USMLE Step 1 Quiz: Cardiac Physiology And Hemodynamics

What this quiz covers

This quiz focuses on Cardiac Physiology And Hemodynamics, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Aortic arch chemoreceptors
2. Carotid sinus and aortic arch baroreceptors (correct answer)
3. Atrial stretch receptors
4. Juxtaglomerular cells

Explanation: In hemorrhagic shock, the decrease in blood pressure leads to reduced stretch of the baroreceptors located in the carotid sinus and aortic arch. This reduced stretch decreases the firing rate of afferent nerves (glossopharyngeal from carotid sinus, vagus from aortic arch). This signal is interpreted by the brainstem as hypotension, leading to a coordinated response of decreased parasympathetic outflow and increased sympathetic outflow to the heart and blood vessels, causing tachycardia and vasoconstriction.

Question 2

1. Aortic and pulmonic valves
2. Mitral and tricuspid valves (correct answer)
3. Aortic and mitral valves
4. Pulmonic and tricuspid valves

Explanation: The first heart sound (S1, 'lub') is a high-frequency sound caused by the closure of the atrioventricular valves (mitral and tricuspid) at the beginning of isovolumetric contraction. This event marks the start of ventricular systole. The second heart sound (S2) is caused by the closure of the semilunar (aortic and pulmonic) valves.

Question 3

1. Capillary hydrostatic pressure (correct answer)
2. Interstitial hydrostatic pressure
3. Capillary oncotic pressure
4. Interstitial oncotic pressure

Explanation: Right-sided heart failure causes a backup of blood in the systemic venous circulation, leading to increased central venous pressure. This elevated pressure is transmitted back to the systemic capillaries, increasing the capillary hydrostatic pressure. According to the Starling equation, this increased hydrostatic pressure overcomes the opposing oncotic pressure, leading to a net filtration of fluid out of the capillaries and into the interstitial space, resulting in edema.

Question 4

1. Phrenic nerve
2. Sympathetic cardiac nerves
3. Vagus nerve (correct answer)
4. Glossopharyngeal nerve

Explanation: Endurance training leads to an increase in resting parasympathetic (vagal) tone. The vagus nerve (CN X) innervates the sinoatrial (SA) and atrioventricular (AV) nodes. Increased vagal activity releases acetylcholine, which slows the intrinsic firing rate of the SA node, resulting in resting bradycardia. This is a normal physiologic adaptation in well-conditioned athletes.

Question 5

1. Increased end-systolic volume (correct answer)
2. Decreased peak systolic pressure
3. Increased stroke volume
4. Decreased end-diastolic volume

Explanation: A pure alpha-1 agonist like phenylephrine causes systemic vasoconstriction, which increases total peripheral resistance and, consequently, left ventricular afterload. With increased afterload, the ventricle must generate a higher pressure to eject blood, and it is unable to eject as much blood per beat. This results in a smaller stroke volume and a larger volume of blood remaining in the ventricle at the end of systole, i.e., an increased end-systolic volume.

Question 6

1. Mean systemic filling pressure
2. Venous return (correct answer)
3. Resistance to venous return
4. Cardiac contractility

Explanation: Placing a patient in the Trendelenburg position uses gravity to augment the movement of blood from the venous capacitance vessels of the lower body and splanchnic circulation towards the heart. This action increases the volume of blood returning to the right atrium, thereby increasing venous return and preload. By the Frank-Starling mechanism, this can transiently increase cardiac output and blood pressure.

Question 7

1. Albumin (correct answer)
2. Gamma globulins
3. Fibrinogen
4. Transferrin

Explanation: Albumin is the most abundant plasma protein and is synthesized exclusively by the liver. It is the main determinant of plasma colloid oncotic pressure, the force that holds fluid within the vascular space. In severe liver disease (cirrhosis), the liver's synthetic function is impaired, leading to hypoalbuminemia. The resulting decrease in oncotic pressure allows fluid to shift from the capillaries into the interstitial space, causing edema and ascites.

