Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 841: What is the PRIMARY mechanism by which the Na+-Ca2+ exchanger functions in cardiac muscle cells?

A. Na+-Ca2+ exchanger requires ATP directly
B. Na+-Ca2+ exchanger acts to remove Ca2+ from heart muscle cells (Correct Answer)
C. The Na+-Ca2+ exchanger operates in reverse mode during normal cardiac contraction
D. The Na+-Ca2+ exchanger primarily moves Ca2+ into cardiac muscle cells during systole

Explanation: ***Na+-Ca2+ exchanger acts to remove Ca2+ from heart muscle cells.*** - The primary function of the **Na+-Ca2+ exchanger (NCX)** in cardiac muscle is to **extrude calcium from the cell** into the extracellular space. - It uses the electrochemical gradient of **sodium (Na+)** which flows into the cell, to power the removal of **calcium (Ca2+)** from the cell, contributing to muscle relaxation during diastole. *The Na+-Ca2+ exchanger operates in reverse mode during normal cardiac contraction* - While it can theoretically operate in reverse, its **primary physiological role** during normal cardiac contraction is forward mode (Ca2+ extrusion). - Reverse mode operation (Ca2+ influx) is typically seen under specific conditions, such as **pathological states** or severely altered intracellular Na+ concentrations. *Na+-Ca2+ exchanger requires ATP directly* - The **Na+-Ca2+ exchanger** is a **secondary active transporter** and does not directly use ATP. - Its energy comes from the **electrochemical gradient of Na+**, which is maintained by the **Na+/K+-ATPase** (primary active transport, which *does* use ATP). *The Na+-Ca2+ exchanger primarily moves Ca2+ into cardiac muscle cells during systole.* - Moving **Ca2+ into the cell** during systole would primarily be the role of **L-type calcium channels** on the sarcolemma. - The NCX's main role is to **reduce intracellular Ca2+** after contraction, facilitating relaxation during diastole.

Question 842: What is the mechanism by which M2 receptors mediate the inhibition of the heart by the vagus nerve?

A. Inhibition of adenylate cyclase (Correct Answer)
B. Activation of phospholipase C
C. Inhibition of cAMP breakdown
D. Opening of voltage-gated calcium channels

Explanation: ***Inhibition of adenylate cyclase*** - M2 receptors are **Gαi-protein coupled receptors**, and their activation leads to the inhibition of **adenylate cyclase**. - This inhibition reduces the intracellular concentration of **cAMP**, which in turn decreases the activity of **protein kinase A (PKA)**, leading to a decrease in heart rate and contractility. - Additionally, M2 receptors activate **inwardly rectifying potassium channels (IKACh)**, which hyperpolarizes the cell membrane and further slows heart rate. *Activation of phospholipase C* - This mechanism is characteristic of **M1 and M3 muscarinic receptors (Gq-coupled)**, which activate phospholipase C, leading to increased IP3 and DAG. - This pathway is primarily involved in smooth muscle contraction and glandular secretion, not direct cardiac inhibition by M2 receptors. *Inhibition of cAMP breakdown* - Inhibition of **cAMP breakdown** would lead to an increase in cAMP levels, which would stimulate cardiac function. - This effect is mediated by drugs like **phosphodiesterase inhibitors**, which block the enzyme responsible for cAMP degradation, and is opposite to the effect of M2 receptor activation. *Opening of voltage-gated calcium channels* - Opening of voltage-gated calcium channels would **increase** calcium influx, leading to increased contractility and heart rate. - This is the mechanism of action of **sympathetic stimulation via β1-adrenergic receptors**, not parasympathetic M2 receptor activation, which has the opposite effect.

Question 843: Cerebral blood flow is most directly increased by?

A. Increase in PO2
B. Increase in PCO2 (Correct Answer)
C. Decrease metabolic rate
D. Increase in metabolic rate

Explanation: ***Increase in PCO2*** - An increase in **arterial PCO2** (partial pressure of carbon dioxide) causes **cerebral vasodilation**, leading to a direct increase in cerebral blood flow. - This is a potent regulatory mechanism to ensure adequate **carbon dioxide removal** and **oxygen supply** to the brain. *Increase in PO2* - An increase in **arterial PO2** (partial pressure of oxygen) causes **mild cerebral vasoconstriction**, which would tend to decrease cerebral blood flow, not increase it. - Cerebral blood flow is generally **less sensitive** to changes in PO2 within the normal range compared to PCO2. *Decrease metabolic rate* - A decrease in the brain's **metabolic rate** would typically lead to a **decrease in local demand** for oxygen and nutrients, resulting in **decreased cerebral blood flow**. - Cerebral blood flow is intrinsically linked to the metabolic needs of brain tissue. *Increase in metabolic rate* - An increase in the brain's **metabolic rate** would lead to an **increase in demand** for oxygen and glucose, which in turn causes **vasodilation** and an increase in cerebral blood flow. - However, this is an indirect effect, whereas an increase in PCO2 directly causes vasodilation.

