Chapter 18

What is the ligamentum arteriosum?

A remnant of the ductus arteriosus The ligamentum arteriosum is a remnant of the ductus arteriosus. The ductus arteriosus is a fetal shunt that connects the pulmonary trunk to the aorta.

Which of these structures conduct(s) action potentials the slowest?

AV node Action potentials slow down as they pass through the AV node. This gives the atria time to finish contracting before the ventricles are depolarized.

Which of the following structures receives the depolarization wave from the atria and passes impulses onto the ventricles?

AV node The AV node receives the depolarizing wave from the atria and delays and coordinates that distribution to the ventricles.

Which part of the intrinsic conduction system delays the impulse briefly before it moves on to the ventricles?

AV node Yes, the AV node slows down the impulse giving the atria time to contract before the ventricles contract.

Increased pressure in the ventricles would close what valve(s)?

AV valves only Yes, increased pressure in the ventricles would close the AV valves.

Identify the ear like flaps that are attached to the top chambers of the heart.

Auricle The auricles are flap like extensions of the atria that are observed on the external surface of the heart.

Determine which of the following electrocardiogram (ECG) tracings is missing P waves but is otherwise regular.

B illustrates a QRS complex and a T wave, but it is missing the P wave.

In contrast to a skeletal muscle cell action potential, why does the action potential for a cardiac muscle cell contain a "plateau" phase?

Cardiac muscle cells contain slow Ca2+Ca2+ channels in their sarcolemma that continue to allow influx of Ca2+Ca2+ ions after Na+Na+ channels are inactivated. Fast voltage-gated Na+Na+ channels are open only for a very brief time before they are inactivated, but the resulting depolarization opens slow Ca2+Ca2+ channels that start a positive feedback cycle that temporarily keeps the membrane potential high.

What fibrous structure functions to anchor the atrioventricular valves in a closed position?

Chordae tendineae Chordae tendineae are the fibrous cords that attach the atrioventricular valves to papillary muscles. They help anchor the atrioventricular valves in a closed position during ventricular contraction.

The base of the heart is located at the bottom of the heart.

False The base is located at the top of the heart. It is unusual to see the largest portion of an organ at the top. Typically, the base is at the bottom of an organ, as in the lungs.

The moderator band is found on both the right and left side of the heart.

False The moderator band is a cord like structure found in the right ventricle. It contains portions of the cardiac conduction system that help convey the electrical message for the ventricles to contract. It works as a shortcut within the cardiac conduction system.

Why does a graph of the membrane potential of living pacemaker cells never demonstrate a flat line?

Hyperpolarization of pacemaker cells triggers the opening of slow Na+ channels and starts a new slow depolarization phase At the end of an action potential, as repolarization and hyperpolarization occur, slow Na+ channels open, causing the membrane potential to immediately start to increase again (called the pacemaker potential), eventually triggering another action potential.

Which of the following is correct about the filling of the ventricles? View Available Hint(s)

Most blood flows passively into the ventricles through open AV valves. Yes, most of the ventricular filling is passive; atrial contraction adds just a little more blood.

Contraction of the atria results from which wave of depolarization on the ECG tracing?

P wave Yes, the P wave represents atrial depolarization, which leads to atrial contraction.

Name the ridged bundles of muscle found projecting inside the right atrium.

Pectinate muscles The pectinate muscles are ridge like elevations of muscle. They are parallel to each other and resemble teeth of a comb. They are easily observed inside the right atrium.

Describe the pressures in the atria and ventricles that would cause the opening of the AV valves.

Pressure in the atria would be greater than the pressure in the ventricles. Yes, higher pressure in the atria than in the ventricles forces the AV valves to open and blood moves into the ventricles.

What is the function of the coronary circulation?

Provide a blood supply to the heart The coronary circulation provides oxygenated blood to the heart through coronary arteries. Coronary veins return the deoxygenated from the heart to the coronary sinus on the posterior surface of the heart. The coronary sinus empties the deoxygenated blood directly into the right atrium where it joins deoxygenated from the body.

Identify the valve located at the exit of the right ventricle.

Pulmonary semilunar valve The pulmonary valve is the semilunar valve found between the right ventricle and the pulmonary trunk. Deoxygenated blood travels through the pulmonary valve as it exits the right side of the heart on its way to the lungs.

