Cardiovascular System — MCQs

The following data were obtained from a man weighing 70 kg: Aorta oxygen (O2) content is 20.0 vol%, femoral vein O2 content is 16 vol%, coronary sinus O2 content is 10 vol%, and pulmonary artery O2 content is 15 vol%. What is the cardiac output of this man, given a total body O2 consumption of 400 ml/min?

Q898

All are effects of the parasympathetic system on the heart except?

Q899

Vagal stimulation of the heart causes

Q900

What is the normal range for portal vein pressure?

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 891: Which of the following ECG changes is not associated with hypokalemia?

A. Prolonged QRS interval
B. Tall T wave (Correct Answer)
C. Prominent U waves
D. Depressed ST segment

Explanation: ***Tall T wave*** - **Tall, peaked T waves** are characteristic of **hyperkalemia**, reflecting rapid repolarization. - In **hypokalemia**, T waves are typically **flattened** or **inverted**, not tall. *Prolonged QRS interval* - A **prolonged QRS interval** can occur in severe hypokalemia due to slowed ventricular conduction. - This change indicates more severe electrolyte imbalance impacting the **depolarization phase** of ventricular myocytes. *Depressed ST segment* - **ST segment depression** is a common finding in hypokalemia, often associated with a reduced resting membrane potential. - This can indicate **myocardial ischemia** or simply the electrical instability caused by low potassium levels. *Prominent U waves* - **Prominent U waves** are a hallmark ECG finding in hypokalemia, often appearing as a deflection immediately following the T wave. - They are thought to represent delayed repolarization of **Purkinje fibers** or specific ventricular muscle cells.

Question 892: Volume receptors are primarily located in which of the following?

A. Carotid sinus and aortic arch
B. Right and left atria (Correct Answer)
C. Pulmonary arteries only
D. Renal juxtaglomerular apparatus

Explanation: **Correct: *Right and left atria*** - The **atria** contain **low-pressure baroreceptors** (volume receptors) that primarily sense changes in circulating blood volume. - These receptors respond to stretching of the atrial walls, which occurs with increased blood volume, signaling the need for fluid excretion. *Incorrect: Carotid sinus and aortic arch* - These locations house **high-pressure baroreceptors** that primarily regulate **arterial blood pressure**, not circulating volume. - They respond to changes in the stretch of the arterial walls caused by blood pressure fluctuations. *Incorrect: Renal juxtaglomerular apparatus* - This apparatus primarily senses changes in **renal perfusion pressure** and **sodium delivery** to the distal tubule. - It plays a crucial role in regulating blood pressure and fluid balance through the **renin-angiotensin-aldosterone system (RAAS)**, but its primary role is not as a volume receptor. *Incorrect: Pulmonary arteries only* - While pulmonary baroreceptors exist, they primarily monitor **pulmonary arterial pressure** and do not serve as the main volume receptors for the systemic circulation. - Volume receptors are distributed more broadly within the low-pressure venous system.

Question 893: During the cardiac cycle, the opening of the aortic valve takes place at the:

A. Beginning of systole
B. End of isovolumetric contraction (Correct Answer)
C. End of diastole
D. End of diastasis

Explanation: ***End of isovolumetric contraction*** - The **aortic valve opens** when the pressure in the left ventricle exceeds the pressure in the aorta, marking the end of **isovolumetric contraction** and the beginning of ventricular ejection. - During **isovolumetric contraction**, the mitral and aortic valves are closed, and ventricular pressure rises rapidly without a change in volume. *Beginning of systole* - The **beginning of systole** is marked by the closure of the mitral valve, initiating **isovolumetric contraction**, not the opening of the aortic valve. - Aortic valve opening occurs later in systole, after ventricular pressure has built sufficiently. *End of diastole* - The **end of diastole** is when the ventricles are maximally filled with blood, just before contraction begins. - At this point, the mitral valve is typically still open, and the aortic valve is closed. *End of diastasis* - **Diastasis** is the period of slow passive filling of the ventricles during mid-diastole. - The **end of diastasis** is followed by atrial contraction, which occurs before systole and before the aortic valve opens.

Question 894: Which of the following normally has a slowly depolarizing "prepotential"?

