Chapter 9: Cardiac Physiology

Human Physiology

The circulatory system has three main components: ___, __, and ____

Heart; blood vessels; blood

The ___ are passageways for the distribution of pumped blood throughout the body

blood vessels

The ___ is a transport medium, serving the needs of body cells.

blood

The ___ establishes a pressure gradient to pump the blood

heart

The ___ circulation is a loop of blood vessels between the heart and the lungs.

The ___ circulation is the circuit of blood vessels between the heart and other body systems.

systemic

Draw pulmonary and systemic circuit diagram

Draw pulmonary and systemic circuit diagram

___  is the ability of an excitable cell to rhythmically initiate its own actions potentials

Autorhythmicity

___ do the mechanical work of pumping, and make 99% of the cardiac muscle cells. Normally do not initiate their own action potentials. -90 mV at rest (close to K+ potential). Also called myocardial cells, cardiac muscle cells

Contractile cells

___ are specialized for initiating and conducting the action potentials responsible for contraction of the working cell. Do not have a resting potential. Display pacemaker activity

Autorhythmic cells

___ is the self-excitable activity of an excitable cell in which its membrane potential gradually depolarizes to threshold on its own

Pacemaker activity

___ is a self-induced slow depolarization to threshold occurring in a pacemaker cell as a result of shifts in passive ionic fluxes across the membrane accompanying automatic change in channel permeability

Pacemaker potential

Ion flow during contraction: Pacemaker cells “leaky” to Sodium (the ___ node)

SA

Ion flow during contraction: ___ enters, reaches threshold, and causes a depolarization

Na+

Ion flow during contraction: ___ flows inward

Ca2+

Ion flow during contraction: ___ flows out

K+

To elaborate, the initial phase of the slow depolarization to threshold is caused by net Na+ entry through a type of voltage-gated channels found only in cardiac pacemaker cells. Typically voltage-gated channels open when the membrane becomes less negative (depolarizes), but these unique channels open when the potential becomes more negative (hyperpolarizes) at the end of repolarization from the previous action potential. Because of their unusual behavior, they are called funny, or I_f channels. When one action potential ends and the I_f channels open, the resultant depolarizing net inward Na+ current through these open channels starts immediately moving the pacemaker cell's membrane potential toward threshold once again.

Na+ role in pacemaker potential

The second ionic mechanism that contributes to this pacemaker potential is a progressive reduction in the passive outward flux of K+. In cardiac autorhythmic cells, permeability to K+ does not remain constant between action potentials as it does in nerve and skeletal muscle cells. The K+ channels that opened during the falling phase of the preceding action potential slowly close at negative potentials. This slow closure gradually diminishes the outflow of positive potassium ions down their concentration gradient. As a result, the rate of K+ efflux slowly declines at the same time the slow inward leak of Na+ occurs through the open I_f channels, further contributing to the drift toward threshold.

K+ role in pacemaker potential

The third ionic contribution to pacemaker potential is increased Ca²⁺ entry. In the second half of the pacemaker potential, the I_f, channels close and transient Ca²⁺ channels (T-type Ca²⁺ channels), one of two types of voltage-gated Ca²⁺ chan-nels, open before the membrane reaches threshold. ("T" stands for transient.) The resultant brief influx of Ca²⁺ further depolarizes the membrane, bringing it to threshold, at which time the transient Ca²⁺ channels close.

Once threshold is reached, the rising phase of the action potential occurs in response to activation of a long-lasting, voltage-gated Ca²⁺ channel (L-type Ca²⁺ channel; "L" standing for long-lasting) and a subsequently large influx of Ca²⁺. The Ca²⁺ -induced rising phase of a cardiac pacemaker cell differs from that in nerve and skeletal muscle cells, where Na+ influx rather than Ca²⁺ influx swings the potential in the positive direction.

Ca2+ role in pacemaker potential

The falling phase is the result, as usual, of the K+ efflux that occurs when K* permeability increases on activation of voltage-gated K+ channels, coupled with closure of the L-type Ca²⁺ channels. After the action potential is over, slow closure of these K+ channels contributes to the next slow depolarization to threshold.

