13 THE CARDIVASCULAR SYSTEM: CARDIAC FUNCTION
An Overview of the Cardiovascular System
The cardiovascular system consists of three components: 1) the heart: a muscular pump that drives the flow of
blood through blood vessels; 2) blood vessels: conduits through which the blood flows; 3) blood: a fluid that
circulates around the body, carrying materials to and from the cells.
The heart performs sensory and endocrine functions that help regulate cardiovascular variables such as blood
volume and pressure. The blood vessels are not just conduits for blood, but are also important sensory and
effector organs that regulate blood pressure and the distribution of blood to various parts of the body. The blood
also transports hormones, serving as communications link acting in conjunction with the nervous system.
Regulation of the cardiovascular system involves interactions with several other organ systems, including the
nervous system, the endocrine system, and the kidneys.
A muscular organ whose function is to generate the force that propels blood through the blood vessels.
4 chambers: 2 upper chambers (atria: receive blood that comes back to the heart from the vasculature) and 2
lower chambers (ventricles: receive blood from the atria and generate the force that pushes the blood away from
the heart and trough the blood vessels).
The heart can be functionally separated into left and right halves. The atria and ventricles on either side are
separated by a wall called the septum that prevents blood in the left heart from mixing with blood in the right
heart. Separating the atria is referred to the interatrial septum; for ventricles is the interventricular septum.
The wider upper pole (end) of the heart is known as the base; the narrower lower pole is the apex.
When blood moves through the body, it travels in a circular pattern through a system of blood vessels (heart to
organs to heart). This system is called vasculature and is a closed system.
As blood flows away from the heart, blood vessels branch repeatedly and reduce diameter. As blood flows back
to the heart, vessels converge and increase in diameter.
Blood leaving the heart to the organs and tissues goes through large vessels called arteries (branch repeatedly
within the organs and tissues).
The smallest arteries branch into smaller vessels called arterioles, which carry blood to the smallest vessels,
From the capillaries, blood moves to larger vessels called venules, which lead to larger vessels called veins,
which carry blood back to the heart.
The most numerous cells in the blood are erythrocytes (red blood cells). They contain hemoglobin.
Also have leukocytes (white blood cells), which come in a variety of types and help the body defend itself against
Have platelets, which are not cells but instead cell fragments for blood clotting.
The liquid portion of the blood, called plasma, is made up of water containing dissolved proteins, electrolytes,
and other solutes.
The Path of Blood Flow Through the Heart and Vasculature
SERIES FLOW THROUGH THE CARDIOVASCULAR SYSTEM
There are two divisions: the pulmonary circuit, which consists of all blood vessels within the lungs and those
connecting the lungs with the heart; the systemic circuit, which has the rest of the blood vessels in the body.
The right heart supplies blood to the pulmonary circuit. Left heart supplies to the systemic circuit. The heart is
two separate pumps housed within a single organ.
The pulmonary and systemic circuit both possess capillary beds (dense networks of capillaries) for gas and
In pulmonary capillaries, O moves into the blood from air in the lungs while CO leaves. The blood is then
oxygenated (red blood) 13 THE CARDIVASCULAR SYSTEM: CARDIAC FUNCTION
The capillary beds on the systemic circuit are located in all organs and tissues except the lungs. Cells consume
oxygen and generate carbon dioxide. The blood is then deoxygenated (blue blood).
Path of blood flow:
1. Left vent pumps oxy blood into the aorta, a major artery whose branches carry blood to capillary beds of
all organs and tissues in the systemic circuit.
2. Blood becomes deoxy in systemic tissues, travels back to the heart in the venae cavae (2 large veins that
carry blood into the right atrium). Superior vena cava carries blood from parts above the diaphragm.
Inferior vena cava carries blood from parts below the diaphragm.
3. From the right atrium, blood passes through the tricuspid valve into the right vent.
4. Right vent pumps blood into the pulmonary trunk which branches into the pulmonary arteries, carries
deoxy blood to the lungs.
5. Blood becomes oxy in the lungs and travels to the left atrium in the pulmonary veins.
6. From the left atrium, blood passes through the bicuspid valve into the left vent.
PARALLEL FLOW WITHIN THE SYSTEMIC OR PULMONARY CIRCUIT
In the systemic circuit, blood doesn’t flow from one organ directly to the next. Blood travels through the aorta
and the arteries that branch off it to reach only one organ at a time before flowing through veins that converge to
either the superior or inferior vena cava.
The heart muscle obtains most of its nourishment from blood via the coronary arteries, which branch off the
aorta near its base and run through the heart muscle. Decrease in blood flow through the coronary arteries can
lead to a heart attack.