Electrical impulses pass through the conduction system of the heart in the following sequence:

SA node, AV node, bundle of His, bundle branches, and Purkinje fibers The impulses of the heart originate at the SA node (pacemaker). The impulse is then transmitted to the AV node (atrioventricular node), where the impulse slows down to allow the atria to completely contract and thereby fill the adjacent ventricles. The AV node then transmits the impulse to the bundle of His, which branches into left and right bundle branches. The bundle branches give rise to the Purkinje fibers, which transmit the impulse to the ventricle walls and stimulate ventricular contraction.

Besides cardiac muscle, what muscle tissue type is involuntary and displays autorhythmicity (also called "automaticity")? Besides cardiac muscle, what muscle tissue type is involuntary and displays autorhythmicity (also called "automaticity")?

Smooth muscle

Which of the following is true during ventricular systole?

The AV valves are closed. At the beginning of ventricular systole, the one-way AV valves are forced shut. The AV valves remain shut throughout ventricular systole. This prevents blood from flowing back into the atria when the ventricles contract.

The P wave on an electrocardiogram represents __________.

The P wave reflects the depolarization of the atria.

What would happen to the SA node if a chemical blocker was used to reduce transport of Na+ into the pacemaker cells?

The SA node would depolarize more slowly, reducing the heart rate. Diffusion of Na+ into the pacemaker cell causes a gradual depolarization of the cell membrane, called the pacemaker potential. If the rate of depolarization slows down, it will take longer for the membrane to reach threshold and trigger the next action potential, which will reduce heart rate.

What is the role of the atrioventricular bundle?

The atrioventricular bundle provides the only pathway for electrical signals to pass from the atria to the ventricles The atrial cardiac cells are not connected by gap junctions to the ventricular cardiac cells. The only means by which the electrical signals of the heart can pass from atria to ventricles is the atrioventricular bundle.

labeling the heart 7

The base is located at the top of the heart. It is unusual to see the largest portion of an organ at the top. Typically, the base is at the bottom of an organ, as in the lungs.

Which of the following is equivalent to the ventricular volume during isovolumetric contraction?

The end diastolic volume (EDV) Isovolumetric contraction occurs at the beginning of ventricular systole when the ventricular volume is at its maximum value−the end diastolic volume (EDV). Because no volume changes occur during isovolumetric contraction, ventricular volume remains at this value throughout the phase.

Isovolumetric relaxation is characterized by which of the following?

The semilunar and AV valves are closed. During isovolumetric relaxation, all valves into and out of the ventricles remain closed. This prevents blood from flowing backward through the heart.

During the ventricular ejection phase of the cardiac cycle, which of the following is true?

The semilunar valves are open. During ventricular ejection, blood flows from the ventricles into the arteries. To do so, the blood must pass through the semilunar valves, which must be open during this phase. Narrowing of the left semilunar valve reduces blood flow out of the heart, a disease condition known as aortic stenosis.

When the atria contract, which of the following is true?

The ventricles are in diastole. Atrial contraction fills each of the ventricles to their maximum capacity−the end diastolic volume (EDV). This occurs towards the end of ventricular diastole while the ventricles are still relaxed.

Name the irregular ridges of muscle lining the ventricles.

Trabeculae carneae Trabeculae carneae are the small elevated ridges of muscle arranged in a web type fashion found in both the right and left ventricles.

Identify the right atrioventricular valve.

Tricuspid valve The tricuspid valve is the atrioventricular valve located on the right side of the heart. It separates the right atrium from the right ventricle.

The first branch off the arch of the aorta is the brachiocephalic artery in both the sheep and the human.

True In both the sheep and human, the brachiocephalic artery is the first major vessel to exit the arch of the aorta. Further branching of the brachiocephalic artery and the arch of the aorta differs between a sheep and a human.

Blood on the right never mixes with blood on the left, once the heart is fully developed.

True Once fetal modifications (i.e. foramen ovale, ductus arteriosus) close following delivery of the infant, blood on the right side of the heart should not mix with blood on the left side of the heart.

What does the QRS wave of the electrocardiogram (ECG) represent?

Ventricular depolarization is represented by the QRS complex of an ECG.

Atrial pressure is greater than ventricular pressure during which phase of the cardiac cycle?

atrial contraction Blood always flows from high to low pressure. During atrial contraction, blood flows from atria (high pressure) to ventricles (low pressure). Similarly, this same pressure gradient exists during ventricular filling.

Identify the valve found between the left atrium and left ventricle.

bicuspid valve (mitral valve) The atrioventricular valve on the left is the bicuspid valve. The mitral valve is also called the mitral valve.