Final phase of pacemaker potential

Draw Ion flow during contraction graph

Draw Ion flow during contraction graph

Draw pacemaker cells and autorhythmic graph

Draw pacemaker cells and autorhythmic graph

Specialized noncontractile cardiac cells capable of ___ lie in the following specific sites: sinoatrial node (SA node), atrioventricular node (AV node), bundle of His (atrioventricular bundle), and purkinje fibers

autorhythmicity

The ___, a small, specialized region in the right atrial wall near the opening of the superior vena cava. 70-80 APs per minute. Known as the pacemaker of the heart.

sinoatrial node (SA node)

The ___ , a small bundle of specialized cardiac muscle cells located at the base of the right atrium near the septum, just above the junction of the atria and ventricles. 40-60 APs per minute.

atrioventricular node (AV node)

The ___, a tract of specialized cells that originates at the AV node and enters the interventricular septum. Here, it divides to form the right and left bundle branches that travel down the septum, curve around the tip of the ventricular chambers, and travel back towards the atria along the outer walls. 20-40 APs per minute.

bundle of His (atrioventricular bundle)

___, small terminal fibers that extend from the bundle of His and spread throughout the ventricular myocardium much like small twigs of a tree branch.  20-40 APs per minute.

Purkinje fibers

Once an action potential occurs in any cardiac muscle, it is propagated through the rest of the myocardium via gap junctions and the specialized conduction system.

Once an action…

An action potential initiated at the SA node first spreads throughout both atria. Its spread is facilitated by two specialized atrial conduction pathways, the interatrial and internodal pathways. The AV node is the only point where an action potential can spread from the atria to the ventricles. From the AV node, the action potential spreads rapidly throughout the ventricles, hastened by a specialized ventricular conduction system consisting of the bund of His and Purkinje fibers.

An action potential…

Atrial excitation and contraction should be complete before the onset of ventricular contractions. During cardiac relaxation, the AV valves are open, so venous blood entering the atria continues to flow directly into the ventricles. Almost 80% of ventricular filling occurs by this means before atrial contraction. When the atria do contract, more blood is squeezed into the ventricles to complete ventricular filling. Ventricular contraction then occurs to eject blood from the heart into arteries.

Atrial excitation…

Excitation of cardiac muscle fibers should be coordinated to ensure that each heart chamber contraction as a unit to pump efficiently

Excitation of cardiac…

An action potential originating in the SA node first spreads through both atria, primarily from cell to cell via gap junction.

SA node

The ___ extends from the SA node within the right atrium to the left atrium. Because this pathway rapidly transmits the action potential from the SA node to the pathway’s termination in the left atrium, a wave of excitation can spread across the gap junction throughout the left atrium the same as excitation is similarly spreading through the right atrium. This incur that both atria become depolarized to contract simultaneously.

interatrial pathway

The __ extends from the SA node to the AV node. The AV node is the only point of electrical contact between the atria and ventricles; in other words, because the atria and ventricles are structurally connected by electrically nonconductive fibrous tissue, the only way an action potential in the atria can spread to the ventricles is by passing through the AV node. The internodal conduction pathway directs the spread of an action potential originating at the SA node to the AV node to ensure sequential contraction of the ventricles following atria contraction. Hastened by this pathway, the action potential arrives at the AV node within 30 sec of SA node firing.

internodal pathway

The action potential is conducted relatively slowly through the AV node. This slowness is advantageous because it allows time for complete ventricular filling. The impulse is delayed about 100 msec (the AV nodal delay), which enables the atria to become completely depolarized and to contract, emptying their contents into the ventricles, before ventricular depolarization and contraction occur.

Conduction between the atria and the ventricle

After the AV nodal delay, the impulse travels rapidly down the septum via the right and left branches of the bundle of His and throughout the ventricular myocardium via the Purkinje fibers. The network of fibers in this ventricular conduction system is specialized for rapid propagation of action potentials. Its presence hastens and coordinates the spread of ventricular excitation to ensure that the ventricles contract as a unit. The action potential is transmitted through the entire Purkinje fiber system within 30 msec.

Ventricular excitation

Although this system carries the action potential rapidly to a large number of cardiac muscle cells, it does not terminate on every cell. The impulse quickly spreads from the excited cells to the rest of the ventricular muscle cells by means of gap junctions.

cardiac muscle cells

Purkinje fibers can transmit an action potential 6 times faster than the ventricular syncytium of contractile cells could

6

Rapid conduction of the action potential down the bundle of His and its swift, diffuse distribution throughout the Purkinje network lead to almost simultaneous activation of the ventricular myocardial cells in both ventricular chambers, which ensures a single, smooth, coordinated contraction that can efficiently eject blood into both the systemic and pulmonary circulations at the same time.