The parallel arrangement of organs in the systemic circuit confers 2 distinct advantages:
o Separate artery feeds each organ, each receiving fully oxygenated blood. Blood leaving the tissue returns
to the lungs to be reoxygenated before it returns to the systemic circuit.
o Blood reaches the organs via parallel paths; therefore blood flow to the organs can be independently
regulated. Blood flow can be adjusted to math the constantly changing metabolic needs of organs.
Anatomy of the Heart
A membranous sac called the pericardium, which contains pericardial fluid that lubricates the heart as it beats,
surrounds the heart.
To generate the blood flow, the heart consists primarily of cardiac muscle.
For blood to flow in the correct direction, 4 valves prevent backflow.
MYOCARDIUM AND THE HEART WALL
Epicardium: outer layer of connective tissue. Myocardium: a middle layer of cardiac muscle. Endothelium: an
inner layer of epithelial cells. The endothelial layer extends throughout the entire cardiovascular system.
The heart’s pumping action is conferred by the rhythmic contraction and relaxation of the myocardium.
The ventricular muscle is substantially thicker than atrial muscle. The ventricles pump blood over relatively long
distances through the vasculature; they work harder to pump a given volume of blood.
The left ventricular muscle is much thicker than the right since it pumps blood to all the organs in the body
except the lungs. The right ventricle pumps blood only to the lungs.
The pressure to pump blood at a given rate to the whole body is greater than that to pump blood to the lungs.
The heartbeat is a wave of contraction that sweeps through heart muscle fibers in an orderly, coordinated
fashion. Atria contract first, putting blood in the ventricles. Ventricles contract, putting blood in the organs.
The atrial muscle (atrial myocardium) and ventricular muscle (ventricular myocardium) are physically anchored
to and separated by a layer of fibrous connective tissue (fibrous skeleton of the heart).
VALVES AND UNIDIRECTIONAL BLOOD FLOW
The heart chambers contract in a cardiac cycle. Atria contract first, then the ventricles causing pressures in the
chambers to fluctuate and, change the direction of pressure gradients for blood to flow.
The atrioventricular valves (AV valves) permit blood to flow from the atrium to the ventricle but not in the
opposite direction. When atrial pressure is higher than ventricular pressure, the valves open. When ventricular
pressure becomes higher than the valve closes. 13 THE CARDIVASCULAR SYSTEM: CARDIAC FUNCTION
The AV valve on the left consists of two flaps or cusps of connective tissue and is called the bicuspid valve or
mitral valve. The right AV valve has 3 cusps and is called the tricuspid valve.
Semilunar valves are located between the ventricles and arteries. The aortic valve is located between the left
ventricle and the aorta. The pulmonary valve is located between the right ventricle and the pulmonary trunk.
There function is similar to AV valves, prevent backflow.
The aortic and pulmonary valves open when ventricular pressure is greater than arterial pressure (when the
ventricles contract). This allows blood to leave the ventricle and enter the arteries.
When the ventricles relax and ventricular pressure becomes lower than arterial pressure, the valves close,
thereby preventing blood from flowing back into the ventricles from the arteries.
Electrical Activity of the Heart
THE CONDUCTION SYSTEM OF THE HEART
Cardiac muscle contractions are triggered by signals originating from within the muscle itself. Contractile
activity of cardiac muscle is called myogenic.
For skeletal muscle is neurogenic, since it originates in neurons.
The ability of the heart to generate signals that trigger its contractions on a periodic basis is called
autorhythmicity. Cause by a small percentage of muscle cells called autorhythmic cells that generate little
contractile force but coordinate and provide rhythm to the heartbeat.
Pacemaker cells: initiate action potentials and establish the heart rhythm.
Conduction fibers: transmit action potentials through the heart in a highly coordinated manner.
The pacemaker cells and the conduction fibers make up the conduction system.
Contractile cells: cells that generate contractile force.
PACEMAKER CELLS OF THE MYOCARDIUM
o Contractions of the heart are initiated by pacemaker cells, which spontaneously generate action
potentials. They determine the pace of the heartbeat by firing action potentials.
o Concentrated in the sinoatrial node (SA node), located in the wall of the upper right atrium close to the
superior vena cava. Also atrioventricular node (AV node), located near the tricuspid valve in the
o The SA node have a faster inherent rate of spontaneous depolarization. The SA node drives the
depolarization in the AV node by conduction fibers.
CONDUCTION FIBERS OF THE MYOCARDIUM
o Specialized to conduct the action potentials generated by the pacemaker cells through the myocardium,
o They are larger in diameter and can conduct action potentials more rapidly than ordinary fibers.
o Action potentials can travel 4m/s in certain parts of the conduction system. 0.3-0.5m/s in most cardiac
INITIATION AND CONDUCTION OF AN IMPULSE DURING A HEARTBEAT
1. An action potential is initiated in the SA node. Impulses travel to the AV node by intermodal pathways
(conduction fibers that run through the walls of the atria). Also spread through the atrial muscle by
2. The impulse is conducted to cells of the AV node, which transmit action potentials less rapidly.the
impulse is momentarily delayed by 0.1s (AV nodal delay).
