aorta semilunar valve à aorta (Distributed by the systemic circulatory route to the rest of the body)

As the blood circulates through the tissues of the body, it becomes deoxygenated and then returns to the right atrium through the venae cava.

Coronary circulation

System of blood vessels that serves the heart muscle

• Oxygenated blood flows directly into the coronary arteries from the aorta.

• Blood is carried in these arteries to various regions of the heart muscle.

• Blood flows through capillaries serving the cardiac myofibers, where 02 is exchanged for C02.

• Deoxygenated blood is collected by the coronary veins that drain into the coronary sinus and then into the right atrium.

• Only the inner one-tenth millimeter of the endocardial surface can obtain significant nutrition directly from the blood inside the cardiac chamber

• Most of the coronary venous blood flow from the left ventricular muscle returns to the right atrium of the heart by way of the coronary sinus.

• The normal coronary blood flow in the resting person averages 70 ml/min/100 g of heart weight, or about 225 ml/min, which is about 4% to 5% of the total cardiac output.

  • During strenuous exercise, the heart in the young adult increases its cardiac output fourfold to sevenfold.

• Coronary capillary blood flow in the left ventricle muscle is low during systole, which is opposite to flow in vascular beds elsewhere in the body.

  • The reason for this phenomenon is strong compression of the intramuscular blood vessels by the left ventricular muscle during systolic contraction.

  • During diastole, the cardiac muscle relaxes and no longer obstructs blood flow through the left ventricular muscle capillaries, so blood flows rapidly during all of diastole

Heart rate

• Increasing Heart Rate Decreases Duration of the Cardiac Cycle.

• When heart rate increases, the duration of each cardiac cycle decreases, including the contraction and relaxation phases (systole and diastole).

  • The duration of systole decreases, but not by as great a percentage as diastole.

  • Example: At a normal heart rate of 72 beats/min, systole comprises about 0.4 of the entire cardiac cycle. At three times the normal heart rate, systole is about 0.65 of the entire cardiac cycle.

    • This means that the heart beating very rapidly does not remain relaxed long enough to allow complete filling of the cardiac chambers before the next contraction.

Cardiac output

The amount of blood ejected by the left (or right) ventricle into the aorta (or pulmonary artery) per minute.

Most commonly it refers to the amount of blood pumped by the left ventricle to the body per minute.

Cardiac Output (CO; volume per minute) = Stroke Volume (SV; volume per beat) × Heart Rate (HR; beats/min)

CO = SV × HR / ↑stroke volume or ↑ heart rate = ↑ CO, and vice versa.

• CO in average resting human is approximately 5 liters/min

  • The maximum percentage that CO can be increased above resting CO is called cardiac reserve.

    • CO can potentially be increased ≈5× resting CO, which would be 5 × 5 L/min = 25 L/min in vigorously exercising humans.

    • In horses, resting cardiac output is ≈13.5 L/min (HR = 30 bpm; SV = 450 mL/beat) and can increase to ≈300 L/min during vigorous activity in a Thoroughbred.

Arteries and Arterioles

• The function of the arteries is to transport blood under high pressure to the tissues. For this reason, arteries have strong vascular walls, and blood flows at a high velocity in the arteries.

• Arterioles are the last small branches of the arterial system; they act as control conduits through which blood is released into the capillaries. Arterioles have strong muscular walls that can close the arterioles completely or, by relaxing, can dilate the vessels severalfold; thus, arterioles can vastly alter blood flow in each tissue in response to its needs.

Capillaries and venules

• The function of the capillaries is to exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and interstitial fluid. To serve this role, the capillary walls are thin and have numerous minute capillary pores permeable to water and other small molecular substances.

• The venules collect blood from the capillaries and gradually coalesce into progressively larger veins.

Veins

• The veins function as conduits for transport of blood from the venules back to the heart.

• The veins also serve as a major reservoir of extra blood. Because the pressure in the venous system is low, the venous walls are thin. Even so, they are muscular enough to contract or expand and thereby serve as a controllable reservoir for the extra blood, either a small or a large amount, depending on the needs of the circulation.

Microcirculation

Each nutrient artery entering an organ branches six to eight times before the arteries become small enough to be called arterioles.

• Then, the arterioles branch two to five times, reaching diameters of 5 to 9 micrometers at their ends, where they supply blood to the capillaries.

• At the point where each true capillary originates from a metarteriole, a smooth muscle fiber usually encircles the capillary.

  • This structure is called the precapillary sphincter, and it can open and close the entrance to the capillary.

The venules are larger than the arterioles and have a much weaker muscular coat.

• The pressure in the venules is much less than that in the arterioles.

• The metarterioles and precapillary sphincters are in close contact with the tissues they serve.

• The local conditions of the tissues—such as the concentrations of nutrients, end products of metabolism, and hydrogen ions—can cause direct effects on the vessels to control local blood flow in each small tissue area.

Blood flow in capillaries

• Blood usually does not flow continuously through the capillaries. Instead, it flows intermittently, turning on and off every few seconds or minutes.

  • This happens because of the intermittent contraction of the metarterioles and precapillary sphincters (vasomotion).

• Despite the fact that blood flow through each capillary is intermittent, so many capillaries are present in the tissues that their overall function becomes averaged.

Water and nutrient exchange between the blood and interstitial fluid

Diffusion through the capillary membrane is the most important means of transferring substances between plasma and interstitial fluid

• As the blood flows along the lumen of the capillary, tremendous numbers of water molecules and dissolved particles diffuse back and forth through the capillary wall, providing continual mixing between the interstitial fluid and plasma.

• Electrolytes, nutrients, and waste products of metabolism all diffuse easily through the capillary membrane.

• Proteins are the only blood constituents that do not readily pass through the capillary membrane

• Lipid-soluble substances diffuse directly through the cell membranes of the capillary endothelium

  • Such substances include oxygen and carbon dioxide

Intercellular cleft: thin-slitted, curving channel that lies between adjacent endothelial cells.

• The width of the capillary intercellular cleft pores, 6 to 7 nanometers, is about 20 times the diameter of the water molecule, which is the smallest molecule that normally passes through the capillary pores.

• The diameters of plasma protein molecules, however, are slightly greater than the width of the pores.

• Other substances, such as sodium ions, chloride ions, glucose, and urea, have intermediate diameters.

• The permeability of the capillary pores for different substances varies according to their molecular diameters

The size of the pores varies greatly depending on the tissue

1. In the brain, the junctions between the capillary endothelial cells are tight junctions that allow only extremely small molecules such as water, oxygen, and carbon dioxide to pass into or out of the brain tissues.