Rapid conduction

Draw figure 9-8 p.311

Draw figure 9-8 p.311

Draw conduction pathways in heart diagram

Draw conduction pathways in heart diagram

Conduction and timing in the heart: There is a lag time between ___ in the heart with muscle contraction

depolarization

Conduction and timing in the heart: There is a lag between ___ of atria and ventricles of the heart

contraction

Draw Conduction and timing in the heart graph

Draw Conduction and timing in the heart graph

Innervation of the heart: Heart innervated by the ___ (Cranial nerve X) and by the __ (increase heart rate)

Vagus nerve; sympathetic trunk

Draw Innervation of the heart diagram

Draw Innervation of the heart diagram

The action potential in cardiac ___ cells varies considerably in ionic mechanisms and shape from the SA node (autorhythmic cells) potential, despite being initiated by the nodal pacemaker cells

contractile

1. AP of contractile cell: Like nerve and skeletal muscle cells, contractile cells reach an action potential via voltage gated Na+ channels. However, the rate at which Na+ rushes in is quicker, having the membrane potential rapidly reverse to a positive value of +20mV to +30mV.

1. AP…

2. AP of contractile cell: At peak potential, another subclass of K+ channels transiently opens. The resultant limited efflux of K+ through these transient channels brings about a brief, small repolarization as the membrane becomes slightly less positive.

2. AP…

3a. AP of contractile cell: The membrane potential is maintained at this near peak positive level for several hundred milliseconds, producing a plateau phase of the action potential. This plateau is maintained by two voltage-dependent permeability changes: activation of "slow" L-type Ca2+ channels and a marked decrease in K+ permeability in the cardiac contractile cell membrane. These changes occur in response to the sudden change in voltage during the rising phase of an action potential.

3a…

3b. AP of contractile cell: Opening of the L-type Ca2+ channel results in a slow inward diffusion of Ca2+ because Ca2+ is in greater concentration in the ECF. This continuous influx of positively charged Ca2+ prolongs the positivity inside the cell and is primarily responsible for the plateau portion of the action potential.

3b…

3c. AP of contractile cell: The plateau effect is enhanced by the concomitant decrease in K+ permeability on close of both the briefly opened transient K+ channels and the leaky K+ channels open at resting potential. The resultant reduction in outward movement of positively charged K+ prevent rapid depolarization of the membrane and this contributes to the prolongation of the plateau phase.

3c…

4. AP of contractile cell: The rapid falling phase of the action potential results from inactivation of the Ca2+ channels and delayed activation of “ordinary” voltage-gated K+ channels. The decrease in Ca2+ permeability diminishes the slow, inward movement of positive Ca2+, whereas the sudden increase in K+ permeability simultaneously promotes rapid outward diffusion of positive K+. Thus, as in other excitable cells, the cell returns to resting potential as K+ leave the cell. At resting potential, the “ordinary” voltage-gated K+ channels close and the leaky K+ channels open once again.

4…

Draw contractile cell’s action potential graph

Draw contractile cell’s action potential graph

Contractile cardiac muscle has a long refractory period that lasts about _ msecs because of the prolonged plateau phase of the action potential. Meaning cardiac contraction cannot be restimulated until contraction is almost over, making summation of contraction and tetanus of cardiac muscle impossible beforehand.

250

Contractile cells inability of summation of contraction and tetanus is important because the pumping of blood requires alternate periods of contraction (emptying) and relaxation (filling) a prolonged ___ contraction would prove fatal, The heart chambers could be filled and emptied again.

tetanic

Refractory period of contractile cell: The chief factor responsible for the long refractory periods is inactivation (during the prolonged ___ phase) of the double-gated Na+ channels (that were activated during the initial Na+ influx of the rising phase and are in their closed and not capable of opening conformation).

plateau

A contractile cell cannot produce another action potential until Na+ channels have recovered from their inactivation process (when the membrane has already ___ to resting)

repolarized

A long refractory period prevents ___ of contractile cardiac muscle

tetanus

Draw contractile cell AP and refractory period graph

Draw contractile cell AP and refractory period graph

The EKG has 3 parts; ___wave/phase, complex, wave/phase

P; QRS; T

The _ wave represents atrial depolarization. Not SA nodal depolarization, as that does not generate enough activity to reach the recorded surface.

P

The _ complex represent ventricular depolarization

QRS

The _ wave represent ventricular depolarization

T

Draw labeled EKG

Draw labeled EKG

There is no wave representation for ___ repolarization as it normally occurs simultaneously with ventricular depolarization and is masked by the QRS complex.

atrial

The P wave is much smaller than the QRS complex, because the atria have a much smaller muscle mass than the ventricle, and consequently generate less ___ activity.

electrical

Three points in time no net current flow is taking place on the heart miniature, so the EKG remains at baseline: PR segment, _ segment, and TP interval.