3. From the AV node, the impulse travels through the atrioventricular bundle (bundle of His), a compact
bundle of muscle fibers located in the interventricular septum. The AV node and bundle of His are the
only electrical connection between the atria and the ventricles, separated by the fibrous skeleton.
4. The signal travels only to the atrioventricular bundle before it spilts into left and right bundle branches.
5. Then impulses travels through an extensive network of branches referred to as Purkinje fibers, which
spread through the ventricular myocardium from the apex upward toward the valves. Then from the
fibers, the impulse travel through the rest of the myocardial cells.
SPREAD OF EXCITATION THROUGH THE HEART MUSCLE
As a wave of excitation spreads, contraction of the muscle follows. 13 THE CARDIVASCULAR SYSTEM: CARDIAC FUNCTION
Starts at the SA node, then to the atria. The wave funnels through the atrioventricular bundle by way of AV node,
which acts as a kind of bottleneck due to the relative slowness of impulse conduction in this region.
The delay allows the wave to spread completely through the atria before it reaches the ventricles, ensuring
complete atrial contraction before ventricular contraction starts.
Ventricular contraction would work against the pumping action of the atria is there was no delay.
Once impulses reach the bundle branches and the Purkinje fibers, they are carried relatively quickly to the lower
portion of the ventricles. Then the wave fans out. After ventricular contraction begins at the apex and spreads
THE IONIC BASIS OF ELECTRICAL ACTIVITY IN THE HEART
ELECTRICAL ACTIVITY IN PACEMAKER CELLS
o A cardiac contractile cell fires an action potential only when it is depolarized to threshold by a stimulus.
o The stimulus is a circulating electrical current that originates in neighboring cells that fire action
potentials. The current enters through gap junctions and exits by through the plasma membrane,
o Pacemaker cells can fire action potentials without an external stimulus, and do so in a regular, periodic
fashion because it doesn’t have a steady resting potential.
o After an action potential, a pacemaker cell immediately begins to depolarize slowly and continues to do
so until its membrane potential reaches threshold, triggering another action potential.
o The membrane potential returns to -60 to -70mV and then begins another round of slow depolarization
until another action potential is triggered.
o Slow depolarizations that lead to the action potentials are called pacemaker potentials.
o In pacemaker cells and other cardiac muscle cells, electrical signals are caused by changes in plasma
membrane ion permeability by the opening/closing specific ion channels
o As a membrane’s permeability to a particular ion increases relative to that of other ions, the membrane
potential moves toward the equilibrium potential of that ion.
o Important permeability involves Na, K, and Ca. The intracellular fluid is rich in potassium but poor in Na
and Ca compared to the extracellular fluid.
o Equilibrium potential of K is negative (-94mV), and for Na and Ca is positive (+60mV, +130mV).
o Therefore increased Na and Ca permeability makes the membrane potential more positive. Increasing K
permeability becomes more negative.
o The slow depolarization that occurs in the early stages of the pacemaker potential is due to closing of K
channels and opening of funny-channels (open after repolarization and allow Na and K ions to cross the
o K channels open during repolarization, and then close when the membrane returns to polarized state.
o With K channels closed and funny channels open during the early stages of the pacemaker potential, K
movement out of the cell decreases, Na movement into the cell increases causing depolarization.
o Funny channels open for a brief time, they close when membrane potential reaches -55mV.
o The initial depolarization triggers the opening of voltage-gated T-type Ca channels (transient) for a
short period of time before inactivating.
o Increasing the Ca, depolarizes the cell. This triggers the opening of L-type Ca channels (long-lasting),
which stays open longer and inactivate slowly.
o Result is a large increase Ca P , causing rapid depolarization. This causes Ca channels to open and some
Na to flow into the cell. This allows Ca channels to open to let Na in, incrNato depolarize.
o The depolarization triggers the opening of K channels, causing a riseKin P slowly after the increase in
PCa pulling the membrane potential to rest.
o The repolarization removes the stimulus for Ca channel opening. This reducesCaand decreases the
flow of Ca into the cell, along with increasKng P to repolarize.