Question 8

1. Presence of valves
2. Thicker tunica media
3. Higher compliance (correct answer)
4. Smaller luminal diameter

Explanation: Compliance describes the distensibility of a blood vessel (change in volume for a given change in pressure). Veins have much thinner, less muscular walls than arteries, making them significantly more compliant. This high compliance allows them to accommodate large changes in blood volume with only small changes in pressure. This enables them to function as a volume reservoir, or capacitance system, for the circulation.

Question 9

1. Increased heart rate and increased stroke volume (correct answer)
2. Increased heart rate and decreased stroke volume
3. Decreased heart rate and increased stroke volume
4. Increased heart rate with no change in stroke volume

Explanation: Cardiac output is the product of heart rate and stroke volume (CO = HR x SV). During exercise, sympathetic nervous system activity increases. This leads to an increased heart rate (positive chronotropy) and increased myocardial contractility (positive inotropy), which enhances stroke volume. Furthermore, the skeletal muscle pump increases venous return, which boosts preload and further increases stroke volume via the Frank-Starling mechanism. Thus, the rise in cardiac output is a result of increases in both heart rate and stroke volume.

Question 10

1. Interstitial fluid hydrostatic pressure
2. Capillary hydrostatic pressure
3. Interstitial fluid colloid osmotic pressure
4. Capillary colloid osmotic pressure (correct answer)

Explanation: In nephrotic syndrome, massive proteinuria leads to hypoalbuminemia. Since albumin is the primary determinant of plasma oncotic pressure, its loss significantly reduces the capillary colloid osmotic pressure (also known as oncotic pressure, $\pi_c$). This force normally pulls fluid into the capillaries. When it is reduced, the balance of Starling forces is disrupted, favoring net filtration of fluid out of the capillaries and into the interstitium, causing generalized edema.

Question 11

1. Reactive hyperemia
2. Myogenic autoregulation
3. Metabolic autoregulation (active hyperemia) (correct answer)
4. Parasympathetic regulation

Explanation: Metabolic autoregulation, specifically active hyperemia, is the process by which blood flow to a tissue is matched to its metabolic activity. When a tissue like the myocardium becomes more metabolically active or ischemic, it produces vasodilator metabolites (e.g., adenosine, K+, H+, CO2). These substances act locally on arteriolar smooth muscle to cause vasodilation, thereby increasing blood flow and oxygen delivery to meet the tissue's needs.

Question 12

1. A shift of the end-systolic pressure-volume relationship (ESPVR) curve upward and to the right
2. A decrease in the peak systolic pressure achieved
3. An increase in the end-systolic volume
4. A shift of the end-systolic pressure-volume relationship (ESPVR) curve upward and to the left (correct answer)

Explanation: The end-systolic pressure-volume relationship (ESPVR) defines the maximal pressure the ventricle can generate at any given left ventricular volume and is a load-independent measure of myocardial contractility. An increase in contractility (positive inotropy) allows the ventricle to generate more pressure for a given volume. This causes the ESPVR line to become steeper and shift upward and to the left. The pressure-volume loop will also shift leftward to a smaller end-systolic volume, reflecting a larger stroke volume.

Question 13

1. Mean systemic filling pressure (correct answer)
2. Myocardial contractility
3. Arteriolar tone
4. Heart rate

Explanation: Mean systemic filling pressure (Pmsf) is the pressure in the vascular system when blood flow ceases, reflecting the overall 'fullness' of the circulation. It is directly proportional to the stressed blood volume. Administering an IV fluid bolus increases the stressed volume, which in turn increases the Pmsf. This raises the pressure gradient for venous return (venous return = [Pmsf - Right Atrial Pressure] / Resistance to venous return), leading to increased preload and, via the Frank-Starling mechanism, increased cardiac output.