Question 844: Inhibition of heart by vagus is mediated by which receptors?

A. M1
B. M2 (Correct Answer)
C. NN
D. NM

Explanation: ***M2*** - The **vagus nerve** primarily mediates its inhibitory effects on the heart through **muscarinic M2 receptors**. - Activation of M2 receptors by **acetylcholine** (released from the vagus nerve) decreases heart rate and contractility. *M1* - **M1 receptors** are primarily found in neuronal tissue and glands, playing a role in **gastric acid secretion** and cognitive functions. - They are not the primary muscarinic subclass responsible for vagal inhibition of the heart. *NN* - **NN receptors** are **nicotinic receptors** found on postganglionic neurons in autonomic ganglia. - They are involved in **ganglionic transmission** and are not directly responsible for efferent vagal effects on the heart. *NM* - **NM receptors** are **nicotinic receptors** found at the **neuromuscular junction** of skeletal muscles. - Their activation leads to **skeletal muscle contraction**, and they have no role in regulating heart function.

Question 845: Inotropic effect of thyroid hormone is by ?

A. Membrane receptors
B. cAMP
C. cGMP
D. Enhancement of Catecholamines (Correct Answer)

Explanation: ***Enhancement of Catecholamines*** - Thyroid hormones **potentiate the effects of catecholamines** (like adrenaline and noradrenaline) on the heart, leading to increased heart rate and contractility, which is an **inotropic effect**. - This occurs by increasing the number and sensitivity of **beta-adrenergic receptors** on cardiac muscle cells. *Membrane receptors* - While thyroid hormones do have some rapid, non-genomic effects that may involve **membrane receptors**, their primary and well-established inotropic effect is mediated indirectly through catecholamine sensitivity. - The classic action of thyroid hormones is via intracellular receptors that modulate gene expression, not direct membrane receptor signaling for inotropic effects. *cAMP* - **cAMP** is a common second messenger for many hormones, particularly those acting via G protein-coupled receptors. - While catecholamines themselves act through cAMP to exert their cardiac effects, thyroid hormones *enhance the action* of catecholamines rather than directly using cAMP as their primary inotropic mechanism. *cGMP* - **cGMP** is a second messenger often associated with nitric oxide signaling and vasodilation, contributing to cGMP-dependent protein kinases. - It is not the primary mediator for the *positive inotropic effect* of thyroid hormones on the heart.

Question 846: The ST Segment of an ECG corresponds to which phase of the action potential?

A. Rapid repolarization
B. Final repolarization
C. Plateau phase (Correct Answer)
D. Rapid depolarization

Explanation: ***Plateau phase*** - The **ST segment** of the ECG represents the period when the ventricles are completely depolarized and corresponds to the **plateau phase (phase 2)** of the ventricular myocardial action potential. - During this phase, there is a balance between **calcium influx** and **potassium efflux**, maintaining the depolarized state and contributing to the sustained contraction of the ventricles. *Rapid depolarization* - This phase, represented by the **QRS complex** on the ECG, signifies the rapid influx of sodium ions into the ventricular cells. - It corresponds to **phase 0** of the action potential, where there is a sharp upstroke. *Rapid repolarization* - This corresponds to **phase 3** of the ventricular action potential, where potassium ions rapidly exit the cell, leading to repolarization. - On the ECG, this phase is represented by the **T wave**. *Final repolarization* - This is **not a standard electrophysiological term** in cardiac action potential nomenclature. - The complete repolarization process is represented by the **T wave** (phase 3), which returns the ventricle to its resting potential (phase 4). - The term may cause confusion as it doesn't correspond to a specific phase or ECG component.

Question 847: Aortic valve closure occurs in which part of cardiac cycle?

A. Beginning of isovolumetric contraction
B. During rapid ventricular filling
C. Beginning of ventricular ejection
D. Beginning of isovolumetric relaxation (Correct Answer)

Explanation: ***Beginning of isovolumetric relaxation*** - Aortic valve closure marks the end of **ventricular systole** and the start of **isovolumetric relaxation**, as blood ceases to be ejected and the ventricle begins to relax while remaining closed. - This event corresponds to the **second heart sound (S2)** and signifies the beginning of a period where ventricular volume remains constant, but pressure drops. *Beginning of isovolumetric contraction* - This phase begins with the closure of the **mitral and tricuspid valves** (first heart sound, S1), as ventricular pressure rises but volume remains constant before ejection. - The aortic valve is still closed at this point, as ventricular pressure is not yet high enough to open it. *Beginning of ventricular ejection* - This phase begins when the **aortic valve opens** as ventricular pressure exceeds aortic pressure, allowing blood to be ejected from the left ventricle. - Aortic valve closure occurs *after* ejection, not at its beginning. *During rapid ventricular filling* - Rapid ventricular filling occurs when the **mitral valve opens** (following isovolumetric relaxation), allowing blood to flow from the atria into the ventricles. - During this phase, the aortic valve is closed, but its closure happened earlier, at the beginning of isovolumetric relaxation.