ELECTRICAL ACTIVITY IN CARDIAC CONTRACTILE CELLS
o Gap junctions connect the pacemaker and conducting cells to contractile cells.
o Action potentials in cardiac contractile cells vary in shape and sped of propagation because they vary in
type and number of ion channels they possess.
o 2 important events to characterize cardiac action potentials: 1) P decreases as a result of a voltage-
gated K channel that closes when depolarized. 2) Depolarization causes the opening of voltage-gated
calcium channels to trigger muscle contractions. 13 THE CARDIVASCULAR SYSTEM: CARDIAC FUNCTION
CARDIAC ACTION POTENTIAL
o Phase 0: depolarization of the membrane triggers the opening of voltage-gated Na channels, which
raises PNaand increases the flow of Na into the cell. Membrane potential becomes more positive, which
triggers more Na channels to open, increasing P ,Naore depolarization. Rapid rise to +30 to +40mV.
o Phase 1: Na channels begin to inactivate, which reduces P causing the membrane potential to fall
toward the negative due to less Na moving in and K moving out. To depolarize and to counteract the Na
channel inactivation there are: closing of voltage-gated K channels, decreasing the flow of K out of the
cell, and also opening L-type Ca channels, raising the PCaand increases the flow of Ca into the cell.
o Phase 2: plateau phase. K channels stay close lowering the P tK below resting value, and P Caremains
elevated. Both keep the membrane at the depolarized state.
o Phase 3: P Kraduallyincreases because of the delayed rectifier channels, which open in response to
depolarization bringing the membrane potential to a negative value. They slowly open in phase 1 and 2
but affect the membrane potential in phase 3. The stimulus is removed that kept the inward rectifier
channels closed in phase 2, raising the PKmore. It also removes the stimulus that kept the Ca channels
open, lowering the P Ca
o Phase 4: resting potential. P , P , P are at resting values. Because P is greater than P the
K Na Ca K Na/Ca
membrane potential is -90mV, which is close to the equilibrium potential of K.
EXCITATION-CONTRACTION COUPLING IN CARDIAC CONTRACILE CELLS
Similar to stimulation in smooth muscle cells.
1. The stimulus is a current that triggers an action potential through the gap junctions.
2. It spreads through the plasma membrane, down the T tubules causing voltage-sensitive Ca channels on
the SR to open to release Ca into the cytosol.
3. The action potential triggers the opening of voltage-gated Ca channels on the plasma membrane,
allowing Ca to enter the cell (plateau phase)
4. Ca acts on voltage-sensitive channels that trigger CA release from the SR and stimulates them to stay
open longer. More CA for each action potential (calcium-induced calcium release).
5. Cytosolic Ca triggers contraction of cardiac muscle. Ca bidns to troponin, shifting tropomyosin off of
actin, and crossbridge cycling occurs.
6. 95% of Ca that binds to troponin come from the SR. 5% comes from extracellular fluid.
7. Relaxation of cardiac muscle requires removal of CA from the cytosol. Use a Ca-ATPase on the
membrane of the sarcoplasmic reticulum, transporting Ca from the cytosol to the lumen of the SR. there
are also Ca-ATPase in the plasma membrane to transport Ca from cytosol to the interstitial fluid. cardiac
muscle have Na-Ca exchanger in the plasma membrane.
8. Without Ca bound to troponin, tropomyosin shifts back on the myosin-binding sites on actin and the
muscle fiber relaxes.
RECORDING THE ELECTRICAL ACTIVITY OF THE HEART WITH AN ELECTROCARDIOGRAM
Electrocardiogram (ECG) is a noninvasive means for monitoring the electrical activity of the heart. The recorded
electrical events cause the contraction of cardiac muscle.
The ECG is a record of the overall spread of electrical current through the heart as a function of time during the
Have electrodes on the skin. Electrical activity generated in nervous/muscle tissue spreads through the body
(body fluids = conductors).
More synchronized activity gives larger amplitudes.
Equilateral triangle: right arm, left arm, and left leg (Einthoven’s triangle). Electrodes are connected to
oscilloscope or chart recorder.
Certain pairs of electrodes are called leads. One electrode in each lead is positive, one is negative.
Lead I: detects the potential at the left arm – the right arm.
Lead II: detects the potential at the left leg – the right arm.
Lead III: detects the potential at the left leg – left arm.
The direction of the recorded waveforms depends on whether the difference between the two electrodes is
positive (upward deflection) or negative (downward deflection). Ex: depolarization towards positive electrode
causes an upward deflection; towards negative electrode causes a downward deflection. 13 THE CARDIVASCULAR SYSTEM: CARDIAC FUNCTION
P wave: upward deflection due to atrial depolarization.
QRS complex: a series of sharp upward and downward deflections due to ventricular depolarization. Correlated
with phase 0.
T wave: an upward deflection caused by ventricular repolarization. Correlated with phase 3.
Atrial depolarization is generally not detected in an ECG recording because it occurs at the same time as QRS
Between the waves is a horizontal line, called the isoelectric line (no changes in electrical activity).
The ECG reflects patterns