What does the ECG wave tracing represent?

electrical activity in the heart Yes, the ECG waves show the depolarization and repolarization in various areas of the heart.

Which of the following is equivalent to the ventricular volume during isovolumetric relaxation?

end systolic volume (ESV) Isovolumetric relaxation occurs at the beginning of ventricular diastole when the ventricular volume is at its minimum value−the end systolic volume (ESV). Because no volume changes occur during isovolumetric relaxation, ventricular volume remains at this value throughout the phase.

Name the inner lining of the heart.

endocardium The endocardium is the smooth, slick lining of the chambers of the heart. Endocardium is a continuation of endothelium that lines blood vessels. Both endocardium and endothelium are composed of simple squamous epithelial tissue, which minimizes friction between the wall of the chamber/vessel and the blood.

Which of these is a condition marked by premature ventricular contraction?

extrasystole Premature ventricular contraction (PVC) is a type of extrasystole. Extrasystole, or premature contraction, can be caused by ectopic pacemakers (also called ectopic foci). An ectopic focus can result when the SA node is defective. Alternatively, some drugs (such as caffeine and nicotine) can generate an ectopic focus.

Oxygenated blood flows through the right side of the heart.

false Oxygenated blood returning from the lungs is received by the left atrium, then to the left ventricle before being pumped out to the body. This pathway of blood flow is known as the systemic circulation.

When threshold is reached at the SA node (an autorhythmic cell), what channels open causing further depolarization of the membrane?

fast calcium Yes, unlike nerve cells or cardiac muscle cells, fast calcium channels are responsible for the depolarization phase of the autorhythmic cell action potential. When the fast calcium channels open, calcium rushes into the cell making it less negative (or more positive).

In what direction does blood flow through the heart?

from a region of high pressure to a region of low pressure Blood moves through the heart from atria to ventricles and out large arteries, always from areas of high pressure to areas of lower pressure through one-way valves.

Action potentials generated by the autorhythmic cells spread to the contractile cells through what structures in the membrane?

gap junctions Yes, action potentials generated by the autorhythmic cells spread waves of depolarization to contractile cells through gap junctions. If the depolarization causes the contractile cells to reach threshold, they will in turn generate an action potential

What causes the aortic semilunar valve to close?

greater pressure in the aorta than in the left ventricle Yes, backflow of blood in the aorta (towards the left ventricle) closes the aortic semilunar valve.

Put the phases of the cardiac cycle in the correct order, starting after ventricular filling.

isovolumetric contraction, ventricular ejection, isovolumetric relaxation Yes, the ventricles must contract and eject blood before they relax and fill again.

Identify the most muscular chamber.

left ventricle the left ventricle is responsible for pumping blood out to the body. This requires a lot of force. The left ventricle is the most muscular in order to pump the blood with adequate force.

What heart chamber pushes blood through the aortic semilunar valve?

left ventricle The aortic valve is located between the left ventricle and the aorta. The left ventricle pumps oxygenated blood out of the heart through the aortic valve to the body.

One of the changes that occurs in the pacemaker potential (unstable resting membrane potential) in the SA node (an autorhythmic cell) is a decreased efflux of what ion?

potassium Yes, if there is a decreased efflux of potassium while there is a normal influx of sodium, the inside of the cell would become less negative. Thus, threshold would be reached. The ability of these autorhythmic cells to spontaneously depolarize is what results in the pacemaker potential.

What is the main function of heart valves?

prevent the backward flow of blood Heart valves are one-way valves that prevent blood from flowing backward. For example, the AV valves only allow blood to flow from the atria to the ventricles. And the semilunar valves only allow blood to flow from the ventricles to the aorta and pulmonary trunk. When valves do not completely close, blood flows backward through the heart, creating an abnormal "sloshing" sound known as a heart murmur.

The anatomy of the intrinsic conduction system causes contraction of the ventricles to begin at the apex and move superiorly. Why is this important?

so blood is forced upward, toward the semilunar valves Beginning contraction at the apex causes blood to be forced upward, through the semilunar valves, and into the aorta and pulmonary trunk. This makes ventricular ejection more efficient, much like squeezing a tube of toothpaste up from the bottom is more efficient at dispensing the toothpaste than is squeezing from the top.

In order to cause cardiac muscle contraction, the contractile cells must also depolarize. What causes the depolarization of the contractile cells?

the flow of positive ions from adjacent cells Yes, the flow of positive ions from the autorhythmic cells (or adjacent cells) brings the membrane to threshold initiating depolarization of the contractile cell.

