Chapter 20- the heart

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.1

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.

In a single cardiac contraction, or heartbeat, the entire heart contracts in series--first the atria and then the ventricles. What is the first step in this process?

An action potential is generated by the sinoatrial node. -- Each heartbeat begins with an action potential generated at the SA node, and the rest of the conducting system then propagates and distributes this electrical impulse.

Premature contractions are common and typically not dangerous. If the premature contractions are due to aberrant Purkinje fibers, which of the following should you NOT conclude?

Atrial premature contractions occur -- Depolarizing Purkinje fibers or ventricular myocardial cells that reach threshold before the SA node trigger premature ventricular contractions (PVCs). Epinephrine, some stimulatory drugs, and some ions can increase the number of PVCs.

Which of the following is correct about the filling of the ventricles?

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.

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.

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?

K+ -- 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.

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.

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.

Which chamber receives blood from the superior and inferior vena cavae?

Right atrium -- Yes, the right atrium receives unoxygenated blood from the systemic circuit.

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

Several distinct abnormalities of cardiac electric activity characterize cardiac arrhythmias. Which of the following is NOT one of these abnormalities?

Tachycardia with a heart rate of less than 60 bpm -- Fibrillations, premature contractions facilitated by ectopic pacemakers, and tachycardias both characterize cardiac arrhythmias.

Heart valves are in what state during isovolumetric contraction?

The AV valves and semilunar valves are closed. -- Isovolumetric contraction is the brief period of time at the beginning of ventricular systole where both the AV valves and semilunar valves are closed. Because these closed valves prevent blood from exiting the ventricles, the volume of the ventricles stays the same despite contraction of the heart muscle. This period of contraction is quite brief - lasting less than 20 msec or about 2 percent of the full cardiac cycle.

What happens during atrial systole?

The chambers contract and push blood into the ventricles. -- Fluids move from an area of higher pressure to an area of lower pressure. In the cardiac cycle, the pressure within each chamber rises during systole and falls during diastole.

The heart is actually (one, two, or three) pumps?

Two pumps -- Yes, the right side of the heart pumps to/from the lungs (pulmonary circuit) and the left side of the heart pumps to/from the rest of the body (the systemic circuit).

An increase in sympathetic stimulation of the heart would increase stroke volume by increasing __________.

contractility -- Increased sympathetic activity increases heart contractility. This causes cardiac fibers to contract more forcefully at all levels of preload. Regardless of end diastolic volume, this mechanism increases stroke volume by reducing end systolic volume.

How would a decrease in blood volume affect both stroke volume and cardiac output?

decreased stroke volume and no change in cardiac output -- Yes, a decreased blood volume would decrease the end diastolic volume, thus lowering the stroke volume. Although this would initially lead to a decrease in the cardiac output, heart rate would increase because of increased activity of the sympathetic nervous system in an effort to maintain cardiac output.

What is the relaxed state of the ventricle called?

diastole -- The resting state of the atrium or ventricle is referred to as diastole. The heart chambers are in diastole for the majority of the cardiac cycle.

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 would increase heart rate?

epinephrine and norepinephrine -- Yes, secreted by the adrenal medulla as a result of sympathetic stimulation, these hormones act as part of the sympathetic response, increasing heart rate.

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).

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.

How would an increase in the sympathetic nervous system increase stroke volume?

increased contractility -- Yes, an increase in sympathetic nervous system activity would increase contractility (by increasing available calcium), thus increasing stroke volume. Contractility causes an increase in stroke volume by decreasing end systolic volume; it does not change end diastolic volume.

By what mechanism would an increase in venous return increase stroke volume?

increased end diastolic volume -- Yes, an increase in venous return increases the end diastolic volume. The fibers are stretched more, resulting in an increase in the force of contraction (preload, or the Frank-Starling Mechanism).

Which of the following would increase cardiac output to the greatest extent?

increased heart rate and increased stroke volume -- Yes, cardiac output = heart rate x stroke volume.