2. In the liver, the clefts between the capillary endothelial cells are nearly wide open so that almost all dissolved substances of the plasma, including the plasma proteins, can pass from the blood into the liver tissues.

3. The pores of the gastrointestinal capillary membranes are midway in size between those of the muscles and those of the liver.

4. In the glomerular capillaries of the kidney, numerous small oval windows called fenestrae penetrate all the way through the middle of the endothelial cells so that tremendous amounts of small molecular and ionic substances (but not the large molecules of the plasma proteins) can filter through the glomeruli without having to pass through the clefts between the endothelial cells

Fluid filtration across capillaries

• The hydrostatic pressure in the capillaries tends to force fluid and its dissolved substances through the capillary pores into the interstitial spaces.

• Conversely, osmotic pressure caused by the plasma proteins (colloid osmotic pressure) tends to cause fluid movement by osmosis from the interstitial spaces into the blood.

• This osmotic pressure exerted by the plasma proteins normally prevents significant loss of fluid volume from the blood into the interstitial spaces

Net exchange pressure across capillary walls at any point in a capillary can be determined using the following equation referred to as the Starling Hypothesis:

Net exchange pressure = (PC + πIF) – (PIF + πP)

Where:

• PC = Capillary hydrostatic pressure

• PIF = Interstitial fluid hydrostatic pressure

• πP = Plasma colloid osmotic pressure

• πIF = Interstitial colloid osmotic pressure

(PC + πIF) represents the net outward driving pressures.

(PIF + πP) represents the net inward driving pressures.

Hydrostatic Pressure

Pressure exerted on vessels walls by fluid (blood – “outward” or interstitial fluid – “inward”)

Net filtration pressure (NFP) = PC – PIF

• Capillary hydrostatic pressure (PC) is the blood pressure in the capillary bed. It varies depending on arterial pressure, venous pressure, precapillary and post-capillary resistance.

• In general, PC is highest on the arteriolar end of the capillary bed (37 mm Hg) and lowest on the venular end (17 mm Hg).

• Interstitial fluid hydrostatic pressure (PIF) is extremely low (≈1 mm Hg) throughout the capillary bed under normal conditions.

Colloid Osmotic (Oncotic) Pressure

Osmotic pressure created by the presence of non-filterable proteins in the plasma.

• Plasma colloid osmotic pressure (πP) = 25 mm Hg.

• Interstitial colloid osmotic pressure (πIF) = 0 mm Hg.

Note: Other than proteins, most solutes (like ions and small molecules) diffuse easily between plasma and IF, and therefore do not contribute to colloid osmotic pressure.

So, using net exchange pressure to determine net direction of bulk flow at the both ends of a capillary bed:

Net exchange pressure = (PC + πIF) – (PIF + πP)

Net exchange under normal conditions:

On the arteriolar end of capillary bed:

• Net exchange pressure = (37 + 0) – (1 + 25) = +11 mm Hg

The positive net exchange pressure means that ultrafiltration is occurring on the arteriolar end of the capillary bed.

On the venular end of capillary bed:

• Net exchange pressure = (17 + 0) – (1 + 25) = -9 mm Hg

The negative net exchange pressure means that reabsorption is occurring on the venular end of the capillary bed.

Note: In most capillary beds, net ultrafiltration (capillary to interstitial space) is greater than net reabsorption (interstitial space to venule) resulting in accumulation of fluid in the interstitial space. On average, ultrafiltration from all capillary beds in humans exceeds reabsorption by about 3 liters/day or about 9/10 of what was filtered in the capillaries is reabsorbed in the venules. This non-absorbed fluid is “picked up” by the lymphatic system. (More on this later!)

Abnormal imbalance of forces at the capillary membrane

• If the mean capillary pressure rises significantly above the average value of 17 mm Hg, the net force tending to cause filtration of fluid into the tissue spaces rises.

  • Fluid will begin to accumulate in the interstitial spaces and edema will result

• Conversely, if the capillary pressure falls very low, net reabsorption of fluid into the capillaries will occur instead of net filtration, and the blood volume will increase at the expense of the interstitial fluid volume.

Edema

Excessive accumulation of interstitial fluid in the interstitial spaces within a tissue.

Example: pulmonary edema is excess accumulation of fluid in the interstitial spaces of the lungs.

Edema may result for a variety of reasons. Some common causes include:

• High arterial blood pressure, Blood volume (pregnancy).

• Venous obstruction.

• Leakage of plasma proteins into interstitial fluid.

• Loss of plasma proteins in the urine due to leakage of plasma proteins into filtrate in the renal corpuscle of the kidney.

• Decreased plasma protein concentration due to severe starvation or liver failure.

  • Kwashiorkor is a form of severe protein malnutrition observed in malnourished children.

• One sign of this disease is a severely swollen belly, which is primarily due to fluid accumulation in the peritoneal cavity (ascites = peritoneal edema).

  • This form of edema results from lower concentration of plasma proteins and the corresponding lower plasma colloid osmotic pressure (πP).

• Obstruction of lymphatic drainage = Lymphedema.

Lymphatic system

• The lymphatic system represents an accessory route through which fluid can flow from the interstitial spaces into the blood.

• Most importantly, the lymphatics can carry proteins and large particulate matter away from the tissue spaces, neither of which can be removed by absorption directly into the blood capillaries.

• This return of proteins to the blood from the interstitial spaces is an essential function, without which the animal would die within about 24 hours.

Lymphatic System – Three general functions:

1. Transports excess interstitial (tissue) fluid back into the bloodstream.

2. Transports absorbed fat from small intestine to the bloodstream.

3. Help provide immunological defenses against pathogens in lymphatic tissues like lymph nodes, tonsils, and spleen.

• Lymphatic vessels are found in all tissues except cartilage, bone, epithelium, and CNS tissues.

• The smallest vessels of the lymphatic system are the lymphatic capillaries, which are closed-ended vessels found widely dispersed in the capillary beds of most tissues.

  • Lymphatic capillaries are highly permeable to almost all dissolved chemicals (along with particulates, microbes, and cells).

• Once interstitial fluid containing its solutes, microbes, cells, and particulates enters the lymphatic capillaries, it is referred to as lymph.

• Lymphatic capillaries merge to form successively larger lymph vessels called lymphatics.

  • As the lymph moves through these vessels, it circulates through lymph nodes, which are oval or kidney-shaped organs that act as lymph filters to remove foreign particles like bacteria and viruses and to help orchestrate immune responses to pathogens.

