A&p Ch. 18 - Heart & Cardiovascular Function

Cardiac tamponade refers to the accumulation of fluid in the pericardial cavity, which is the space between the heart and the pericardium (the sac surrounding the heart). This fluid buildup puts pressure on the heart, restricting its movement and affecting its ability to pump blood effectively. It can be a life-threatening condition requiring immediate medical intervention. Mitral valve prolapse, pleural effusion, cardiomyopathy, and pericarditis are all different conditions that do not specifically involve fluid accumulation in the pericardial cavity.

The volume of blood ejected from each ventricle during a contraction is called the stroke volume. This is the amount of blood that is pumped out of the heart with each heartbeat. It is calculated by subtracting the end-systolic volume (the amount of blood left in the ventricle after contraction) from the end-diastolic volume (the amount of blood in the ventricle before contraction). The stroke volume is an important measure of heart function and can be used to calculate the cardiac output, which is the amount of blood pumped by the heart in one minute.

The correct answer is 100,000. The heart is responsible for pumping blood throughout the body, and it beats around 100,000 times each day. This continuous pumping ensures that oxygen and nutrients are delivered to all the cells in the body and waste products are removed. The heart's consistent beating is essential for maintaining overall health and proper functioning of the body.

Acetylcholine slows the heart because it opens potassium ion channels in SA node cells and causes the pacemaker potential to depolarize more slowly. This allows more time for the heart to relax between contractions and results in a slower heart rate.

As blood leaves the right ventricle, it passes through the pulmonary valve. The pulmonary valve is located between the right ventricle and the pulmonary trunk. Its main function is to prevent the backflow of blood from the pulmonary trunk into the right ventricle. This allows the blood to flow in one direction, from the heart to the lungs, where it can be oxygenated.

The rupture of the papillary muscles in the left ventricle can lead to several complications, including mitral regurgitation, mitral valve prolapse, and bicuspid regurgitation. This is because the papillary muscles are responsible for anchoring the mitral and bicuspid valves, and their rupture can cause the valves to malfunction. Therefore, all of the options provided can be potential consequences of papillary muscle rupture in the left ventricle.

The difference between the systolic and diastolic pressures is called the pulse pressure. This is because the pulse pressure represents the force generated by the contraction of the heart during systole and the relaxation of the heart during diastole. It is an important measure of cardiovascular health and can provide information about the elasticity of blood vessels and the overall functioning of the heart.

Activation of beta-one receptors causes heart rate to increase. Beta-one receptors are found in the heart and when they are stimulated, they increase the heart rate. This is because beta-one receptors are responsible for the release of adrenaline, which leads to an increase in heart rate and contractility.

The right atrium receives blood from all of the above sources, including the coronary sinus, superior vena cava, and inferior vena cava. The coronary sinus drains deoxygenated blood from the heart muscle, while the superior vena cava brings deoxygenated blood from the upper body, and the inferior vena cava brings deoxygenated blood from the lower body.

The resistance would be greater in a vessel 10 microns in diameter compared to a vessel 1 mm in diameter. This is because resistance is inversely proportional to the cross-sectional area of the vessel. A smaller diameter vessel has a smaller cross-sectional area, resulting in higher resistance to the flow of fluid or electricity. Therefore, the vessel with a diameter of 10 microns would have a greater resistance than the vessel with a diameter of 1 mm.

At the circled label "5" on the graph, the occurrence is peak systolic pressure. This means that it represents the highest pressure in the arteries during systole, which is the phase of the cardiac cycle when the heart contracts and pumps blood into the arteries. The peak systolic pressure indicates the maximum force exerted by the heart to push blood out into the circulatory system.

The term "near the left fifth intercostal space" refers to a specific location on the chest. The apex of the heart is located in this region, making it the correct answer. The pericardial cavity refers to the space surrounding the heart, the visceral pericardium is the inner layer of the pericardium, the aorta is a major blood vessel that carries oxygenated blood from the heart, and the right atrium is one of the chambers of the heart. However, none of these descriptions specifically match the location near the left fifth intercostal space.