A. Sinoatrial node (Correct Answer)
B. Atrial muscle cells
C. Bundle of His
D. Purkinje fibers

Explanation: ***Sinoatrial node*** - The **sinoatrial (SA) node** is the **normal primary pacemaker** of the heart, with cells that exhibit characteristic **slow depolarization** during phase 4 of their action potential, known as the **prepotential** or pacemaker potential. - This prepotential is primarily due to the **funny current (If)**, T-type Ca²⁺ channels, and decreasing K⁺ efflux, which gradually brings the membrane potential to threshold. - The SA node has the **fastest intrinsic rate** (60-100 bpm), which is why it normally dominates cardiac rhythm. *Atrial muscle cells* - Atrial muscle cells are **contractile cells** and do not spontaneously depolarize; they require an external stimulus from the SA node or other pacemaker cells. - Their action potential phase 4 maintains a **stable resting membrane potential** of about -90 mV, without prepotential. *Bundle of His* - The Bundle of His does possess **latent pacemaker activity** with a slowly depolarizing prepotential, but with a much slower intrinsic rate (40-60 bpm) than the SA node. - Under **normal conditions**, the SA node fires first and suppresses the Bundle of His through **overdrive suppression**, so it does not normally initiate the heartbeat. - It only becomes the dominant pacemaker if both the SA and AV nodes fail. *Purkinje fibers* - **Purkinje fibers** also have intrinsic pacemaker activity with a slowly depolarizing prepotential, but with the slowest rate (20-40 bpm) among cardiac pacemaker tissues. - Like the Bundle of His, they are **subsidiary pacemakers** that are normally suppressed by faster upstream pacemakers through overdrive suppression. - They only initiate rhythm in pathological conditions when higher pacemakers fail.

Question 895: The largest component of the total peripheral resistance is due to:

A. Venules
B. Arterioles (Correct Answer)
C. Capillaries
D. Precapillary sphincters

Explanation: ***Arterioles*** - **Arterioles** are the primary site of **resistance** in the cardiovascular system due to their relatively small diameter and the significant ability of their **smooth muscle** walls to constrict or dilate. - This resistance plays a crucial role in regulating **blood flow** to various organs and contributes to **mean arterial pressure**. *Venules* - **Venules** are primarily involved in collecting blood from capillaries and have relatively low resistance compared to arteries and arterioles. - While they contribute to capacitance, their impact on **total peripheral resistance** is minimal. *Capillaries* - Although **capillaries** have very small diameters, their sheer number in parallel reduces the overall resistance of the capillary bed. - The primary function of capillaries is **exchange** of nutrients and waste, not primarily resistance. *Precapillary sphincters* - **Precapillary sphincters** control blood flow *into* capillaries from arterioles, acting as gates. - While they regulate flow to specific capillary beds, they are not the largest *component* of total systemic resistance; the **arterioles themselves** are.

Question 896: Which of the following is the main factor for the ductus arteriosus closure postnatally?

A. Increase in partial pressure of oxygen (PaO2) (Correct Answer)
B. Elevated levels of circulating prostaglandins
C. Reduction in pulmonary vascular resistance
D. Postnatal increase in systemic vascular resistance

Explanation: ***Increase in partial pressure of oxygen (PaO2)*** - The **increase in PaO2** after birth causes a profound relaxation of the **pulmonary arterioles** and constriction of the **ductus arteriosus**. - This is the most crucial physiological change, leading to the **functional closure** of the ductus within hours of birth. *Postnatal increase in systemic vascular resistance* - While systemic vascular resistance (SVR) does increase postnatally as the **placental circulation** ceases, it is not the primary direct cause of ductus arteriosus closure. - An increased SVR contributes to the **pressure gradient** changes in the heart, but the oxygen-mediated constriction is more direct and powerful for the ductus itself. *Elevated levels of circulating prostaglandins* - **Prostaglandins**, particularly PGE2, are responsible for **maintaining the patency** of the ductus arteriosus *in utero*. - After birth, the **decrease in prostaglandin levels** (due to lung metabolism and removal of the placenta) is essential for closure, but elevated levels would actually keep it open. *Reduction in pulmonary vascular resistance* - A reduction in **pulmonary vascular resistance (PVR)** is indeed a significant postnatal change, allowing for increased pulmonary blood flow. - While this change alters blood flow dynamics, the direct cause of ductus arteriosus constriction is the **increased PaO2**, not solely the fall in PVR.

Question 897: The following data were obtained from a man weighing 70 kg: Aorta oxygen (O2) content is 20.0 vol%, femoral vein O2 content is 16 vol%, coronary sinus O2 content is 10 vol%, and pulmonary artery O2 content is 15 vol%. What is the cardiac output of this man, given a total body O2 consumption of 400 ml/min?