  • The lymphatics eventually merge into the right and left thoracic ducts that drain into the subclavian veins near where these veins connect with the internal jugular vein.

• Lymphatics contain a system of one-way valves and movement of lymph is driven by skeletal muscle pumping, lymphatic vessel smooth muscle contractions, and tissue pressure.

  • There is no dedicated pump for lymph circulation in mammals.

• Throughout the course of the day, flow of lymph varies considerably.

  • It is highest during exercise and lowest during periods of inactivity.

Lymphatic system circulation

• Average lymph flow per day is about equal to the animal’s volume of plasma (3 liter/day in humans).

• Lymphedema results from blockage of the lymphatics leading to edema in the most compliant tissues (usually subcutaneous) in the area of the body where the blockage occurs.

  • Example: Elephantiasis is an example of what happens during severe lymphatic blockage. It is caused by the larvae of the filariasis worm, which invade and block the lymphatic system causing massive lymphedema in extremities and other soft tissues.

• The fluid that returns to the circulation by way of the lymphatics is extremely important because substances of high molecular weight, such as proteins, cannot be absorbed from the tissues in any other way.

• At the junctions of adjacent endothelial cells, the edge of one endothelial cell overlaps the edge of the adjacent cell in such a way that the overlapping edge is free to flap inward, thus forming a minute valve that opens to the interior of the lymphatic capillary.

  • However, this fluid has difficulty leaving the capillary once it has entered because any backflow closes the flap valve

Blood, Erythrocytes and Hemoglobin
Blood is the suspension of cells in an extracellular fluid (ECF) called plasma that circulates in
Blood, Erythrocytes and Hemoglobin

Blood is the suspension of cells in an extracellular fluid (ECF) called plasma that circulates in

the cardiovascular system.

Functions

1. Transportation.

   • Delivers nutrients, acquired by the digestive system, to the cells of the body

   • Carries oxygen (02) from lungs to tissues.

   • Carries carbon dioxide (C02) from tissues to the lungs.

   • Brings waste products (e.g. nitrogenous wastes like urea and uric acid) from tissues to the kidneys for excretion.

   • Carries hormones secreted from endocrine glands to their target tissues to regulate cellular functions.

   • Plays a role in temperature regulation by bringing heat from deeper structures to the surface.

2. Regulation

   • Buffers such as bicarbonate (HC0-3) and proteins in plasma help maintain a stable pH in body fluids and tissues.

   • Plasma proteins (e.g. albumins) help maintain osmotic balance of body fluids.

3. Protection.

   • Platelets and clotting factors in blood prevent loss of body fluid from damaged blood vessels.

   • Many components of the immune system (e.g. white blood cells, antibodies, interferon, and complement) that protects against infections are found in blood.

Composition

Centrifugation of a blood sample in the presence of an anticoagulant (i.e., EDTA, citrate,

heparin, or oxalate) results in three layers:

1. Packed red cells - red blood cells (RBC) found at the bottom of the centrifugation tube.

   • Hematocrit (also called PCV for packed cell volume)

     • The percentage of the total blood volume represented by packed red blood cells.

     • Normal hematocrit values are: 40-54 % for men, 37-47% for women, 27-45% for sheep.

2. Packed white blood cells

• Layer just above the packed red cells that contains the white blood cells (WBC) and platelets.

1. Plasma and serum

• Plasma is the clear fluid layer at the top of the centrifuge tube that is colorless to yellow (yellow color is due to bilirubin and carotenes).

  • Fluid layer that results from the removal of formed elements from blood in the presence of an anticoagulant.

  • It contains clotting proteins like fibrinogen.

• Serum is the fluid layer that results when a blood sample is allowed to spontaneously clot.

  • Therefore, serum differs from plasma in that it does not contain clotting proteins like fibrinogen.

Composition of plasma: Blood makes up 8% of total mass of an organism and plasma

makes up approximately 55% of total blood volume.

Plasma is an extracellular fluid (ECF) and contains:

1. Water- 92%

2. Solutes-8%

3. Plasma proteins - 7% of plasma. Important for:

   • Maintaining total fluid balance

   • Transporting substances

   • Clotting

   • Plasma proteins are primarily produced by the liver.

   • Albumins constitute 60% of plasma proteins.

     • Albumins serve as transport and are important in maintaining osmotic and pH (act as buffers) balance in the body.

   • Transported substances include some water-insoluble hormones, free fatty acids, bile acids, trace elements (iron, copper, cobalt, manganese, and zinc) and some drugs (e.g. penicillin).

   • Globulins - 35% - Include antibodies, which are proteins used in immune system.

   • Fibrinogen - 4% - A large protein that is converted into insoluble fibrin strands during clot formation.

4. Other proteins - 1%

5. Other solutes - 1%

• Electrolytes or ions including Na+ (primary cation), Cl- (primary anion)

• Organic nutrients and regulatory substances including lipids, glucose, amino acids, and vitamins.

• Organic waste products including urea, uric acid, bilirubin, and creatinine.

• Dissolved gases - C02, 02

Formed elements of the blood

Make up approximately 45% of total blood volume. There are three types of formed elements:

1. Erythrocytes (red blood cells, RBC) - 99.9% of total cells present.

   • Major function is transport of gases (02, C02).

2. Leukocytes (white blood cells, WBC)

   • Function in immune defense.

3. Thrombocytes (platelets)

   • Function in hemostasis, which is the stoppage of bleeding from damaged blood vessels.

Erythrocytes

• Shape: Biconcave disc. This shape allows for:

  • All intracellular hemoglobin to be close to the cell surface

  • Increased flexibility for squeezing through narrow capillaries (smallest capillaries can be 3 μm in diameter).

• Enucleated - Nucleus lost as cells mature.

• RBCs do not contain mitochondria, so 02 carried by these cells is not used to generate energy (ATP) in the RBC.

  • RBCs use anaerobic respiration (glycolysis) for energy production.

• Mature RBCs are essentially "bags of hemoglobin".

  • Hemoglobin (Hb) is an iron-containing protein responsible for most 02 (98.5%) and some C02 (23%) transport by the blood.

• Hemoglobin is made up to two parts:

• Globin - protein portion consisting of 4 subunits: 2 α subunits and 2 ß- subunits.

• Heme - iron-containing pigment found in each of the four protein subunits. Each heme binds one molecule of 02, so one hemoglobin molecule binds up to 4 02 molecules.

