Heart

the heart is the center of the cardiovascular system. Why is the heart considered a double pump?

the heart, while only a single organ, works as a double pump.

it propels blood through the lungs (pulmonary circulation) and the rest of the body (systemic circulation) simultaneously.

How much blood does the heart pump per day?

at rest: 1,800 gallons per day through about 60,000 miles of blood vessels

general description of the heart

hollow cone shaped organ, weighing about 10 ounces in the adult

consist of 2 upper chambers, the atria, and 2 lower chambers, the ventricles

location of the heart

rest on the muscular diaphragm separating the thoracic and abdominal cavities

thoracic space in which it sits is known as the mediastinum

apex

2/3 of the heart mass lies to the left of the midline, with its apex, formed at the tip of the left ventricle, lying 9cm (3.25 inches) from the midline, deep to the 5th intercostal space

base

opposite to the apex is the base of the heart. it lies superior and posterior in the mediastinum and is formed mostly by the left atrium

pericardium

tripled-layered bag that surrounds and protects the heart, confining it to its position within the mediastinum, yet allowing it freedom of movement for contraction.
the pericardium consist of 2 major portions: fibrous and serous (parietal vs. visceral)

fibrous

the outer fibrous pericardium is a tough, inelastic, fibrous connective tissue attached to the great vessels associated with the heart, the diaphragm and at the roots of the lungs

-it serves to anchor the heart within the mediastinum
-prevent over-stretching of the heart during exercise
-offers some degree of protection

serous (parietal vs. visceral)

the inner serous pericardium is a thinner and more delicate membrane that forms a double layer around the heart.
it is subdivided into 2 layers: parietal and visceral

parietal

the outer portion of the serous pericardium is fused to the inside surface of the fibrous pericardium

visceral

the inner visceral layer of the serous pericardium, also known as the *epicardium or the outer wall of the heart itself, adheres tightly to the surface of the heart muscle (the myocardium)

pericardial cavity

small space btn. the parietal and the visceral layers

it contains a small amount of pericardial fluid, secreted by the serous pericardium that is used for lubrication to reduce friction as the heart moves

epicardium

the wall of the heart itself is subdivided into 3 layer.

epicardium: the outermost layer- also known as the visceral layer of the serous pericardium.

it is a thin, transparent membrane that imparts a slippery texture to the outer surface of the heart

myocardium

middle layer: consist of cardiac muscle cells and is responsible for the pumping action of the heart
the cardiac muscle fibers are involuntary, striated and branched, swirling diagonally around the heart in interlacing bundles to form 2 large networks- atria and the ventricles

intercalated discs

where each cardiac muscle cell contacts neighboring cells by transverse thickenings of the sarcolemma
within which are gap junctions that electrically couple the cells so that they work as a unit (functional syncytium)

cardiac skeleton

this dense fibrous connective tissue that is in the form of a figure-8, completely separates the two muscle masses (atria and ventricles)
the fibrous tissue uncouples the electrical activity of the atria and ventricles so they can work independently

endocardium

inner most layer: simple squamous epithelium overlying a thin connective tissue
this becomes continuous with the endothelium of the blood vessels

heart chambers

the interior of the heart is divided into four compartments that receive the circulating blood

right and left atrium

the two superior chambers- each of which has an appendage called the auricle that increases the volume of the atrium and is used during exercise

right and left ventricle

the two lower chambers

coronary sulcus and the interventricular sulci

externally, the heart chambers a delineated from one an other by a series of grooves within which lie coronary arteries and coronary veins

coronary sulcus

separates the atria from the ventricles

anterior and posterior interventricular sulci

separate the two ventricles front and back

role of cardiac skeleton

connective tissue that effectively separates the upper atria from the lower ventricles so they can work independently

interatrial septum

internally the chambers of the heart are separated by muscular walls called septa

-interatrial septum separates the atria and bears a prominent feature called the *fossa ovalis, the remnant of the fetal *foramen ovale- an opening that allowed blood to pass from right atrium to left atrium

interventricular septum

separates the ventricles and is divided into 2 portions: superior *membranous and inferior *muscular interventricular septum

atrial wall thickness

the myocardium of the atria is relatively thin since it has only to move blood into the ventricles, and therefore needs to generate only a small amount of pressure

ventricular wall thickness

the myocardium of the ventricles is considerably thicker since it must move blood to the lungs (right ventricle) or to the rest of the body (left ventricle).
the left ventricle wall is the thickest since is must generate the largest amount of pressure

basic pattern of blood flow from the body through the 3 veins

-superior vena cava brings blood from most of the upper body to the heart (head, neck, upper extrem. and thorax)