Intercalated discs are specialized structures found in cardiac muscle cells. They contain gap junctions, which allow for the transfer of ionic currents and action potentials from cell to cell. These electrical signals are essential for coordinating the contraction of the heart muscle. Therefore, intercalated discs play a crucial role in transferring both ionic currents and action potentials, as well as facilitating the force of contraction in cardiac muscle cells. Hence, the correct answer is "all of the above".

Chamber 16 receives oxygenated blood from the pulmonary circuit.

The wall of the left ventricle is thicker than the right because the left ventricle does more work than the right ventricle. This is because the left ventricle pumps blood to the entire body, while the right ventricle only pumps blood to the lungs. Additionally, the left ventricle pumps against greater resistance than the right ventricle, as the systemic circulation has a higher resistance compared to the pulmonary circulation. Lastly, the left ventricle produces a higher pressure than the right ventricle to overcome the greater resistance and pump blood effectively throughout the body.

If the pacemaker cells in the SA node become more permeable to potassium ions, it means that more potassium ions will be able to flow out of the cells. This will cause the cells to hyperpolarize, meaning that the membrane potential will become more negative. Hyperpolarization of the membrane makes it more difficult for the cells to reach the threshold for firing an action potential, thus decreasing the heart rate. Therefore, the correct answer is that both the heart rate will decrease and the membrane will hyperpolarize.

The blood flowing into the heart from the venae cavae flows next through the tricuspid valve.

Abnormally slow depolarization of the ventricles would most change the shape of the QRS complex in an ECG tracing. The QRS complex represents the depolarization of the ventricles, so if this depolarization is abnormally slow, it would result in a prolonged or widened QRS complex. This change in shape indicates a delay in the electrical conduction through the ventricles, which could be caused by various conditions such as bundle branch block or ventricular tachycardia.

Total peripheral resistance refers to the resistance encountered by blood flow in the systemic circulation. It is influenced by various factors, including blood vessel diameter, flow characteristics, and blood viscosity. The length of a blood vessel also affects resistance, as longer vessels offer more resistance. However, the osmolarity of interstitial fluids does not directly impact total peripheral resistance. Osmolarity refers to the concentration of solutes in a solution, and while it may affect fluid movement between the vascular and interstitial spaces, it does not directly influence the resistance encountered by blood flow in the systemic circulation.

The AV node does deliver the stimulus to the AV bundle, which is indeed located within the interventricular septum. Therefore, both parts of the statement are true.

Blocking the beta-one adrenergic receptors will decrease heart rate because these receptors are responsible for the stimulation of the heart. When they are blocked, the sympathetic nervous system's influence on the heart is reduced, leading to a decrease in heart rate.

Venoconstriction refers to the narrowing of the veins, which reduces the amount of blood within the venous system. This reduction in blood volume in the venous system leads to an increase in blood volume in the arterial and capillary systems. Therefore, the correct answer is "reduces; increases".

During the cardiac cycle, the QRS complex of the ECG precedes the increase in ventricular pressure. This is because the QRS complex represents ventricular depolarization, which occurs before ventricular contraction and the subsequent increase in ventricular pressure. Additionally, the first heart sound coincides with the QRS complex of the ECG. This is because the first heart sound, also known as S1, is produced by the closure of the atrioventricular valves at the beginning of ventricular systole, which aligns with the QRS complex on the ECG.

The first heart sound, also known as S1, is caused by the closure of the AV valves. These valves, which include the tricuspid valve on the right side of the heart and the mitral valve on the left side, close to prevent the backflow of blood from the ventricles to the atria during ventricular contraction. This closure produces a sound that can be heard as the "lub" sound of the heartbeat. Therefore, the correct answer is AV valves close.

The structure labeled "8." in Figure 18-1 is the papillary muscles.