What structure in the intrinsic cardiac conduction system determines heart rate?

the sinoatrial (SA) node The SA node is composed of pacemaker cells that initiate and set the initial pace, or the sinus rhythm, of the heartbeat.

What does the QRS complex represent in the ECG wave tracing?

ventricular depolarization Yes, the QRS complex represents depolarization in the ventricles, which have greater mass than the atria.

sovolumetric relaxation and ventricular filling (two phases of the cardiac cycle) take place during __________.

ventricular diastole Yes, both occur during ventricular diastole when the ventricles are not actively contracting and ejecting blood.

Most of the decrease in ventricular volume takes place during which phase of the cardiac cycle?

ventricular ejection As the ventricles contract, blood is forced through the semilunar valves and out into the arteries, resulting in a reduction in ventricular blood volume. At the end of this phase, ventricular volume is at a minimum−the "end-systolic volume" (ESV).

Ventricular pressure is greater than aortic pressure during which phase of the cardiac cycle?

ventricular ejection While pressure in the ventricle is greater than pressure in the aorta, the semilunar valve remains open. This allows blood to be ejected from the ventricle.

Most of the increase in left ventricular volume takes place during what phase of the cardiac cycle?

ventricular filling During ventricular filling the AV valves remain open, which allows blood to flow from the atria into the ventricles. The passive flow of blood during this phase (before atrial contraction) accounts for roughly 80 percent of the increase in ventricular volume.

Repolarization of an autorhythmic cell is due to the opening of which channels?

voltage-gated potassium channels Yes, opening of voltage-gated potassium channels causes positive potassium ions to move out of the cell. This efflux of potassium causes the cell to become more negative inside thus, repolarizing the cell.

At what point in the cardiac cycle does the AV valve open?

when atrial pressure becomes greater than ventricular pressure Blood flows through the heart in one direction (atria-ventricles-large arteries) and from high to low pressure. When pressure in the atrium becomes greater than ventricular pressure, the AV valve opens; and blood flows from the atrium into the ventricle.

At what point in the cardiac cycle does the semilunar valve close?

when pressure in the ventricle becomes less than aortic pressure When pressure in the ventricle drops below aortic pressure, the semilunar valve shuts. This prevents blood from flowing backward through the heart.

At what point in the cardiac cycle does the semilunar valve open?

when ventricular pressure becomes greater than aortic pressure When pressure in the ventricle exceeds pressure in the aorta, the semilunar valve opens. This allows blood to be ejected from the ventricle.

At what point during the cardiac cycle does the AV valve close?

when ventricular pressure becomes greater than atrial pressure When ventricular pressure rises above atrial pressure, the AV valve closes. This prevents blood from flowing backward through the heart.

A person notices his or her heart beat because he or she senses blood being pumped by the heart. Excessive caffeine intake can lead to irregular heart rhythms (arrhythmias) that patients perceive as "skipped beats." Given that caffeine is a stimulant, which of the following mechanisms best explains the reason for the feeling that the heart skipped a beat?

Purkinje fibers initiate spontaneous action potentials, which cause the ventricles to contract early. Action potentials normally originate in the SA node because the pacemaker cells there depolarize faster than pacemaker cells located elsewhere in the heart. However, certain drugs, such as caffeine, nicotine, or cocaine, can stimulate other pacemaker cells to speed up and temporarily "escape" the SA node rhythm. If this ectopic focus consists of Purkinje fibers, then ventricular contraction will occur prematurely, prior to ventricular filling. Without proper filling, this abnormal contraction pumps little blood and is not sensed. However, the following normal beat generated from the SA node has augmented filling and is sensed, resulting in the missed beat sensation.

Which chamber of the heart exits into the pulmonary trunk?

Right ventricle The right ventricle pumps deoxygenated blood through the pulmonary semilunar valve into the pulmonary trunk to be distributed to the lungs.

Which part of the intrinsic conduction system normally initiates the depolarizing impulse that causes a heartbeat?

SA node Like the rest of the intrinsic conduction system, the SA node contains pacemaker cells that spontaneously depolarize. The cells within the SA node, however, depolarize faster than the other cells within the system. This causes action potentials to initiate in the SA node rather than in any of the other structures.

Which part of the conduction system initiates the depolarizing impulse, which spreads throughout the heart?

SA node The SA Node spontaneously depolarizes, causing the wave of depolarization that spreads through the rest of the conduction system and heart.