-the inferior vena cava brings blood from all parts of the body inferior to the diaphragm

-the coronary sinus receives blood from the coronary veins draining the heart itself and delivers it to the right atrium

-from the right atrium blood moves into the right ventricle, it is pumped into the pulmonary trunk, which divides into the right and left pulmonary arteries, each of which carries deoxygenated blood to its respective lungs

-oxygenated blood from the lungs passes to the left atrium via 4 pulmonary veins

-blood then passes from the left atrium into the left ventricle, from which it is pumped into the aorta for distribution throughout the systemic circulation

what are heart valves?

as each chamber of the heart contracts, it pushes a portion of its blood into a ventricle or into a great artery
to prevent backflow of blood, the heart is equipped with valves, formed from the connective of the cardiac skeleton and covered with endocardium
they open and close by *pressure changes

two types of heart valves

1. atrioventricular (AV) valves
2. semilunar (SL) valves

atrioventricular (AV) valves

lie btn. the atria and the ventricles

the tricuspid valve is on the right side and the bicuspid (mitral) valve is on the left

each cusp of an AV valve is roughly shaped like a triangle; base is attached to the heart wall and apex is pointed down into the ventricle

chordae tendineae

tendinous cords of connective tissue that are attached to the apices of the cusp- which anchor the valves down inside the ventricle wall by attaching to papillary muscles

mechanism by which the atrioventricular valves open and close

-in order for blood to pass from atrium to ventricle, the AV valve must be open with its pointed ends extending into the ventricular cavity, the papillary muscles are relaxed and the chordae tendineae slackened

-contraction of the ventricular myocardium increases the pressure within the ventricle, forcing blood toward the opening between atrium and ventricle

-the pressure change and the force of the blood drive the cusp of the AV valve upward until their edges meet and close the opening, thus preventing backflow of blood into the atrium

-at the same time, the papillary muscles contract, adding further tension to the chordae tendineae, preventing the cusp from everting or swinging upward into the atrium and thus allowing blood to flow back into the atrium

semilunar valves (SL)

2nd type of heart valves

locations of SL valves

pulmonary truck and aorta just as each vessel emerges from its respective ventricle

shape of SL valves

consist of 3 half-mooned shaped cusps that are attached to the artery wall like a pocket is attached to a shirt, with a free upper margin

functioning of the SL valves

-when blood is ejected from the ventricle into the artery, the cusps are pushed flat against the artery wall, allowing blood to pass

-after contracting, when arterial pressure becomes greater than ventricular pressure, blood backfills the SL valve cusp, filling the pockets and causing the free margins to bulge outward from the wall of the vessel

-when edges of the 3 bulging cusp meet each other, the valve is closed and cannot return to the heart

coronary circulation

myocardium has its own blood supply and does not rely upon diffusion of nutrients from the blood circulating through the chambers to meet its needs.
-two coronary arteries branching from the ascending aorta, right and left, are responsible for total blood flow to the myocardium

left coronary artery

emerges from the aorta to the left of the pulmonary trunk and almost immediately divides into 2 branches:
-anterior interventricular artery (left anterior descending or LAD)
-circumflex artery

LCA- supply area

via its branches is responsible for most of the blood supply to the anterior myocardium of both ventricles and to the left atrium

right coronary artery

emerges from the aorta to the right of the pulmonary trunk and passes in the groove btn. the right atrium and right ventricle
it gives rise to 2 arteries:
-marginal artery
-posterior interventricular artery

RCA- supply area

supplies blood to the right atrium and the posterior myocardium of the ventricles

anastomoses of the heart

there are a great many of interconnection (anastomoses) btn the branches of the coronary arteries, particularly where the anterior and posterior interventricular arteries meet
-anastomoses provide a \# of alternate routes for blood flow- should one path become blocked (detour!)