During ventricular systole, the AV valves are closed. This is because ventricular systole refers to the contraction of the ventricles, which causes an increase in pressure within the ventricles. As the ventricles contract, the AV valves close to prevent the backflow of blood into the atria. This closure of the AV valves ensures that blood is forced into the arteries and prevents any regurgitation of blood into the atria. Therefore, during ventricular systole, the AV valves being closed is the correct answer.

Arteries are blood vessels that carry oxygenated blood away from the heart to the rest of the body. They have thick and elastic walls that allow them to withstand the high pressure generated by the pumping action of the heart. Arteries have the highest blood pressure among the given options because they receive blood directly from the heart and are responsible for distributing it throughout the body.

Veins have the lowest blood pressure compared to arteries, arterioles, capillaries, and venules. This is because veins carry deoxygenated blood back to the heart, and the pressure exerted by the heart has decreased significantly by the time the blood reaches the veins. Additionally, veins have thinner walls and less smooth muscle compared to arteries, which also contributes to their lower blood pressure.

The QRS complex represents the depolarization of the ventricles on an electrocardiogram. This complex is a series of waves that indicate the contraction of the ventricles, which is the main pumping action of the heart. The P wave represents the depolarization of the atria, while the T wave represents the repolarization of the ventricles. The S wave is not directly related to ventricular depolarization. The PR complex is not a recognized term in electrocardiography.

The correct answer is "all of the above." The mitral valve closes when left ventricular pressure exceeds left atrial pressure, which typically occurs at the beginning of ventricular systole. Additionally, the mitral valve and the tricuspid valve usually close at the same time, as they are both atrioventricular valves.

The correct answer is Systole; diastole or Ejection; filling. In the context of the question, "contraction" is the opposite of "relaxation". Systole refers to the contraction phase of the cardiac cycle, while diastole refers to the relaxation phase. Similarly, ejection refers to the contraction of the ventricles and the expulsion of blood, while filling refers to the relaxation of the heart chambers and the filling of blood. Both pairs of terms represent the opposite actions of contraction and relaxation.

In this question, the blood flow is directly related to the cross-sectional area of the vessel. The larger the diameter of the vessel, the greater the cross-sectional area, and therefore, the higher the blood flow. Additionally, the length of the vessel does not affect the blood flow. Therefore, the vessel with a diameter of 1.0 cm will have the highest blood flow, regardless of its length.

The heart pumps approximately 6,000 milliliters of blood each minute. This is because the heart is responsible for circulating blood throughout the body, supplying oxygen and nutrients to the cells. The average adult has a blood volume of about 5 liters, and the heart completes one full cycle of pumping all the blood in the body in about one minute. Therefore, the heart pumps approximately 6,000 milliliters (or 6 liters) of blood each minute to maintain proper circulation.

The visceral pericardium refers to the innermost layer of the pericardium, which is the protective sac surrounding the heart. It is also known as the epicardium. The epicardium is a thin layer of connective tissue that covers the outer surface of the heart and helps to protect and lubricate it. It is responsible for providing a smooth surface for the heart to beat against and helps to reduce friction between the heart and surrounding structures. Therefore, the correct answer is epicardium.

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If blood pressure doubled, it means that there is an increase in the force exerted by the blood against the walls of the vessel. This increased pressure would result in an increase in the blood flow through the vessel. Therefore, the correct answer is that the blood flow would be doubled.

The blood colloid osmotic pressure is mainly determined by the concentration of plasma proteins. This pressure is essential for maintaining fluid balance in the blood vessels. Plasma proteins, such as albumin, exert an osmotic force that helps to draw fluid back into the blood vessels from the surrounding tissues. When the concentration of plasma proteins is low, as in conditions like liver disease or malnutrition, the blood colloid osmotic pressure decreases, leading to fluid accumulation in the tissues and causing edema. Therefore, the concentration of plasma proteins is crucial for maintaining the blood colloid osmotic pressure.