A. 10 L/min
B. 8 L/min (Correct Answer)
C. 6 L/min
D. 5 L/min

Explanation: ***8 L/min*** - The cardiac output is calculated using the **Fick principle**: CO = Total body O2 consumption / (Arterial O2 content - Mixed venous O2 content). - In this case, **Arterial O2 content is 20 vol%** and **Mixed venous O2 content (pulmonary artery) is 15 vol%**. So, CO = 400 ml/min / (20 vol% - 15 vol%) = 400 ml/min / 5 ml O2/100 ml blood = 400 / 0.05 = 8000 ml/min = **8 L/min**. *10 L/min* - This result would be obtained if the arteriovenous oxygen difference was smaller, specifically 4 vol% (400 / 0.04 = 10000 ml/min). - This calculation does not correctly use the given **mixed venous O2 content** from the pulmonary artery. *6 L/min* - This result would be obtained if the arteriovenous oxygen difference was larger, specifically 6.67 vol% (400 / 0.0667 ≈ 6000 ml/min). - This calculation misrepresents the **actual O2 extraction** from the arterial blood. *5 L/min* - This result would be obtained if the arteriovenous oxygen difference was 8 vol% (400 / 0.08 = 5000 ml/min). - This choice indicates an incorrect application of the **Fick principle** or misidentification of the relevant oxygen content values.

Question 898: All are effects of the parasympathetic system on the heart except?

A. Negative chronotropic
B. Negative dromotropic
C. All are seen
D. Negative inotropic (Correct Answer)

Explanation: ***Negative inotropic*** - While the parasympathetic system (via the **vagus nerve**) primarily affects the **sinoatrial (SA) and atrioventricular (AV) nodes** to decrease heart rate and conduction velocity, it has a **minimal direct effect on ventricular contractility** (inotropy) in humans. - The ventricles are less densely innervated by parasympathetic fibers compared to the atria, so acetylcholine's direct negative inotropic effect is **clinically insignificant** in a healthy heart. - This is the **EXCEPTION** - not a significant parasympathetic effect on the heart. *Negative chronotropic* - The parasympathetic system, primarily through **acetylcholine** acting on **muscarinic receptors** in the SA node, decreases the heart rate (chronotropy). - This slows the rate of spontaneous depolarization of pacemaker cells. - This **IS** a major parasympathetic effect. *Negative dromotropic* - Parasympathetic stimulation also slows the conduction velocity through the **AV node** (dromotropy). - This increases the PR interval on an ECG and can lead to various degrees of AV block in extreme cases. - This **IS** a major parasympathetic effect. *All are seen* - This option is incorrect because the **negative inotropic effect** is NOT a significant parasympathetic effect on the heart. - While negative chronotropic and negative dromotropic effects are prominent features of parasympathetic activity, the direct influence on ventricular contractility is minimal.

Question 899: Vagal stimulation of the heart causes

A. Increased R-R interval in ECG (Correct Answer)
B. Increased cardiac output
C. Increased heart rate
D. Increased force of heart contraction

Explanation: ***Increased R-R interval in ECG*** - Vagal stimulation releases **acetylcholine**, which acts on **M2 muscarinic receptors** in the heart, particularly in the SA and AV nodes. - This leads to a decrease in heart rate, which manifests as an **increase in the R-R interval** on an ECG. *Increased heart rate* - Vagal stimulation is part of the **parasympathetic nervous system**, which generally **decreases heart rate**. - **Sympathetic stimulation**, not vagal stimulation, is responsible for increasing heart rate. *Increased force of heart contraction* - While vagal innervation affects atrial contractility, its primary effect on ventricular contractility is minimal compared to **sympathetic stimulation**. - Increased force of contraction is mainly mediated by **catecholamines** from the sympathetic nervous system. *Increased cardiac output* - Cardiac output is the product of heart rate and stroke volume; a decrease in heart rate due to vagal stimulation would generally **decrease cardiac output**, assuming stroke volume remains constant or does not significantly increase to compensate. - Vagal stimulation primarily aims to **conserve energy** and *slow down heart activity*, not increase overall output.

Question 900: What is the normal range for portal vein pressure?

A. 1-3 mm Hg
B. 3-5 mm Hg
C. 5-10 mm Hg (Correct Answer)
D. 10-15 mm Hg

Explanation: ***5-10 mm Hg*** - The normal **portal vein pressure** typically ranges from 5 to 10 mmHg. - Pressures above this range are indicative of **portal hypertension**, a common complication of **cirrhosis** and other liver diseases, which can lead to varices and ascites. *1-3 mm Hg* - This range is significantly lower than the **normal portal vein pressure**. - Such low pressures are not typically observed in the **portal venous system** under normal physiological conditions. *3-5 mm Hg* - This range is still considered to be on the lower end and borders on **hypotension** within the portal system. - While it's relatively close to the lower limit of normal, it doesn't represent the typical **physiological range** of portal vein pressure. *10-15 mm Hg* - Pressures in this range are usually considered **elevated** and fall within the spectrum of **portal hypertension**. - While slight elevations might occur transiently, a sustained pressure in this range indicates an underlying issue, such as **cirrhosis** or **post-hepatic obstruction**.

Practice by Chapter

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Cardiac Cycle

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Cardiac Output and Its Regulation

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