coronary veins

1. great cardiac veins
2. middle cardiac veins
3. small cardiac veins
4. coronary sinus

great cardiac veins

located in the anterior interventricular sulcus, along side the anterior interventricular branch of the right coronary artery.
-it drains blood from the myocardium of the anterior aspect of the heart

middle cardiac vein

lies in the posterior interventricular sulcus, alongside the posterior interventricular branch of the right coronary artery
-it drains blood from the myocardium of the posterior aspect of the heart

small cardiac vein

found in the groove btn. the right atrium and the left ventricle
-it drains blood from the myocardium of both areas

coronary sinus

lies in the groove btn. the left atrium and the ventricles.
-receives venous blood from the great, middle and small cardiac veins, and then opens into the right atrium

how does the heart stimulate itself to beat?

using inherent and rhythmic electrical activity

what is the role of the ANS and the ES in the regulation of heart beat?

they can ONLY modify the heart beat (slow it down or speed it up)

-DO NOT establish the fundamental rhythm

what certain muscle cells repeatedly create spontaneous action pot. that then trigger contractions?

autorhythmic cells

describe development of these cells

during heart dev. about 1% of the forming cardiac mus cells lose the ability to contract and become instead autorhythmic (self excitable)

2 essential functions of these cells

1. act as a pacemaker- setting basic rhythm for the entire heart

2. form the conduction system- the route for conducting the impulses throughout the heart muscle- this system assures that the cardiac chambers contract in a coordinated and timely fashion, thus making the heart and effective pump

name and describe each of components of the cardiac conducting system

1. the sinoatrial (SA) node is a collection of autorhythmic cells in the posterior right atrial wall, just inferior to the opening of the superior vena cava.
it is the primary pacemaker- setting the basic rhythm of the heart at about 90-100 beats per min.

2. the atrioventricular (AV) node is a collection of autorhythmic cells at the junction of the four chambers, within the cardiac skeleton.
it is the secondary pacemaker- setting the basic rhythm of the heart at about 40-50 beats per min

3. the atrioventricular (AV) bundle extends from the AV node into the membranous interventricular septum

4. at the muscular septum the AV bundle divides into right and left bundle branches, each traveling though its respective ventricle.

5. extending from the bundle branches are Purkinje fibers, which pass the action potentials to the ventricular myocardium and cause the muscle cells to contract

why are the cells of the SA and AV node autorhythmic?

bc the cells maintain a membrane potential (like all muscle cells) but they are "leaky" to Na+ ions. -As Na+ leaks into the cells, they then reach threshold and create an action potential- this occurs at a certain rate for each node, so each creates its own rhythm (SA\=90-100/min; AV\=40-50/min)

how do action potentials of the SA node influence the rest of the atrial muscle cells?

since SA nodes are a part of the atrial myocardium and all of the cells are "connected" by intercalated disc, the action pot. spread away from the SA node, sweeping across the atrial muscle via gap junctions, and causing the cells to contract

describe the timing of events that occur in the conduction system as depolarization sweeps across the heart muscle

-at the beginning of a cardiac cycle, SA generates an act. pot. (0.00 sec)
this act. pot. moves across the atrial muscle like a wave, reaching the AV node at 0.04 secs

-the AV node, which depolarizes more slowly, passes the act. pot. into the AV bundles at 0.16 sec.
this 0.12 delay at the AV node allows the atria to finish their contraction sequence b4 the ventricles can begin theirs.

-the action potentials move down the AV bundle, the bundle branches, and the Purkinje fibers, causing the interventricular septal muscle to begin contracting 1st (0.17 sec)

-the apex region of the ventricles contracts (0.18 sec) and the contraction sequence continues to move superiorly until finally the superior-most parts of the ventricles contract at 0.21-0.22 sec

-this arrangement allows the atria to move blood into the ventricles b4 the ventricles begin to contract, and then causes the ventricles to contract in such a way as to eject blood into the great arteries

electrocardiogram

impulse conduction through the heart generates electrical currents that can be detected at the body surface
-a recording of the electrical changes that accompany each cardiac cycle (ECG)

-this is a composite of action potentials produced by all the heart muscle fibers during each heartbeat.
in a typical recording there are several clearly recognizable and named parts

P wave

1st small upward deflection- that represents atrial depolarization as it spreads from the SA node and across both atria

QRS complex

2nd wave- begins as small deflection down, followed by a large deflection up, ends with small deflection down.
it represents ventricular depolarization- the spread of excitation through the ventricles