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.

Ventricular diastole begins with the closing of the semilunar valves. What phase of the cardiac cycle happens between this event and the later opening of the AV valves?

isovolumetric relaxation -- The prefix iso- means "same" so this is a period of "same volume". While the semilunar valve and AV valve are both closed, blood may not enter nor exit the ventricle. Therefore ventricular volume remains constant during this time. This period of relaxation is quite brief - lasting less than 20 msec or about 2 percent of the full cardiac cycle.

Which heart chamber receives blood from the pulmonary veins?

left atrium -- Yes, the left atrium receives oxygenated blood from the pulmonary veins.

Which chamber pumps oxygenated blood out the aorta to the systemic circuit?

left ventricle -- Yes, the left ventricle pumps oxygenated blood out the aorta to the entire body (systemic circuit).

With the force of the head-on collision, the patient's chest collided with the steering wheel, causing fractures of the anterior chest wall, including the sternum and ribs. The heart itself, located just deep to the sternum, was injured and began bleeding into the pericardial cavity. What is normally found in the pericardial cavity?

pericardial fluid -- The pericardial cavity normally contains 15-50 mL of pericardial fluid, which acts as a lubricant, reducing friction between the opposing surfaces as the heart beats.

Which heart chamber pumps unoxygenated blood out of the pulmonary trunk?

right ventricle -- Yes, the right ventricle pumps unoxygenated blood out the pulmonary trunk to the lungs.

Dr. Jim knew this victim was suffering from cardiac tamponade due to blood in the pericardial space. The blood in the pericardial cavity compressed the heart, squeezing the atria and ventricles so the atria could not fill. What vessels fill the right atria?

superior and inferior vena cava -- The superior/inferior vena cava fill the right ventricle and the pulmonary veins fill the left ventricle.

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.

The one-way nature of the left AV valve prevents blood flow from _________.

the left ventricle to the left atrium -- The atrioventricular valves are one-way valves that allow blood to flow into the ventricle but prevent blood flow back into the atrium. This maintains the movement of blood forward, toward the systemic circulation.

The decrease in left ventricular pressure at the end of ventricular systole causes __________.

the semilunar valve to close -- The semilunar valve closes when left ventricular pressure drops below aortic pressure. This occurs at the end of ventricular systole and marks the end of ventricular ejection. Blood in the aorta collects in the cusps of the semilunar valve and holds the valve shut while pressure in the ventricle continues to decrease and ventricular diastole begins.

Which of the following most correctly describes end-diastolic volume?

the volume of the ventricle when it is most full -- The end-diastolic volume is the ventricular volume at the end of ventricular diastole. At this point, all ventricular filling (including that from atrial systole) has occurred, and the ventricular volume is maximal.

The majority of ventricular filling occurs while the ventricles and atria are in what state(s)?

ventricular and atrial diastole -- The majority of ventricular filling occurs while both the ventricle and atrium are relaxed - while both chambers are in diastole. Ventricular filling also occurs during atrial systole, but this phase only contributes about 30 percent to ventricular filling.

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.

Isovolumetric 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.

Which of the following cardiac arrhythmias is associated with immediate, life-threatening danger?

ventricular fibrillation -- Ventricular fibrillations render the pumping activity of the ventricles impossible, and cardiac arrest ensues.

The closing of the left AV valve occurs near the beginning of __________.

ventricular systole -- Ventricular systole is the contraction of the ventricle. This causes the pressure within the ventricle to quickly rise above that in the atrium, which in turn causes the AV valve to close. The closed AV valve ensures that blood is ejected into the aorta and not back into the atrium.

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.

Left ventricular filling occurs __________.

while the AV valve is open -- The AV valve allows blood to flow in one direction - from the atrium to the ventricle. Because the ventricle is filled with blood from the atrium, this valve must be open to fill the ventricle.