ST

The AV nodal delay is represented by the interval of time between the end of P and the onset of QRS; this segment of the EKG is known as the _ segment. Current is flowing through the AV node, but the magnitude is too small for the EKG electrodes to detect.

PR

It's called the PR segment rather than the _ segment because the Q deflection is small and sometimes absent, whereas the R deflection is the dominant wave of the complex.

PQ

ST segment represents when the ventricles are completely depolarized and the cardiac contractile cells are undergoing the ___ phase of their action potential before they repolarize. This segment coincides with the time during which ventricular activation is complete and the ventricles are contracting and emptying. Note that the ST segment is not a record of cardiac contractile activity, as an EKG is a measure of the electrical activity that subsequently triggers mechanical activity.

plateau

The _ interval is when the heart muscle is completely repolarized and at rest and ventricular filling is taking place.

TP

Draw sequence of depolarization diagram

Draw sequence of depolarization diagram

Draw EKG in correlation to heart electrical activity diagram p.318

Draw EKG in correlation to heart electrical activity diagram p.318

Draw EKG paced by AV node vs normal

Draw EKG paced by AV node vs normal

The main deviations from normal that can be found through ___ are (1) abnormalities in rate, (2) abnormalities in rhythm, and (3) cardiac myopathies.

electrocardiography

The ___ can be determined from the distances between two consecutive QRS complexes.

heart rate

A rapid heart rate of more than 100 beats per minutes is called ___

tachycardia

Draw tachycardia EKG

Draw tachycardia EKG

A slow heart rate of fewer than 50 beats per minutes is called ___

bradycardia

Rhythm refers to the regulatory or spacing of the EKG waves. Any variation from the normal rhythm and sequence of excitation of the heart is termed an ___. ___ may result from ectopic foci, alterations in SA node pacemaker activity, or interference with conduction.

Arrhythmia

___ is an abnormally excitable area. Which can cause early depolarization, initiating an action potential that spreads throughout the rest of the heart before the SA node can initiate a normal action potential.

Ectopic focus

Abnormalities in ___ (arrhythmia) also include extrasystoles, ventricular fibrillation, and heart block.

rhythm

___ is a premature ventricular contraction. Can originate from an ectopic focus.

Extrasystole

Draw extrasystole EKG

Draw extrasystole EKG

Ventricular fibrillation is a very serious rhythmic abnormality in which the ventricular musculature exhibits uncoordinated, chaotic contractions. Multiple impulses travel erratically in all directions around the ventricles. The ECG tracing in ventricular fibrillations is very irregular with no detectable pattern or rhythm. When contractions are so disorganized, the ventricles are ineffectual as pumps.

Ventricular fibrillation…

Draw ventricular fibrillation EKGs

Draw ventricular fibrillation EKGs

___ arises from defects in the cardiac conducting system. The atria still beat regularly, but the ventricles occasionally fail to be stimulated and thus do not contract following atrial contraction. Impulses between the atria and ventricles can be blocked to varying degrees. In some forms of heart block, only every second or third atrial impulse is passed to the ventricles. This is known as 2:1 or 3:1 block. In heart block, the atrial rate is normal but the ventricular rate is considerably below normal.

Heart block

___ is characterized by complete dissociation between atrial and ventricular activity, with impulses from the atria not being conducted to the ventricles at all. The SA node continues to govern atrial depolarization, but the ventricles generate their own impulses at a rate much slower than that of the atria. On the ECG, the P waves exhibit a normal rhythm. The QRS and T waves also occur regularly but much more slowly than the P waves and are completely independent of P wave rhythm. Because atrial and ventricular activity is not synchronized, waves for atrial repolarization may appear, no longer masked by the QRS complex.

Complete heart block

Draw complete heart block EKGs

Draw complete heart block EKGs

___ means cell death

necrosis

Damage of the heart muscle

cardiac myopathies

___ is the inadequate delivery of oxygenated blood to the heart tissue.

myocardial ischemia

___ is a condition in which blood vessels supplying an ear of the heart become blocked or ruptured, resulting in necrosis of heart muscle cells there. Also known as a heart attack.

acute myocardial infarction

Concerning a ___, abnormal QRS waveforms appear when part of the heart muscle becomes necrotic. In addition to ECG changes, because damaged heart muscle cells release characteristic enzymes into the blood, the level of these enzymes in the blood provides a further index of the extent of myocardial damage.

heart attack

Draw myocardial infarction EKG

Draw myocardial infarction EKG

The heart alternately ___ to empty and relaxes to fill

contracts

___ is the period of cardiac contraction and emptying

systole

___ is the period of cardiac relaxing and filling

diastole

The ___ cycles consists of alternate periods of systole and diastole

cardiac