Hemoglobin

Presence of hemoglobin in RBCs substantially increases the total O2 carrying capacity of blood.

• Transports 98.5% of the oxygen in the blood

• The remaining O2 (1.5%) is carried dissolved in the plasma (O2 is not very dissolvable in water, more on this later!)

Transport of O2 and CO2 in the body

Percent saturation of Hb à % of total Hb in the blood that is HbO2

• Fully saturated blood - When all Hb present in the blood is HbO2.

  • In the lungs where the partial pressure of oxygen (Similar to a “concentration gradient of oxygen”; PO2) is high, Hb is almost fully saturated (98%).

• Partially saturated blood - Hb consisting of a mixture of free Hb and HbO2

  • Under resting conditions, blood coming to the lungs from tissues is 75% saturated with O2. In contrast, blood coming to lungs from actively contracting muscles drops to 35% saturation.

Oxygen-hemoglobin (O2-Hb) dissociation curve:

The level of saturation of Hb with O2 can be represented in an O2-Hb dissociation curve:

• Useful for understanding how Hb can bind O2 in the lungs during external respiration and then release the O2 to the tissues during internal respiration.

• The physiological basis for this ability is that some conditions (e.g. pH, pCO2, temperature) within the lungs are different than in the tissues.

• These differences affect the affinity of Hb for O2 in a manner that:

  • They promote O2 binding to Hb in the lungs (higher affinity, left shift)

  • They promote the release of O2 from Hb in the tissues (lower affinity; right shift).

Factors that affect affinity (force of attraction) of Hb for O2.

• Factors that cause a right shift of O2-Hb curve:

  • Decrease the affinity of Hb for O2

    •  ↓ pH, ↑ PCO2, ↑ temperature

• Factors that cause a left shift of O2-Hb curve

  • Increase the affinity of Hb for O2

  • ↑ pH, ↓ PCO2, ↓ temperature

Note that if an increase in the presence of a factor causes a right shift, then a decrease in the presence of that same factor will lead to a left shift.

1. pH -> ↓ pH = ↑ [H+] = ↓ affinity of Hb for O2 (right shift)

   • Called the Bohr shift - In an acidic environment (low pH, high [H+]), Hb’s affinity for O2 is lower (right shift) making it easier for O2 to split from Hb.

     • This occurs because excess H+ bind to HbO2, promoting release of O2

     • In the tissues of an organism, pH is lower (more acidic) than in lungs.

     • This lower pH in the tissues promotes release of O2 from Hb during internal respiration, while the higher pH in the lungs promotes O2 binding to Hb during external respiration.

• Tissue acidity (↑ [H+] and ↓ pH) results from two sources:

  • High PCO2 in tissues

    • As CO2 is taken up by an RBC, the CO2 is converted to carbonic acid (H2CO3) using an enzyme in RBC called carbonic anhydrase:

carbonic anhydrase

CO2 + H20 <-----------> H2CO3<---> H+ + HCO3-

• H2CO3 formed in RBC rapidly dissociates into H+ (hydrogen ion) and HCO3- (bicarbonate):

  •  ↑  PCO2 = ↑ [H+] = ↓ pH

• Tissues are constantly producing CO2 by catabolism, so pH in tissues is lower than pH in lungs thus promoting release of O2 from HbO2.

• Lactic acid also lowers pH in actively contracting muscles.

  • Lactic acid is a byproduct of anaerobic metabolism within tissues, which contributes to the “Bohr shift” by lowering pH in highly active tissues especially contracting skeletal muscles

1. Temperature.

   • ↑Temperature = ↓ affinity of Hb for O2 (right shift)

   • Heat is a by-product of catabolic reactions in all cells.

   • Active tissues like contracting muscles liberate more heat, which promotes the release of the needed O2 from HbO2.

2. 2,3-Biphosphoglycerate (2,3-BPG) -> used primarily at high elevation.

   • ↑  ↓ [2,3-BPG] in RBC = ↓ affinity of Hb for O2 (right shift).

   • ↑  [2,3-BPG] in RBC increases in response to an animal going to a higher elevation (an environment with lower PO2)

   • In low oxygen environments (like at high elevation):

     • 2,3-BPG in RBCs binds to hemoglobin and promotes the release of oxygen from the hemoglobin

Transportation of carbon dioxide (CO2) in the blood.

CO2 is carried in the blood in three forms:

1. Dissolved CO2 – 7%

2. Bicarbonate ions (HCO3-) - 70%.

3. Carbaminohemoglobin – 23%

1. Dissolved CO2 – 7% is dissolved directly in plasma.

   • CO2 is 20-fold more soluble in water (plasma) than O2, so a higher percentage of CO2 is carried directly dissolved in plasma.

2. Bicarbonate ions ->

   • In RBCs, CO2 is rapidly converted to carbonic acid (H2CO3) and then to H+ + HCO3- by the following reaction:

carbonic anhydrase

CO2 + H20 <------------------------> H2CO3<---> H+ + HCO3-

• Chloride shift - As [HCO3-] builds up in RBC, bicarbonate is exchanged for plasma Cl- through the band III protein, a passive anion exchanger (antiporter).

  • Thus, Cl- moves into RBC and HCO3- moves into the plasma.

• Movement of HCO3- into the plasma allows the continued conversion of CO2 to HCO3-.

Conversion of HCO3- back into CO2 during external respiration.

• In the lungs, HCO3- is brought back into RBC in exchange for Cl- through the band III protein (“reverse chloride shift”).

• HCO3- is converted back into CO2 and water by the following reaction:

Carbonic anhydrase

H+ + HCO3- <---> H2CO3 <------------------> CO2 + H2O

NOTE: The H+ needed to reform H2CO3 is provided when O2 binds to Hb and causes the release of H+ from Hb.

• The CO2 then diffuses rapidly into the alveoli and is removed by expiration.

1. Carbaminohemoglobin -> 23% of CO2 is carried bound to an amino group on the globin portion of Hb.

   • “Haldane effect” Binding of O2 to Hb facilitates release of CO2 from Hb:

     • In the presence of low PO2 (tissues), more CO2 binds to Hb.

     • In the presence of high PO2 (lungs), less CO2 binds to Hb.

   • Finally, the CO2 dissolved in the plasma simply diffuses across the respiratory membrane into the alveoli for removal by expiration.

Forms of hemoglobin

• Oxyhemoglobin (Hb02) -02 reversibly binds to iron in heme group of hemoglobin.

  • Binding is reversible so that 02 can be released from Hb in the tissues during internal respiration.

  • Bright red color.