Question 14

1. Norepinephrine
2. Epinephrine
3. Acetylcholine (correct answer)
4. Dopamine

Explanation: Carotid sinus massage applies external pressure to the carotid sinus, increasing the firing rate of the baroreceptors. This signals the brainstem that blood pressure is high, resulting in a reflex increase in parasympathetic (vagal) nerve activity to the heart. The postganglionic parasympathetic nerve endings release acetylcholine, which binds to M2 muscarinic receptors on the SA and AV nodes. This action slows the SA node firing rate and decreases AV node conduction, leading to a decreased heart rate.

Question 15

1. Blood viscosity
2. Vessel length
3. Vessel radius (correct answer)
4. Pressure gradient

Explanation: The Poiseuille equation states that flow is directly proportional to the pressure gradient and the vessel radius to the fourth power ($r^4$), and inversely proportional to vessel length and fluid viscosity. Because flow is dependent on the fourth power of the radius, even small changes in vessel radius (as seen in vasodilation or vasoconstriction) have a very large effect on blood flow.

Question 16

1. Afterload
2. Preload (correct answer)
3. Heart rate
4. Myocardial contractility

Explanation: The Frank-Starling mechanism describes the relationship between ventricular preload (end-diastolic volume) and stroke volume. An increase in preload stretches the myocardial sarcomeres to a more optimal length, leading to a more forceful contraction and an increased stroke volume. Afterload is the resistance the heart pumps against and is inversely related to stroke volume. Heart rate affects cardiac output but not directly stroke volume. Myocardial contractility increases stroke volume but is a separate mechanism from the Frank-Starling law.

Question 17

1. Closure of the aortic valve (correct answer)
2. Opening of the mitral valve
3. Peak of ventricular ejection
4. Closure of the mitral valve

Explanation: The dicrotic notch (or incisura) is a prominent feature of the aortic pressure waveform that marks the end of ventricular ejection. It is caused by the closure of the aortic valve. Immediately after the valve snaps shut, there is a brief backflow of blood in the aorta that rebounds off the closed valve cusps, causing a short-lived rise in aortic pressure. This event signals the beginning of diastole.

Question 18

1. The mitral valve is open, and the aortic valve is closed.
2. The mitral valve is closed, and the aortic valve is open.
3. Both the mitral and aortic valves are open.
4. Both the mitral and aortic valves are closed. (correct answer)

Explanation: Isovolumetric contraction is the phase of the cardiac cycle that begins immediately after the mitral valve closes (producing S1) and ends when the rising left ventricular pressure exceeds the aortic pressure, causing the aortic valve to open. During this brief period, the ventricle is contracting as a closed chamber, so the volume of blood within it does not change ('isovolumetric'). Therefore, both the inflow (mitral) and outflow (aortic) valves must be closed.

Question 19

1. 9 mm Hg/L/min
2. 18 mm Hg/L/min (correct answer)
3. 45 mm Hg/L/min
4. 90 mm Hg/L/min

Explanation: Systemic vascular resistance (SVR), also known as total peripheral resistance (TPR), is calculated using a formula derived from Ohm's law: SVR = (MAP - CVP) / CO. In this case, SVR = (93 mm Hg - 3 mm Hg) / 5 L/min = 90 mm Hg / 5 L/min = 18 mm Hg/L/min. This unit is sometimes referred to as a Wood unit.

Question 20

1. Increased sinoatrial node firing rate
2. Decreased myocardial contractility (correct answer)
3. Increased atrioventricular node conduction velocity
4. Decreased ventricular filling time

Explanation: Beta-adrenergic antagonists (beta-blockers) block the effects of norepinephrine and epinephrine at beta-1 receptors in the heart. Stimulation of these receptors normally increases heart rate, contractility, and conduction velocity. By blocking these receptors, the medication produces a negative inotropic effect (decreased myocardial contractility), which reduces stroke volume. It also produces a negative chronotropic effect (decreased heart rate). Both effects contribute to a reduction in cardiac output and blood pressure.