Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 821: Which ion is primarily responsible for initiating the action potential in contractile cardiac muscle cells (atrial and ventricular myocytes)?

A. Calcium (Ca2+)
B. Chloride (Cl-)
C. Potassium (K+)
D. Sodium (Na+) (Correct Answer)

Explanation: ***Sodium (Na+)*** - The rapid influx of **Na+ through fast voltage-gated sodium channels** is responsible for the rapid depolarization phase (Phase 0) of the action potential in contractile cardiac myocytes. - This initial influx quickly brings the membrane potential to a positive value, initiating the action potential. *Potassium (K+)* - While **K+ channels** are crucial for repolarization (Phase 3) and maintaining the resting membrane potential, they do not initiate the action potential. - An efflux of K+ ions causes the membrane potential to become more negative, leading to repolarization. *Calcium (Ca2+)* - **Ca2+ influx** through L-type calcium channels is responsible for the plateau phase (Phase 2) of the action potential, which prolongs the refractory period. - While important for excitation-contraction coupling, Ca2+ does not initiate the rapid depolarization phase. *Chloride (Cl-)* - **Chloride ions** play a more minor role in cardiac action potentials, primarily contributing to some repolarization currents but not to the initial depolarization. - Their primary role is in maintaining cellular osmolarity and charge balance.

Question 822: Immersion syndrome occurs due to ?

A. Vagal response to cold immersion (Correct Answer)
B. Vagal stimulation during immersion
C. Sympathetic response to cold immersion
D. Sympathetic inhibition during immersion

Explanation: ***Vagal response to cold immersion*** - **Cold shock response** from sudden cold water exposure leads to an immediate gasp reflex, hyperventilation, and activation of the **parasympathetic nervous system** via the vagus nerve. - This vagal activation can cause **bradycardia**, arrhythmias, and even cardiac arrest in susceptible individuals. *Vagal stimulation during immersion* - While immersion can stimulate the vagus nerve, it is specifically the **cold temperature that triggers the significant vagal response** leading to immersion syndrome. - This option is too general and doesn't specify the crucial role of **cold** in initiating the syndrome. *Sympathetic response to cold immersion* - The initial response to cold immersion involves a rapid surge in **sympathetic activity**, leading to vasoconstriction and increased heart rate. - However, the dangerous cardiac events associated with immersion syndrome are predominantly mediated by the overwhelming **vagal (parasympathetic) response**, which can override the sympathetic drive. *Sympathetic inhibition during immersion* - Immersion, particularly into cold water, does not cause sympathetic inhibition; rather, it typically leads to an **initial sympathetic surge** as part of the body's stress response. - The critical cardiovascular risk is due to the subsequent strong **parasympathetic (vagal) activation**, not inhibition of the sympathetic system.

Question 823: What is the primary function of the M2 muscarinic receptor in the heart?

A. Causes hyperpolarization of the SA node (Correct Answer)
B. Enhances contractility of the ventricles
C. Increases release of acetylcholine from nerve endings
D. Increases conduction velocity in the AV node

Explanation: ***Causes hyperpolarization of the SA node*** - Activation of M2 receptors in the **sinoatrial (SA) node** leads to an increase in **potassium efflux**, causing the cell membrane to hyperpolarize. - This hyperpolarization makes it more difficult for the SA node cells to reach their threshold for depolarization, thereby **decreasing heart rate**. *Enhances contractility of the ventricles* - The M2 muscarinic receptor primarily mediates **parasympathetic effects**, which generally decrease cardiac function. - Enhancement of contractility is primarily mediated by **beta-1 adrenergic receptors** in the ventricles, part of the sympathetic nervous system. *Increases release of acetylcholine from nerve endings* - M2 receptors act as **autoreceptors** on presynaptic nerve terminals, and their activation typically **inhibits** further acetylcholine release. - This is a feedback mechanism to limit excessive parasympathetic stimulation. *Increases conduction velocity in the AV node* - Activation of M2 receptors in the **atrioventricular (AV) node** **decreases** conduction velocity, leading to a longer PR interval. - This effect contributes to the overall slowing of heart rate by delaying the impulse transmission to the ventricles.

Question 824: What is the average blood supply to the brain in ml/min?