• Deoxyhemoglobin (Hb) - hemoglobin with no 02 bound.

  • Purplish red (maroon) in color.

• Carbaminohemoglobin (HbC02) - Deoxyhemoglobin transporting C02 (about 23% of total C02).

  • Remember, C02 is not bound to iron in heme but to an amino group on the globin portion of Hb.

Abnormal forms of hemoglobin.

• Carboxyhemoglobin - Carbon monoxide (CO) binds almost irreversibly (210 times higher affinity than 02) to heme portion of hemoglobin.

  • This form of Hb can no longer transport 02.

  • Blood has cherry red color.

• Methemoglobin à Iron in hemoglobin has been oxidized to the ferric form (Fe+3) -> incapable of binding 02 à Iron must be in ferrous form to bind oxygen (Fe+2).

  • Blood has chocolate brown color.

  • Often found in animals that have ingested nitrates (fertilizers) or breathed in chlorates (herbicides).

  • Intravenous infusion of methylene blue à reducing agent, can be used to convert methemoglobin back to functional hemoglobin.

Erythropoiesis

Hematopoiesis is "formation of all types of new blood cells". Erythropoiesis is "the production of new RBCs". Both of these processes occur in the bone marrow.

• All blood cells are produced from pluripotent hematopoietic stem cells (hemocytoblasts) in the bone marrow.

  • These stem cells progress through various stages of differentiation and maturation to produce mature blood cells.

• In general, the marrow of essentially all bones produces RBCs until early infancy.

  • The marrow of the long bones, except for the proximal portions of the humeri and tibiae, becomes fatty and produces no more RBCs after puberty-early adolescence.

  • Beyond this age, most RBCs continue to be produced in the marrow of the membranous bones, such as the vertebrae, sternum, ribs, and ilia.

  • Even in these bones, the marrow becomes less productive as age increases.

The development of RBCs

• Blood cells begin their lives in the bone marrow from a single type of cell called the pluripotent hematopoietic stem cells, from which all cells in blood are eventually derived.

• A small portion of them remains exactly like the original multipotential cells and is retained in the bone marrow to maintain their supply. Numbers diminish with age.

• Intermediate stage cells are stem cells committed to a particular line of cells.

  • These are called committed stem cells.

• Different committed stem cells, when grown in culture, will produce colonies of specific types of blood cells.

  • A committed stem cell that produces erythrocytes is called a colony-forming unit– erythrocyte, and the abbreviation (CFU-E)

• Growth and reproduction of the different stem cells are controlled by multiple proteins called growth inducers.

• Growth inducers promote growth but not differentiation of the cells, which is the function of another set of proteins called differentiation inducers.

  • Formation of growth and differentiation inducers is controlled by factors outside the bone marrow.

  • i.e., in RBCs, exposure of the blood to a low oxygen level for a long time causes growth and differentiation induction -> production of greatly increased numbers of RBCs.

• The initial generations of CFU-E to early erythrocyte development contain no hemoglobin and nucleus.

• In the succeeding generations of erythrocyte development, the cells become filled with hemoglobin to a concentration of about 34% à nucleus condenses to a small size, and its final remnant is absorbed or extruded from the cell.

• In the reticulocyte stage (stage previous to mature erythrocyte), the cells pass from the bone marrow into the blood capillaries by diapedesis (squeezing through the pores of the capillary membrane)

  • After 2 days the cell is then a mature erythrocyte.

  • Because of the short life of the reticulocytes, their concentration among all the RBCs is normally slightly less than 1%

Regulation of Erythropoiesis

• Regulation of the rate of erythropoiesis involves a negative feedback system centered in the kidney.

  • Oxygen deficiency (hypoxia) is sensed by specialized kidney cells that respond to low 02 levels by secreting erythropoietin (EPO) into the plasma.

  • Erythropoietin is carried in the blood to the bone marrow where it stimulates faster development of RBCs.

  • It takes approximately three days to observe an increase in the number of RBCs in blood.

  • When hypoxia is no longer detected by kidney cells, EPO secretion decreases and the rate of production of new RBCs slows.

• When a person becomes extremely anemic as a result of hemorrhage or any other condition, the bone marrow begins to produce large quantities of RBCs.

• At very high altitudes, where the partial pressure of oxygen in the air is greatly decreased, insufficient oxygen is transported to the tissues, and RBC production is greatly increased.

  • In this case, it is not the concentration of RBCs in the blood that controls RBC production, but the amount of oxygen transported to the tissues in relation to tissue demand for oxygen.

• Diseases of the circulation that decrease tissue blood flow can also increase the rate of RBC production.

  • This is especially apparent in prolonged cardiac failure and in many lung diseases because the tissue hypoxia resulting from these conditions increases RBC production, with a resultant increase in hematocrit and, usually, total blood volume.

Erythropoietin

• Normally, about 90% of all erythropoietin is formed in the kidneys, and the remainder is formed mainly in the liver.

• Erythropoietin stimulates the production of proerythroblasts from hematopoietic stem cells in the bone marrow.

  • Erythropoietin causes these cells to pass more rapidly through the different erythroblastic stages than they normally do.

• In the absence of erythropoietin, few RBCs are formed by the bone marrow.

• When large quantities of erythropoietin are formed, and if plenty of iron and other required nutrients are available, the rate of RBC production can rise to perhaps 10 or more times normal.

Erythrocyte lifespan and recycling

• Lifespan of RBCs is around 120 days in humans, 130-160 in cattle, 110 – 120 in dogs, 140 -150 in horses, 120 in sheep, 86 in pigs and 35 days in chickens.

• Once the RBC membrane becomes fragile, the cell ruptures during passage through some tight spot (usually spleen capillaries) of the circulation.

• Many RBCs self-destruct in the spleen, where they squeeze through the red pulp of the spleen.

  • The spaces between the structural trabeculae of the red pulp, through which most of the cells must pass, are only 3 micrometers wide, in comparison with the 8- micrometer diameter of the RBC.

  • When the spleen is removed, the number of old abnormal RBCs circulating in the blood increases considerably.

• When RBCs burst and release their hemoglobin, the hemoglobin is phagocytized almost immediately by macrophages in the liver, spleen in bone marrow.

  • During the next few hours to days, the macrophages release iron from the hemoglobin and pass it back into the blood to be carried by transferrin either to the bone marrow for production of new RBCs or to the liver and other tissues for storage in the form of ferritin.

  • Globin portion of Hb is broken down into amino acids that are used for synthesizing new proteins.