Increased levels of ANP (atrial natriuretic peptide) will not cause an increase in blood pressure. ANP is a hormone released by the heart in response to high blood volume and pressure. It acts to decrease blood pressure by promoting vasodilation (widening of blood vessels) and increasing the excretion of sodium and water in the urine, which reduces blood volume. Therefore, increased levels of ANP will actually lead to a decrease in blood pressure, making it the exception among the given options.

The comparison that is false is "Both hearts weigh about the same." This is incorrect because a trained athlete's heart is larger and stronger compared to a non-athlete's heart. Regular exercise causes the heart to adapt and become more efficient, resulting in an increase in heart size and stroke volume. Therefore, the athlete's heart would weigh more than a non-athlete's heart.

During exercise, the body requires more oxygen and nutrients, which leads to an increase in cardiac output and heart blood flow. This is necessary to meet the increased demand of the working muscles. However, skin blood flow decreases during exercise as the body directs more blood towards the muscles and vital organs, prioritizing their needs over the skin. This helps regulate body temperature and prevent overheating during physical activity. The decrease in abdominal viscera and kidney blood flow is not observed during exercise, as these organs still require a sufficient blood supply to function properly.

The right coronary artery and left coronary artery are responsible for delivering blood to the myocardium, which is the muscular tissue of the heart. These arteries supply oxygen and nutrients to the heart muscle, ensuring its proper functioning.

The left ventricle has a thicker wall, is round in cross section, contracts harder, and produces about four to six times more pressure when it contracts compared to the right ventricle. However, it does not pump a greater volume.

The bicuspid valve, also known as the mitral valve, is located between the left atrium and the left ventricle of the heart. Its main function is to prevent the backward flow of blood from the left ventricle into the left atrium during ventricular contraction.

The mean arterial pressure (MAP) is calculated by adding one-third of the difference between the systolic and diastolic blood pressures to the diastolic blood pressure. In this case, the systolic blood pressure is 120 and the diastolic blood pressure is 90. The difference between them is 30, and one-third of that is 10. Adding 10 to the diastolic pressure of 90 gives us a mean arterial pressure of 100 mm Hg.

The sinoatrial node is the natural pacemaker of the heart, responsible for initiating the electrical signals that regulate the heart's rhythm. Located in the upper right atrium, it generates electrical impulses that spread through the atria, causing them to contract and pump blood into the ventricles. From there, the impulses travel to the atrioventricular node and then to the Purkinje fibers, which stimulate the contraction of the ventricles. However, the primary pacemaker is the sinoatrial node, making it the correct answer.

The cusps (leaflets) of atrioventricular valves attach directly to chordae tendineae. Chordae tendineae are strong, fibrous cords that connect the cusps of the atrioventricular valves to the papillary muscles in the ventricles. This connection helps to prevent the valves from inverting or prolapsing during ventricular contraction, ensuring proper blood flow through the heart.

The electrocardiogram (ECG) is a diagnostic tool used to measure the electrical activity of the heart. It provides valuable information about the heart's rhythm, the condition of the conducting system, the effects of drugs and poisons, and the duration of the ventricular action potential. However, it does not directly provide information about the stroke volume, which is the amount of blood pumped out of the heart with each contraction. Stroke volume is typically measured using other techniques such as echocardiography or cardiac catheterization.

The depolarization of the atria refers to the electrical activation of the atrial muscle cells, which leads to contraction. This electrical activity is represented on an ECG as the P wave. The P wave represents the depolarization of the atria, while the other options (QRS complex, QT interval, T wave, and S-T segment) represent different phases or components of the cardiac cycle that are not specifically related to the depolarization of the atria.

Arteriosclerosis is a condition characterized by the hardening and narrowing of arteries, which can lead to various complications such as heart attacks and strokes. The given answer of 50 percent suggests that half of the deaths in the United States are caused by complications related to arteriosclerosis. This indicates the significant impact of this condition on mortality rates in the country.