T wave

3rd wave- small dome-shaped upward deflection that represents ventricular repolarization
the T wave appears just b4 the ventricles begin to relax

P-R interval

measured from beginning of the P wave to the beginning of the QRS complex
it represents the conduction time from the beginning of the atrial excitation to the beginning of ventricular depolarization

S-T segments

begins at the end of the S wave and ends at the beginning of the T wave
it represents the time when ventricular contractile fibers are fully depolarized

quiescent period

time period btn. the end of T wave and the beginning of the next P wave
it represents the time when all heart muscle is at rest

identify 2 phenomena that control blood flow through the heart

1. contraction and relaxation of the myocardium, controlled by the conduction system

2. opening and closing of the AV and SL valves, controlled by the pressure changes in the heart chambers and the great arteries

blood flows from an area of high pressure to an area of low.
the pressure developed within the heart chamber is related to what 2 things?

1. volume of blood within the chamber exerts a fluid pressure

2. the size of the chamber; as a chamber contracts its size- gets smaller and therefore the pressure within increases

venous pressure

fluid pressure exerted by the blood in the veins of the body

atrial pressure

related to 2 things: the fluid pressure exerted by vol. of blood that ventricle contains & size of the atrial chamber as it contracts and relaxes

ventricular pressure

related to 2 things: fluid pressure exerted by vol. of blood the ventricle contains & size on the ventricular chamber as it contracts and relaxes

arterial pressure

is the fluid pressure exerted by blood in the arteries of the body

what happens during the normal cardiac cycle?

2 atria contract- while 2 ventricles relax- then 2 ventricles contract- while 2 atria relax

systole

contraction phase

diastole

relaxation phase

one complete cardiac cycle consist of what?

systole and diastole of both atria + systole and diastole of both ventricles

the cardiac cycle of a resting adult is divided into 3 phases

1. relaxation phase
2. ventricular filling
3. ventricular systole

1st: relaxation phase

-at the end of the heart beat when the ventricles start to relax, all 4 chambers are in diastole
--this is known as the quiescent period

-as ventricles relax, ventricular pressure drops and blood begins to flow from great arteries back toward respective chamber
--this results in closure of the SV valves, giving the 2nd heart sound

-now vol. of blood within the ventricles does not change bc the AV valves are also closed
\-- this period is called isovolumetric relaxation

-as ventricles cont. to relax, their chamber size cont. increasing until ventricular pressure drops below atrial pressure
--as a result the AV valves open

-blood from the atria begins to move into the ventricles, passing through the open AV valves, following the pressure gradient and the ventricles begin to fill

2nd: ventricular filling

-70% of this filling occurs just after the AV valves open bc of blood that had been filling the atria from the venous circulation while they were diastole and the AV valves were closed

-the 1st third of ventricular filling is thus known as the period of rapid ventricular filling- occurring w/o the benefit of atrial systole

-the middle 3rd of the ventricle filling is called diastasis
this occurs as blood flows from the atria slows and a much smaller vol. of blood enter the ventricles

-excitation of the SA node initiates atrial systole, marking the end of the quiescent period- causing the last 3rd of the ventricular filling to occur (remaining 30%) as the atria completely empty themselves

-at the end of the ventricular diastole, each ventricle contains about 130 ml of blood, the *end-diastolic volume (EDV)
throughout this entire phase the AV valves are open and the SL valves are closed

3rd: ventricular systole

-ventricular systole: as the excitation passes into the AV node the throughout the rest of the conduction system, the ventricular myocardium enter the systole

-ventricular pressure, which was already increased by ventricular filling, now begins to rise even higher
as a result, when the ventricle pressure exceeds the atrial pressure, the AV valves close

-this is known as the period of isovolumetric contraction because the volume of the blood within the ventricles remains the same (130ml)

-when ventricular pressure exceeds arterial pressure, the SL valves open and the blood is ejected from the ventricles into the app. artery. this period is known as * ventricular ejection and continues until the ventricles begin to relax

-once relaxation phase has begun again, arterial pressure exceeds ventricular pressure and the SL valves close again.
at this time each ventricle contains about 60 ml of blood, the *end systolic volume (ESV)

stroke volume

the volume moved from each ventricle during ventricular systole (70ml)

(SV\=EDV-ESV) or (SV\=130ml-60ml)

atrial diastole I and ventricular systole occur at the same time. during this time, what events occur in the heart?