Metabolites of non-iron heme and bilirubin formation:

•  In the macrophages, heme is converted to biliverdin (green pigment) by removal of globin and iron.

  • Biliverdin is then converted to bilirubin in the (yellow pigment) in the macrophages, which is then transported in the blood to the liver where it is secreted as a component of the into the small intestine.

    • Jaundice (or icterus) - Abnormal yellowing of skin, mucous membrane, and whites of eyes due to excess accumulation of bilirubin in vascular system.

      • Common sign of liver problems or blockage of the bile ducts by gallstones.

  • Bacteria in the intestine convert bilirubin to urobilinogen.

    • Some urobilinogen is absorbed back into the bloodstream and converted into urobilin (yellow), which is excreted in urine.

    • Most urobilinogen stays in the intestine and is converted to stercobilin (brown).

Abnormalities associated with regulation of RBC production.

• Anemia - Inability of blood to carry sufficient 02 to the body's cells. Causes hypoxia (inability to deliver enough oxygen to the tissues).

  • Results from inadequate numbers of RBCs or a deficiency of hemoglobin.

Examples:

• Blood loss anemia.

• Aplastic anemia, due to bone marrow dysfunction.

• Hemolytic anemia

• Polycythemia - Excess production of RBCs.

  • Polycythemia - increased blood viscosity à elevated blood pressure due to increased total peripheral resistance (TPR).

  • Two types.

    • Polycythemia vera: Overproduction of RBCs in the bone marrow, usually genetic.

    • Secondary polycythemia: Whenever the tissues become hypoxic because of too little oxygen in the breathed air, such as at high altitudes, or because of failure of oxygen delivery to the tissues, such as in cardiac failure, the blood-forming organs automatically produce large quantities of extra RBC's

Platelets (thrombocytes)

• Platelets play a key role in stopping bleeding from a damaged blood vessel.

• Mammalian platelets are produced in the bone marrow from large cells called

Megakaryocytes.

• Fragments of the megakaryocyte break off and enter the circulation as formed elements called platelets.

• Each platelet contains numerous granules in its cytosol but contains no nucleus.

  • Platelet granules contain chemicals (like ADP, thromboxane A2, serotonin, platelet factor) that promote blood clotting and vasoconstriction of damaged blood vessels.

• Platelets have a short lifespan (9-12 days).

  • Removed from the circulation by fixed macrophages in spleen and liver.

Immunity

Immunity is the capacity for the body to resist pathogenic agents. The immune system of the animal’s body can be divided into two components: innate and acquired immunity.

Innate immunity is the inherited defense mechanism of the body to resist pathogens

• It does not distinguish between specific pathogens, also lacks memory.

• Involves general defense mechanisms that protect against a variety of pathogens.

• Defenses are rapidly activated and act as a “first line” of protection against a wide variety of threats.

  • These defenses include protection by:

1. Physical barriers (e.g. epithelial cells)

2. Inflammation or inflammatory response

3. Phagocytes

4. Complement

5. Fever

6. Interferons

7. Immunological surveillance by natural killer cells

Adaptive immunity is an “acquired immunity”. It is developed after the immune system has been exposed to a specific antigen (molecule from pathogen that can stimulate an immune response). It is developed by the lymphocytes (More on this later!).

Leukocytes

General characteristics

• White blood cells (WBCs) have nuclei, but no hemoglobin

• Function: WBCs are important components of the immune system that protects animals against infectious pathogens.

• There are two groups of WBCs:

  • Granular leukocytes (granulocytes) are WBCs that contain granules in their cytoplasm that are visible under a light microscope.

    • Granular leukocytes get their names because of the staining properties of their granules with acidic (eosin -red color) and basic (baso - blue color) dyes.

  • Agranular leukocytes are WBCs that do not contain cytoplasmic granules.

• There are 6 types of WBCs normally present in the blood:

  • Neutrophils (polymorphonuclear, granulocyte),

  • Eosinophils (polymorphonuclear -, granulocyte-red),

  • Basophils (polymorphonuclear – granulocyte-blue),

  • Monocytes (Macrophages, dendritic cells)

  • Lymphocytes (Adaptive immunity)

  • Occasionally plasma cells (Adaptive immunity)

  • Granulocytes and monocytes (macrophages and dendritic cells) protect the body against invading organisms by ingesting them (by phagocytosis) or by releasing antimicrobial or inflammatory substances.

  • The lifespan of granulocytes after being released from the bone marrow is normally 4 to 8 hours circulating in the blood and another 4 to 5 days in tissues

• Most WBCs are specifically transported to areas of serious infection and inflammation, thereby providing a rapid and potent defense against infectious agents.

Concentration of leucocytes in blood

• An adult human has about 7000 (4000 – 10000) WBCs per microliter of blood

• Of the total WBCs, the normal percentages of the different types are approximately the following:

  • Neutrophils: 62.0%

  • Eosinophils: 2.3%

  • Basophils: 0.4%

  • Monocytes: 5.3%

  • Lymphocytes: 30.0%

Genesis of Leucocytes

• All white blood cells are produced and derived from pluripotent stem cells in the bone marrow known as hematopoietic stem cells

• Hematopoietic stem cells undergo differentiation into specific blood cell lineages under the influence of various cytokines and growth factors

  • Leads to the formation of different types of leukocytes

• In response to infection or inflammation, the production of leukocytes can increase rapidly, leading to leukocytosis

• Elevated white blood cell counts are a common marker of infection, injury, or immune response in the body

Immune system activation, generalities

• Immune responses use highly specific receptor proteins to distinguish “self (host)” from “nonself (pathogen)” by recognizing antigens associated with specific pathogens.

  • Antigens -> Foreign (“nonself”) molecules that stimulate the activation of the immune system.

  • Antigens are either macromolecules released by the pathogen or macromolecules on the surface of an invading pathogen.

• Immune defenses are coordinated using cytokines à “Immunohormones”

  • Cytokines à “Immunohormones” are low molecular weight proteins released by immune cells that usually act in a paracrine or autocrine fashion to regulate the intensity and duration of immune defenses.

  • Cytokine effects usually involve stimulation of proliferation or activation of some type of immune cell.

  • Examples of cytokines include interleukins (IL), colony stimulating factors (CSF), interferons, and tumor necrosis factors (TNF).

Monocyte-macrophage cell system (reticuloendothelial system) – Innate system

Monocytes are leukocytes derived from the bone marrow

• They become macrophages or dendritic cells

• A large portion of monocytes becomes attached to the tissues and remains attached for months or even years until they are called on to perform specific local protective functions

• They have the capability to phagocytize large quantities of bacteria, viruses, necrotic tissue, or other foreign particles.