When renin is released from the kidney, it initiates a cascade of reactions that ultimately lead to the activation of angiotensin I. Renin acts on angiotensinogen, a precursor protein, to convert it into angiotensin I. Angiotensin I is then further converted to angiotensin II through the action of an enzyme called angiotensin-converting enzyme (ACE). Angiotensin II is a potent vasoconstrictor, meaning it causes the blood vessels to constrict, leading to an increase in blood pressure. Therefore, the correct answer is that angiotensin I is activated.

The fast depolarization phase of the action potential in cardiac muscle is caused by an increased membrane permeability to sodium ions. This allows sodium ions to rapidly enter the cell, leading to depolarization and the generation of an action potential. This increased permeability is due to the opening of voltage-gated sodium channels in response to a stimulus. The influx of sodium ions triggers the opening of voltage-gated calcium channels, which then allows calcium ions to enter the cell and initiate muscle contraction. Therefore, the correct answer is increased membrane permeability to sodium ions.

The pulmonary valve is the correct answer because it is responsible for allowing blood to leave the right ventricle and enter the pulmonary artery, which carries deoxygenated blood to the lungs for oxygenation. The other options mentioned are incorrect as they are not directly involved in the passage of blood from the right ventricle.

As blood travels from arteries to veins, the pressure drops. This is because the arteries carry oxygenated blood from the heart to the rest of the body at high pressure, while the veins carry deoxygenated blood back to the heart at a lower pressure. The pressure drop occurs due to the resistance encountered by the blood as it flows through the smaller blood vessels and capillaries. This decrease in pressure allows the blood to flow more slowly and efficiently through the veins, facilitating the exchange of nutrients and waste products in the tissues.

The coronary sulcus is a groove that marks the border between the atria and ventricles. This groove is located on the surface of the heart and helps to separate the upper chambers (atria) from the lower chambers (ventricles). It serves as a landmark for identifying the different regions of the heart and is important for understanding the anatomy and function of the heart.

The correct answer is the coronary arteries. The myocardium, which is the muscular tissue of the heart, receives blood supply from the coronary arteries. These arteries branch off from the aorta and provide oxygenated blood to the heart muscle. The coronary arteries ensure that the myocardium receives the necessary nutrients and oxygen to function properly. The other options, such as the coronary sinus or contact with blood in the pumping chambers, are not correct as they do not supply blood to the myocardium.

The contractions of the papillary muscles prevent the atrioventricular valves from reversing into the atria. These muscles are located in the ventricles of the heart and are connected to the atrioventricular valves by chordae tendineae. When the ventricles contract, the papillary muscles also contract, pulling on the chordae tendineae and preventing the valves from flipping back into the atria. This ensures that blood flows in one direction, from the atria to the ventricles, and prevents any backflow of blood into the atria.

All of the factors mentioned will increase the net filtration pressure to move fluid out of capillaries. Decreased plasma albumen reduces the colloid osmotic pressure, allowing more fluid to move out of the capillaries. Increased blood hydrostatic pressure increases the pressure gradient, pushing more fluid out of the capillaries. Decreased tissue hydrostatic pressure reduces the opposing pressure, facilitating the movement of fluid out of the capillaries. Therefore, all of these factors contribute to an increase in net filtration pressure.

Decreasing ejection fraction refers to the percentage of blood pumped out of the heart with each contraction. A lower ejection fraction indicates that the heart is not effectively pumping blood, resulting in a decreased cardiac output. In contrast, increasing stroke volume, ejection fraction, and heart rate all contribute to an increased cardiac output. Therefore, decreasing ejection fraction is the exception as it does not increase cardiac output.