A. 1500 ml/min
B. 2000 ml/min
C. 750 ml/min (Correct Answer)
D. 250 ml/min

Explanation: ***750 ml/min*** - The brain receives approximately **15% of the total cardiac output**, which translates to about 750 ml of blood per minute in a resting adult. - This flow rate is crucial for supplying the brain with adequate **oxygen and nutrients** to maintain its high metabolic demand. *1500 ml/min* - This value is significantly higher than the average blood flow to the brain and would represent an **abnormally increased cerebral blood flow**, potentially seen in specific pathological states. - The brain's metabolic needs, while substantial, do not typically require such a large volume of blood per minute under normal physiological conditions. *2000 ml/min* - This is an **extremely high value** for cerebral blood flow and is not consistent with normal physiological measurements for an adult brain. - Such a high flow rate could lead to **vascular issues** or is indicative of specific disease states rather than normal function. *250 ml/min* - This value represents a **significantly reduced cerebral blood flow**, which would be insufficient to meet the metabolic demands of the brain. - A persistent flow rate this low would likely result in **ischemia** and neuronal damage.

Question 825: What is the correct order of blood flow velocity in the human circulatory system?

A. Vena cava > Aorta > Vein > Artery > Venule > Arteriole
B. Aorta > Artery > Vena cava > Vein > Arteriole > Venule
C. Vena cava > Vein > Capillary > Arteriole > Aorta > Artery
D. Aorta > Artery > Arteriole > Vein > Venule > Capillary (Correct Answer)

Explanation: ***Aorta > Artery > Arteriole > Vein > Venule > Capillary*** - The **aorta** has the highest blood flow velocity (~30 cm/s) as it directly receives blood from the left ventricle - Blood velocity **progressively decreases** through arteries and arterioles as total cross-sectional area increases - **Capillaries have the LOWEST velocity** (~0.03 cm/s) due to their enormous total cross-sectional area (~2500 cm²), allowing optimal time for gas and nutrient exchange - Velocity **increases again** in venules and veins as vessels converge and total cross-sectional area decreases - The key principle: velocity is **inversely proportional** to total cross-sectional area of vessels *Vena cava > Aorta > Vein > Artery > Venule > Arteriole* - This sequence incorrectly places vena cava first when the **aorta has higher velocity** than vena cava - The mixed ordering doesn't follow the anatomical flow pathway *Aorta > Artery > Vena cava > Vein > Arteriole > Venule* - The vena cava is misplaced in this sequence - **Arterioles should have lower velocity than arteries** but higher than capillaries, not placed after veins *Vena cava > Vein > Capillary > Arteriole > Aorta > Artery* - This sequence is completely reversed - **Aorta and arteries have the highest velocities**, not the lowest - Capillaries have the **lowest velocity**, not in the middle of the sequence

Question 826: What is the resting membrane potential in ventricular cardiac muscle?

A. -90 mV (Correct Answer)
B. -70 mV
C. +70 mV
D. +90 mV

Explanation: ***-90 mV*** - The resting membrane potential in **ventricular muscle fibers** is approximately **-90 mV**, due to the high permeability to **potassium ions** at rest. - This **polarized state** is maintained by the **Na+/K+ ATPase pump**, which establishes ion gradients. *-70 mV* - A resting membrane potential of **-70 mV** is characteristic of **neurons** and skeletal muscle cells, not typical cardiac muscle cells. - This value is mainly maintained by the differential distribution of **sodium** and **potassium ions**. *+70 mV* - A potential of **+70 mV** represents a **depolarized state** far from the resting potential, indicative of an action potential peak. - This value would signify an influx of **positive ions**, primarily sodium, into the cell during activation. *+90 mV* - A potential of **+90 mV** is also a **depolarized state** and is not a resting membrane potential for any excitable cell type. - This value would represent a significant influx of positive charge, causing cell excitation.

Question 827: The 'v' wave in JVP is due to:

A. Isovolumetric relaxation
B. Closure of tricuspid valve (Correct Answer)
C. Right atrial relaxation
D. Right atrial contraction