• When appropriately stimulated, they can become mobile macrophages.

• The total combination of monocytes, mobile macrophages, fixed tissue macrophages, and a few specialized endothelial cells in the bone marrow, spleen, and lymph nodes is called the reticuloendothelial system (monocyte-macrophage system).

Types of macrophages:

1. Tissue Macrophages in Skin and Subcutaneous Tissues (Histiocytes)

2. Macrophages in Lymph Nodes

   • Essentially no particulate matter that enters the tissues, such as bacteria, can be absorbed directly through the capillary membranes into the blood.

   • If the particles are not destroyed locally in the tissues, they enter the lymph and flow to the lymph nodes -> large number of macrophages

3. Alveolar Macrophages in the lungs

   • Phagocyte or form a capsule around the particle or agent.

4. Macrophages (Kupffer Cells) in Liver Sinusoids

   • Large numbers of bacteria from ingested food constantly pass through the gastrointestinal mucosa into the portal blood

   • Pass through liver à Very effective particulate filtration system, almost none of the bacteria from the gastrointestinal tract pass from the portal blood into the general systemic circulation.

5. Macrophages of Spleen and Bone Marrow

   • If pathogen reaches general circulation à Macrophages of spleen and bone marrow.

   • Spleen is similar to the lymph nodes, except that blood, instead of lymph, flows through the tissue spaces of the spleen.

Inflammation

Inflammation is characterized by the following:

1. Vasodilation of the local blood vessels à increased local blood flow

2. Increased permeability of the capillaries à leakage of large quantities of fluid into the interstitial spaces

3. Clotting of the fluid in the interstitial spaces because of increased amounts of fibrinogen and other proteins leaking from the capillaries

4. Migration of large numbers of granulocytes and monocytes into the tissue

Swelling of the tissue cells

Various substances cause the mentioned reactions

• Several of these substances strongly activate the macrophage system, and within a few hours, the macrophages begin to devour the destroyed tissues.

• At times, however, the macrophages may also further injure the still-living tissue cells

Walling-Off Effect of Inflammation

• The tissue spaces and the lymphatics in the inflamed area are blocked by fibrinogen clots so that after a while, fluid barely flows through the spaces.

• This walling-off process delays the spread of bacteria or toxic products.

• The intensity of the inflammatory process is usually proportional to the degree of tissue injury.

  • Example: staphylococci invade tissues and release extremely lethal cellular toxins.

    • Inflammation develops rapidly, being walled off and prevented from spreading through the body.

  • Streptococci, in contrast, do not cause such intense local tissue destruction ->the walling-off process develops slowly à many streptococci reproduce and migrate -> far greater tendency to spread through the body and cause death than staphylococci.

Macrophage and neutrophil response during inflammation

1. Tissue Macrophages Provide First Line of Defense Against Infection (once the physical barriers have been breached).

2. Neutrophil Invasion of the Inflamed Area Is a Second Line of Defense

   • Within the first hour after inflammation begins, large numbers of neutrophils begin to invade the inflamed area from the blood.

   • This invasion is caused by inflammatory cytokines and other biochemical products produced by the inflamed tissues.

3. Second Macrophage Invasion into the Inflamed Tissue Is a Third Line of Defense.

   • Along with the invasion of neutrophils, monocytes from the blood enter the inflamed tissue and enlarge to become macrophages.

   • Number of monocytes in the circulating blood is low and the storage pool of monocytes in the bone marrow is much less than that of neutrophils à

     • Buildup of macrophages in the inflamed tissue area is much slower than that of neutrophils

   • Monocytes are still immature cells, requiring 8 hours or more to swell and develop great quantities of lysosomes ->

   • After several days to several weeks, the macrophages finally dominate the phagocytic cells of the inflamed area.

4. Increased Production of Granulocytes and Monocytes by Bone Marrow Is a Fourth Line of Defense.

   • It takes 3 to 4 days before newly formed granulocytes and monocytes reach the stage of leaving the bone marrow.

   • The bone marrow can continue to produce these cells in large quantities for months and even years

Eosinophils

• Often produced in large numbers with parasitic infections -> migrate into tissues diseased by parasites.

• Although most parasites are too large to be phagocytized by eosinophils or any other phagocytic cells, eosinophils attach themselves to the parasites and release substances that can kill them.

Basophils

• Basophils release histamine, bradykinin, serotonin, heparin and several lysosomal enzymes -> allergic manifestations.

Leukocyte number abnormalities

Leukopenia

• The bone marrow produces very few WBCs à Body unprotected

Leukemias

• Uncontrolled production of WBCs can be caused by cancerous mutation of a myelogenous or lymphogenous cell -> greatly increased numbers of abnormal WBCs in the circulating blood.

Leukemia consequences

• Invade the surrounding bone -> pain and tendency for bones to fracture

• Displacement of the normal bone marrow and lymphoid cells by the nonfunctional leukemic cells à Development of infection, severe anemia, and a bleeding tendency caused by thrombocytopenia.

• Excessive use of metabolic substrates by the growing cancerous cells -> Energy and aa are greatly depleted -> Rapid deterioration of the normal protein tissues of the body

Acquired or adaptive immunity

In addition to innate immunity, the body can develop extremely powerful specific immunity against individual invading agents. It is the part of the immune system that forms antibodies and/or activated lymphocytes that attack and destroy the specific invading organism or toxin.

Lymphocytes

• All lymphocytes are initially produced from hemocytoblasts (Lymphoid stem cells) in the bone marrow.

• These lymphocytes then seed other organs (like the thymus gland, spleen, and lymph nodes, gastrointestinal tract) producing self-replacing colonies of lymphocytes in these organs.

• There are two types of lymphocytes:

  • T-Lymphocytes or T-cells are involved in cell-mediated immunity

  • B-Lymphocytes or B-cells are involved in humoral immunity.

• Acquired (or Adaptive) immunity is the product of the body’s lymphocytes.

  • Strategical locations: Lymphoid tissue à gut, throat and pharynx (tonsils and adenoids), peripheral tissues à if pathogens pass this barrier à spleen, thymus, bone marrow.

  • Almost all the lymphocytes that are formed eventually end up in the lymphoid tissue.

Activation of T and B lymphocytes

When an antigen from a pathogen triggers a specific immune response, T-cells (T lymphocytes) are activated first followed by B-cells (B lymphocytes).