Blood returning to the heart from the systemic circuit first enters the right atrium. This is because after oxygenated blood is delivered to the body through the systemic arteries, it returns to the heart through the systemic veins. The superior and inferior vena cava, which are large veins, carry deoxygenated blood from the body back to the heart. They both empty into the right atrium, where blood is then pumped into the right ventricle and eventually sent to the lungs for oxygenation. Therefore, the correct answer is right atrium.

Blood returning to the heart from the pulmonary circuit first enters the left atrium. This is because after oxygenation in the lungs, the oxygen-rich blood is transported back to the heart through the pulmonary veins, which empty into the left atrium. From the left atrium, the blood then passes through the mitral valve into the left ventricle before being pumped out to the rest of the body.

The P wave of the electrocardiogram represents the depolarization of the atria. Depolarization refers to the electrical activation of the heart muscle cells, causing them to contract. In this case, the P wave specifically represents the depolarization of the atria, which are the upper chambers of the heart. This electrical signal initiates the contraction of the atria, allowing them to pump blood into the ventricles. Therefore, the correct answer is depolarization of the atria.

The T wave on an ECG tracing represents ventricular repolarization. This is the phase of the cardiac cycle where the ventricles are recovering and preparing for the next contraction. As the ventricles repolarize, they relax and reset their electrical state, allowing for proper filling of blood before the next contraction.

Stimulation of the aortic baroreceptors triggers a reflex response to decrease blood pressure. This is achieved by increasing the activity of the parasympathetic nervous system, which causes a decrease in heart rate and vasodilation of blood vessels. This response helps to counteract the initial increase in blood pressure. The other options, such as stimulation of the cardioacceleratory center, increased sympathetic stimulation of the heart, stimulation of the vasoconstrictive center, and increased heart rate, would all result in an increase in blood pressure rather than a decrease.

The release of renin is not associated with a decrease in blood pressure. Renin is an enzyme that is released by the kidneys when blood pressure is low. It acts on a protein called angiotensinogen to produce angiotensin I, which is then converted to angiotensin II. Angiotensin II is a potent vasoconstrictor, meaning it causes blood vessels to constrict, leading to an increase in blood pressure. Therefore, the release of renin actually leads to an increase in blood pressure, making it the exception in this list.

When venous return is increased, more blood is returned to the heart, which leads to an increase in stroke volume. This is because an increased venous return results in a higher preload, which stretches the cardiac muscle fibers and allows for a more forceful contraction, thus increasing stroke volume. On the other hand, when diastolic blood pressure is decreased, there is less resistance against which the heart has to pump, allowing for an easier ejection of blood and an increase in stroke volume. Therefore, both an increased venous return and a decreased diastolic blood pressure can independently cause an increase in stroke volume.

The foramen ovale is a small opening in the interatrial septum, which is the wall that separates the right and left atria of the heart. This opening allows blood to bypass the fetal lungs and flow directly from the right atrium to the left atrium. This is important during fetal development because the lungs are not yet functioning and the oxygenation of blood occurs through the placenta. After birth, the foramen ovale typically closes, redirecting blood flow through the lungs.

A prolapsed mitral valve refers to the improper closure of the valve between the left atrium and the left ventricle of the heart. This condition would cause regurgitation, which means that blood would flow backward into the left atrium instead of moving forward into the aorta. As a result, the left ventricle would need to exert increased effort to pump blood out of the heart and into the circulation. Therefore, the correct answer is increased effort by the left ventricle and regurgitation.

The correct answer is capillary hydrostatic pressure. Capillary hydrostatic pressure refers to the pressure exerted by the fluid within the capillaries of the circulatory system. It is responsible for forcing fluids, such as nutrients and oxygen, out of the capillaries and into the surrounding tissues. The given range of pressure, declining from 35 mm Hg to about 18 mm Hg, aligns with the typical values of capillary hydrostatic pressure.

The first blood vessels to branch from the aorta are the coronary arteries. These arteries supply oxygenated blood to the heart muscle itself. They are responsible for delivering the necessary nutrients and oxygen to the heart, allowing it to function properly. Without the coronary arteries, the heart would not receive the blood supply it needs, which could lead to various heart conditions and complications.