Explanation: ***Closure of tricuspid valve*** - The **'v' wave** in the JVP occurs during **ventricular systole** while the **tricuspid valve is closed**. - It represents the **passive filling of the right atrium** with venous return against the closed tricuspid valve, leading to a gradual rise in atrial pressure. - The peak of the 'v' wave occurs just before the tricuspid valve opens at the end of ventricular systole. - Note: The actual **closure** of the tricuspid valve produces the **'c' wave**, while the 'v' wave reflects the consequences of the valve remaining closed during atrial filling. *Right atrial contraction* - **Right atrial contraction** causes the **'a' wave** in the JVP, which is the first positive deflection in the cardiac cycle. - This wave reflects the increase in right atrial pressure as the atrium contracts to propel blood into the right ventricle during late diastole. *Isovolumetric relaxation* - **Isovolumetric relaxation** of the ventricle occurs after semilunar valve closure and before the tricuspid valve opens. - This phase is associated with the **'y' descent**, as ventricular pressure falls below atrial pressure allowing the tricuspid valve to open and blood to flow rapidly into the ventricle. *Right atrial relaxation* - **Right atrial relaxation** follows atrial contraction and contributes to the **'x' descent** in the JVP. - This decline reflects the pressure drop in the right atrium as it relaxes after contraction, coinciding with ventricular systole pulling the tricuspid annulus downward.

Question 828: Dicrotic notch is caused by:

A. Closure of aortic valve (Correct Answer)
B. Opening of aortic valve
C. Opening of mitral valve
D. Closure of mitral valve

Explanation: ***Closure of aortic valve*** - The **dicrotic notch**, also known as the incisura, represents a brief increase in aortic pressure as blood rebounds against the **closed aortic valve**. - This event marks the end of systole and the beginning of diastole in the arterial pressure waveform. *Opening of mitral valve* - The opening of the mitral valve occurs during early diastole and is associated with the **rapid filling of the left ventricle**, not a notch on the arterial pressure waveform. - This event is more relevant to changes in left ventricular and left atrial pressures. *Opening of aortic valve* - The opening of the aortic valve marks the beginning of **ventricular ejection** (systole) and the rapid upstroke of the arterial pressure wave. - It does not cause a notch in the descending limb of the arterial pressure waveform. *Closure of mitral valve* - The closure of the mitral valve occurs at the beginning of **ventricular systole** and is associated with the first heart sound (S1). - This event is primarily reflected in left ventricular pressure changes and does not directly cause the dicrotic notch on the arterial pressure wave.

Question 829: What is the normal capillary wedge pressure?

A. 0-2 mm Hg
B. 5-10 mm Hg (Correct Answer)
C. 15-20 mm Hg
D. 20-30 mm Hg

Explanation: ***5-10 mm Hg*** - The **pulmonary capillary wedge pressure (PCWP)** or **pulmonary artery occlusion pressure (PAOP)** reflects the pressure in the left atrium and, indirectly, the left ventricular end-diastolic pressure (LVEDP). - A normal PCWP range is typically **4-12 mm Hg**, with 5-10 mm Hg being a common, healthy average. *0-2 mm Hg* - This value is **too low** to represent a normal PCWP. - A PCWP this low could indicate **hypovolemia** or severe vasodilation. *15-20 mm Hg* - This value is **elevated** and suggests **left ventricular dysfunction**, **heart failure**, or **volume overload**. - It would indicate increased pressure in the pulmonary circulation, potentially leading to **pulmonary congestion**. *20-30 mm Hg* - These values are **markedly elevated** and are highly indicative of significant **left ventricular failure** and **pulmonary edema**. - Such high pressures reflect severe compromise of the heart's pumping ability.

Question 830: S2 is associated with ?

A. Rapid ventricular filling
B. Atrial contraction
C. Closure of semilunar valves (Correct Answer)
D. Closure of AV valves

Explanation: ***Closure of semilunar valves*** - The **second heart sound (S2)** is produced by the simultaneous **closure of the aortic and pulmonic valves** at the end of ventricular systole. - This event marks the beginning of **ventricular diastole** and prevents blood from flowing back into the ventricles from the aorta and pulmonary artery. *Rapid ventricular filling* - This phase occurs during **diastole**, after the semilunar valves have closed and the AV valves have opened. - It is associated with the **third heart sound (S3)** if present, which is a low-frequency sound of rapid ventricular distention. *Atrial contraction* - **Atrial contraction** occurs late in ventricular diastole and precedes the first heart sound (S1). - It is sometimes associated with the **fourth heart sound (S4)** in cases of decreased ventricular compliance. *Closure of AV valves* - The **closure of the atrioventricular (AV) valves** (mitral and tricuspid) produces the **first heart sound (S1)**. - This event marks the beginning of **ventricular systole** and prevents blood from flowing back into the atria from the ventricles.

Practice by Chapter

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