• T-cells are typically activated by an antigen presented by a phagocytic cell (e.g. macrophages) that have engulfed an antigen. Once activated, T lymphocytes:

  • Attack the antigen or cells displaying the antigen (T killer cells)

  • Stimulate activation of B-cells

    • Most antigens activate both T lymphocytes and B lymphocytes at the same time.

    • Some of the T cells that are formed, called T-helper cells, secrete specific substances (collectively called lymphokines) that activate the specific B lymphocytes.

    • Without the aid of T-helper cells, the quantity of antibodies formed by the B lymphocytes is usually small

    • Activated B-cells mature into:

      • Plasma cells that produce antibodies that bind to antigens and promote destruction of the pathogen

      • Memory B-cells that are responsible for the rapid secondary response to pathogens.

        • Memory cells are clones of the original Lymphocyte that first was presented with an antigen.

        • Each clone of lymphocytes is responsive to only a single type of antigen

        • Indeed, each B lymphocyte has on its cell surface about 100,000 antibody molecules that will react highly specifically with only one type of antigen.

        • When the appropriate antigen comes along, it immediately attaches to the antibody in the cell membrane, activating it.

  • Form memory T-cells that are responsible for the rapid secondary response to pathogens.

    • Similar to B lymphocytes, each T lymphocyte has surface receptor proteins (or T-cell receptors) on the surface of the T-cell membrane, and these are also highly specific for one specified activating antigen.

      • When stimulated it also forms clones of itself in order to respond to a specific antigen

  • An antigen therefore stimulates only those cells that have receptors for the antigen and are already committed to respond to it.

Macrophages in the lymphocyte activation process

• Millions of macrophages are present in the same tissue with lymphocytes (Lymph nodes, spleen, and other lymphoid tissue).

• Invading organisms are first phagocytized and partially digested by the macrophages.

• Macrophages pass antigens by cell to cell contact directly to the lymphocytes, activating them.

• Macrophages, in addition, secrete a special activating substance, interleukin-1, that promotes still further growth and reproduction of the specific lymphocytes.

T Lymphocytes – Cell mediated immunity

The component of the acquired immune response that uses T-lymphocytes to combat viral infections and intracellular bacterial infections.

T-Lymphocytes:

• Lymphocytes that move from bone marrow and seed the thymus gland (maturation site) become T-cells.

• Unlike B-cells, T-cells do not secrete Ab into body fluids (humors). Instead, T-cells provide cell-mediated immunity for an organism.

• T-cell activation involves MHC proteins.

Lymphocytes in the thymus gland.

• One thymic lymphocyte develops specific reactivity against one antigen, and then the next lymphocyte develops specificity against another antigen.

  • Its progeny are specific sensitized T cells that are released into the lymph, carried to the blood, and then circulated through all the tissue fluids and back into the lymph -> sometimes for years.

• Once processed T lymphocytes leave the thymus and spread via the blood throughout the body to lodge in lymphoid tissue.

• The thymus selects which T lymphocytes will be released by first mixing them with virtually all the specific self-antigens from the body’s own tissues.

  • If a T lymphocyte reacts against the body’s own tissues, it is destroyed and phagocytized instead of being released (Happens to 90% of them – very important process to prevent autoimmune diseases).

Release of Activated T Cells from Lymphoid Tissue and Formation of Memory Cells

• On exposure to the proper antigen, the T lymphocytes of a specific lymphocyte clone proliferate and release large numbers of activated, specifically reacting T cells

• Activated T cells are formed and released into the lymph

• New and more T-lymphocyte memory cells are formed (Reinforcement vaccine doses!)

Antigen-Presenting Cells, Major Histocompatibility Complex Proteins, and Antigen Receptors on T Lymphocytes

• Most acquired (adaptive) immune responses usually require assistance from T cells to begin the process. Even most of the activation of B lymphocytes requires T cell activation!

• T-cell responses are extremely antigen-specific

  • T lymphocytes respond to antigens only when they are bound to specific molecules called major histocompatibility complex (MHC) proteins on the surface of antigen-presenting cells.

• There are two major types of antigen presenting cells: macrophages, and dendritic cells (DCs) – Both originated from Monocytes

  • DCs are the most potent antigen-presenting cells

  • Both present antigens to T cells

• MHC proteins bind peptide fragments of antigen proteins that are degraded inside antigen-presenting cells and then transport them to the cell surface.

Two major types of MHC proteins:

• MHC I à Present antigens to cytotoxic T cells

• MHC II à Present antigens to T-helper cells

• The antigens on the surface of antigen-presenting cells bind with receptor molecules on the surfaces of T cells

Types of T Cells

There are three major groups of T cells:

1. T helper cells

   • Most numerous (75% of all T cells)

   • Major regulator of all immune functions

     • Do this by forming a series of protein mediators -> lymphokines

   • When stimulated, naïve T-helper cells (also called CD4+) can differentiate into subsets that produce different lymphokines and perform different functions.

   • In the absence of the lymphokines from the T-helper cells, the remainder of the immune system is almost paralyzed.

     • HIV destroys T helper cells -> body almost totally unprotected against infectious disease -> acquired immunodeficiency syndrome (AIDS)

Functions of T helper cells (work through lymphokines):

• In the absence of T-helper cells, the clones for producing cytotoxic T cells and regulatory T cells are activated only slightly by most antigens.

• The direct actions of antigens to cause B-cell growth, proliferation, formation of plasma cells, and secretion of antibodies are also slight without the help of the T- helper cells.

• Activation of the Macrophage System à Accumulation in inflamed tissue area through chemotaxis, cause more efficient phagocytosis.

• Feedback Stimulatory Effect on T-Helper Cells à Positive feedback

1. Cytotoxic T cells (killer cells)

   • Direct attack cell that is capable of killing microorganisms and, at times, even some of the body’s own cells (cancerous or abnormal cells).

   • Receptor proteins on the surfaces of the cytotoxic T cells (also called CD8+) cause them to bind tightly to the organisms or cells that contain the appropriate binding-specific antigen.

     • Kill the organism or cell secreting by perforins and cytotoxic substances, not by phagocytosis

   • Capable of delivering killing substances and move on to other organisms (sometimes for months).

   • Effective against virus infected cells, cancer cells and some transplant cells (heart).

2. Regulatory or suppressor T cells

   • Capable of suppressing the functions of both cytotoxic and T-helper cells.

   • Believed to prevent the cytotoxic cells from causing excessive immune reactions that might damage the body’s own tissues (immune tolerance).