Increased hematocrit refers to an increase in the proportion of red blood cells in the blood. While this can affect the viscosity of the blood, it does not directly impact blood flow to a tissue. Increased vessel diameter, increased blood pressure, decreased peripheral resistance, and relaxation of precapillary sphincters all contribute to increased blood flow by allowing more blood to reach the tissue.

The main control of peripheral resistance by the vasomotor centers occurs in the arteriole. Arterioles are small blood vessels that regulate blood flow and control the resistance to blood flow. They have smooth muscle in their walls, which can contract or relax to adjust the diameter of the vessel. By constricting or dilating the arterioles, the vasomotor centers can regulate the resistance to blood flow and control blood pressure. This allows for precise control of blood flow to different tissues and organs based on their metabolic needs.

The explanation for the correct answer is that the statement is stating two separate facts. The first part of the statement is false because the cardioacceleratory center actually activates sympathetic neurons, not parasympathetic neurons. However, the second part of the statement is true because the cardioinhibitory center does control parasympathetic neurons. Therefore, both parts of the statement are true, but they relate to different aspects of brainstem control of heart rate.

Calcium channel blockers, like nifedipine, work by blocking calcium channels in the heart muscle cells. Calcium is necessary for the contraction of the heart muscle, so by blocking these channels, the entry of calcium into the cells is reduced. This leads to a decrease in the force of cardiac contraction, making the heart pump with less force. Therefore, the correct answer is "decrease the force of cardiac contraction."

ADH (antidiuretic hormone) and aldosterone are hormones that are released by the body in response to a serious hemorrhage. A serious hemorrhage leads to a significant loss of blood volume, which triggers the release of ADH and aldosterone. ADH helps to retain water in the body by reducing urine production, while aldosterone promotes the reabsorption of sodium and water in the kidneys. Both of these hormones work together to increase blood volume and maintain blood pressure, which is necessary to compensate for the loss of blood during a serious hemorrhage.

During the T wave of the electrocardiogram, the ventricles are both repolarizing and relaxing. Repolarization refers to the process where the ventricles regain their electrical charge after depolarization, which occurs during the QRS complex. Relaxation, also known as diastole, is the phase when the ventricles are filling with blood and preparing for the next contraction. Therefore, during the T wave, the ventricles are undergoing repolarization and relaxation simultaneously.

At the circled label "4" on the graph, isovolumetric contraction occurs. This refers to the phase of the cardiac cycle where the ventricles contract, causing an increase in pressure within the ventricles. However, during this phase, the volume of blood in the ventricles remains constant as both the atrioventricular and semilunar valves are closed. This allows for the build-up of pressure necessary to open the semilunar valves and initiate ejection of blood during the next phase of the cardiac cycle.

When the connection between the SA node and AV node becomes blocked, it disrupts the normal conduction pathway of electrical signals in the heart. The SA node is responsible for initiating the electrical impulses that regulate the heart's rhythm, and the AV node helps to transmit these impulses to the ventricles. If the connection is blocked, the ventricles will no longer receive the signals to contract at the normal pace set by the SA node. As a result, the ventricles will beat more slowly, leading to a decrease in heart rate.

During the plateau phase of the cardiac muscle action potential, the calcium channels remain open. This allows calcium ions to continue flowing into the cell, balancing the outward flow of potassium ions. This sustained influx of calcium helps to maintain the depolarization of the cell membrane and prolongs the action potential, leading to a longer contraction of the cardiac muscle.

The great and middle cardiac veins are responsible for draining blood from the heart muscle. This blood is then collected into a large vein called the coronary sinus. The coronary sinus is located in the coronary sulcus, which is a groove that runs along the surface of the heart. From the coronary sinus, the blood is eventually returned to the right atrium of the heart. Therefore, the correct answer is coronary sinus.