Question 848: Which of the following structures contains baroreceptors that detect changes in blood pressure?

A. Carotid body
B. Carotid sinus (Correct Answer)
C. Aortic body
D. None of the options

Explanation: ***Carotid sinus*** - The **carotid sinus** is a dilation at the bifurcation of the common carotid artery, containing **baroreceptors** sensitive to changes in blood pressure [1]. - These baroreceptors are **mechanoreceptors** that respond to the stretching of the vessel wall due to increased arterial pressure, sending signals to the brainstem to regulate blood pressure. *Carotid body* - The **carotid body** is a chemoreceptor that primarily detects changes in **blood oxygen, carbon dioxide, and pH** levels, not blood pressure [2]. - It plays a crucial role in regulating **respiration** in response to hypoxemia. *Aortic body* - The **aortic body** is a **chemoreceptor** located near the aortic arch that primarily monitors **blood oxygen, carbon dioxide, and pH levels**. - Note: While the aortic body itself is a chemoreceptor, the **aortic arch** (a different structure) does contain baroreceptors [1]. However, this option specifically refers to the aortic body, which is not a baroreceptor. - The aortic body contributes to the regulation of **respiration** in response to hypoxemia, not directly blood pressure. *None of the options* - This option is incorrect because the **carotid sinus** is a well-known site for baroreceptors involved in blood pressure regulation.

Question 849: Which of the following factors is most commonly targeted therapeutically for blood pressure control?

A. Heart rate
B. Peripheral resistance (Correct Answer)
C. Cardiac output
D. Stroke volume

Explanation: ***Peripheral resistance*** - **Peripheral resistance** is primarily determined by the **arteriolar tone**, which can be effectively modulated by various antihypertensive medications. - Medications like **ACE inhibitors**, **ARBs**, **calcium channel blockers**, and **diuretics** all influence peripheral resistance to lower blood pressure. *Heart rate* - While heart rate contributes to **cardiac output** and thus blood pressure, it is not the most common primary target for hypertension management. - **Beta-blockers** reduce heart rate, but they are often used for specific indications beyond essential hypertension, such as angina or post-MI. *Cardiac output* - **Cardiac output** is a product of **heart rate** and **stroke volume**, and while it directly impacts blood pressure, directly targeting cardiac output as a whole is less common than modulating its individual components or peripheral resistance. - Many antihypertensive drugs reduce cardiac output as a secondary effect of reducing blood volume or heart rate, but directly reducing cardiac output is not the primary mechanism for the most common medications. *Stroke volume* - **Stroke volume** is influenced by **preload**, **afterload**, and **contractility**, and while it impacts cardiac output, it is generally less accessible for direct pharmacological manipulation in hypertension management compared to peripheral resistance. - **Diuretics** can indirectly reduce stroke volume by decreasing preload, but this is often considered a mechanism related to volume status rather than a direct myocardial effect.

Question 850: What is the typical oxygen saturation level of venous blood?

A. 30%
B. 50%
C. 70% (Correct Answer)
D. 90%

Explanation: ***70%*** - Venous blood has a lower oxygen saturation compared to arterial blood because tissues have extracted a significant amount of oxygen for **cellular respiration**. - A typical mixed venous oxygen saturation (SvO2) is around **70-75%**, indicating the amount of oxygen remaining after tissues have taken what they need. *30%* - This level of oxygen saturation is **too low** for typical venous blood and would indicate severe tissue hypoperfusion or extreme oxygen extraction. - Such low levels are usually not compatible with normal physiological function for prolonged periods. *50%* - While lower than normal, a 50% venous oxygen saturation is still indicative of **increased oxygen extraction** by tissues, often seen in conditions of increased metabolic demand or decreased oxygen delivery. - It's not the typical resting value for healthy individuals. *90%* - An oxygen saturation of 90% is more characteristic of **arterial blood** (normal arterial saturation is 95-100%). - Venous blood, having already delivered oxygen to tissues, would normally have a lower saturation.

Practice by Chapter

Cardiac Electrophysiology

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Cardiac Cycle

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Cardiac Output and Its Regulation

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Hemodynamics and Blood Flow

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Arterial System Physiology

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Microcirculation and Lymphatics

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Venous Return and Central Venous Pressure

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Cardiovascular Reflexes

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Regional Circulations

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Cardiovascular Responses to Exercise and Stress

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