1. venous pressure \> atrial pressure\= venous return

2. ventricular pressure \> atrial pressure \= AV close

3. atria fill w/ blood \= atrial pressure increases

4. ventricular pressure \> arterial pressure \= SL valves open

5. ventricular ejection of blood into great arteries!

atrial diastole II and ventricular diastole occur at the same time. during this time what events are occurring in the heart?

1. ventricular pressure decreases

2. arterial pressure \> ventricular pressure \= SL valves close

3. atrial pressure \> ventricular pressure \= AV valves open

4. ventricular filling begins (70%), therefore ventric pressure rises

5. venous pressure \> atrial pressure \= atrial

what events occur during atrial systole?

1. atrial pressure \> ventricular pressure \= the other 30% of ventricular filling occurs

2. atrial pressure \> venous pressure \= atrial filling stops

3. resulting in the atrial completely emptying into the ventricles

timing aspects of the cardiac cycle

-bc resting heart rate in normal adult \= 75 beats/min- each cycle takes 0.08 sec

-during the first 0.04 sec, all 4 chambers are at rest * quiescent period

-during the next 0.04 sec, the atria, then the ventricles are in systole

--when heart rate is changed, it is the quiescent period that is altered accordingly (not the act. pot. period)

cardiac output

CO- the amount of blood ejected from the left ventricle into the aorta per min
-same for right ventricle

2 factors that determine cardiac output

1. Stroke volume: (SV) amount of blood ejects from the ventricle per beat

2. heart rate: (HR) \# of heart beats per min

mathematically how is cardiac output determined?

CO\= SV x HR
CO\= (70ml/beat) x (75 beats/min)
CO\= 5,250ml 5L(min)

cardiac output of 5l/min at rest\-- what would happen if body demanded for a increase in blood flow (exercise)?

body demand increases \= cardiac output increases to meet challenge

cardiac reserve

the difference btn the max CO a person can achieve (ex. exercise) and the CO at rest.

a normal adult has a cardiac reserve of about
4-5 times the resting value (21L/min or about 200-300% resting)

2 basic factors that alter cardiac output

1. stroke volume
2. heart rate

therefore, factors that alter either of these will effect the CO

how much blood does a healthy heart pump with each beat

a healthy heart will pump out all of the blood that was moved into its chamber during diastole

at rest this is 50-60% of the total volume bc 40-50% remains in the ventricles

end-diastolic volume

(EDV) the volume of the blood in a ventricle at the end of diastole

end-systolic volume

(ESV) the volume of blood remaining in the ventricle after systole

using EDV and ESV to determine stroke volume

SV \= EDV - ESV

3 factors that regulate stroke volume and ensure that the left and right ventricles pump equal volumes of blood

-preload: stretch of the heart mus b4 it contracts

-contractility: forcefulness of contraction

-afterload: pressure that must be exceeded by the ventricle b4 blood can be ejected from the ventricle into the artery

preload

greater stretching on cardiac muscle cells just b4 they contract increases their force of contraction

Frank-Starling law of the heart

-within physiological limits, the more the heart is filled during diastole, the greater the force of contraction

its effects on stroke volume and cardiac output

the preload depends on the volume of the blood that fills the ventricles at the end of diastole (EDV) and is determined by:
1. length of diastole
2. venous pressure

- when heart rate increases, the duration of ventricular diastole is shortened. less filling time means a smaller EDV and the ventricles may contract b4 they are adequately filled

-on the other hand, when venus pressure increases, a greater volume of blood is forced into the atria and therefore into the ventricles so that the EDV is increased

-the Frank-Starling law equalizes the output of the 2 ventricles and keeps the same volume of blood flowing to both systemic and pulmonary circuits

EX: at the beginning of exercise- left ventricle pumps more than the right, causing the volume of blood returning to the right atrium to increase
this increases right side EDV and the right ventricle contracts more forcefully with the next beat

contractility

length of contraction at any given preload

positive inotropic agent

increases contractility
-these agents stimulation by the sympathetic nervous system- hormones glucagon and epinephrine, increased calcium ions into the EC fluid an the drug digitalis