Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 841: Which of the following statements is true about coronary circulation?

A. Uniform flow during full cardiac cycle
B. Flow rate is approximately 500 ml/min
C. Major flow during diastole (Correct Answer)
D. All of the above

Explanation: ***Major flow during diastole*** - The **coronary arteries** are compressed during **systole** by the contracting myocardium, significantly reducing blood flow to the heart muscle. - During **diastole**, the myocardium relaxes, allowing the coronary arteries to open fully and deliver the majority (70-80%) of oxygenated blood to the heart. - This is the most distinctive feature of coronary circulation. *Flow rate is approximately 500 ml/min* - The typical **coronary blood flow** at rest is approximately **225-250 ml/min** (about 5% of cardiac output at rest). - 500 ml/min is significantly higher than normal resting coronary flow and would represent a pathological or high-demand state. *Uniform flow during full cardiac cycle* - **Coronary blood flow** is highly variable (phasic) throughout the cardiac cycle, being significantly higher during **diastole** and much lower during **systole**. - This non-uniform flow is a unique characteristic of coronary circulation due to mechanical compression from myocardial contraction. *All of the above* - Not all statements are correct, as the flow rate value is incorrect and flow is non-uniform throughout the cardiac cycle. - The **major flow during diastole** is the most accurate and physiologically important statement regarding coronary circulation.

Question 842: How many phases are there in the action potential of cardiac muscles?

A. 2 phases
B. 3 phases
C. 4 phases
D. 5 phases (Correct Answer)

Explanation: ***5 phases*** - The cardiac myocyte action potential is classically described in **five phases** (phases 0, 1, 2, 3, and 4), which encompass depolarization, repolarization, and the resting state. - Each phase is characterized by specific ion channel activities leading to distinct electrical changes essential for proper cardiac function. *2 phases* - Action potentials in nerve cells typically follow a simpler two-phase model: **depolarization** and **repolarization**. - This model does not account for the additional plateau and resting phases characteristic of cardiac muscle cells. *3 phases* - Some simplified models might describe three phases (depolarization, repolarization, and a resting phase), but this still **omits specific nuances** of cardiac repolarization and the sustained plateau phase. - This simplification leaves out the early repolarization and the critical plateau phase (phase 2), which is vital for the prolonged contraction of the heart. *4 phases* - While some sources might refer to four phases, they typically combine certain repolarization steps or omit the distinct early repolarization phase. - This description would likely miss the **early, rapid repolarization phase (phase 1)**, understating the complex ion movements.

Question 843: What is the normal O2 extraction ratio of tissues?

A. 5 percent
B. 15 percent
C. 25 percent (Correct Answer)
D. 40 percent

Explanation: ***25 percent*** - The normal **O2 extraction ratio** (or **oxygen utilization coefficient**) is approximately 25%, meaning tissues extract about one-fourth of the oxygen delivered by arterial blood. - This ratio is crucial for understanding **tissue oxygenation** and can increase significantly during times of high metabolic demand, such as exercise. *5 percent* - An O2 extraction ratio of 5% is **too low** for normal physiological function, indicating that tissues are receiving much more oxygen than they are utilizing. - Such a low ratio would be seen only in situations of **excessive oxygen delivery** or **severely reduced metabolic demand**. *15 percent* - While 15% represents some oxygen extraction, it is **below the normal physiological range** for resting tissues. - An extraction ratio of 15% would mean the tissues are not extracting sufficient oxygen to meet their typical metabolic needs efficiently. *40 percent* - An O2 extraction ratio of 40% is **higher than the normal resting value** and suggests increased oxygen demand by the tissues. - This level of extraction is typically seen during **strenuous exercise** or in conditions of **reduced oxygen delivery** where tissues compensate by extracting more oxygen from available blood.

Question 844: What does the ST Segment of an ECG correspond to?

A. Ventricular depolarization
B. Plateau phase between ventricular depolarization and repolarization (Correct Answer)
C. Atrial depolarization
D. AV Conduction

Explanation: ***Plateau phase between ventricular depolarization and repolarization*** - The **ST segment** represents the electrically neutral period between ventricular depolarization and repolarization, corresponding to the **plateau phase (phase 2)** of the ventricular action potential. - During this phase, the entire ventricular myocardium is depolarized, and there is minimal electrical activity, typically causing the ST segment to be **isoelectric**. *Ventricular depolarization* - This electrical event is represented by the **QRS complex** on the ECG, not the ST segment. - The QRS complex signifies the rapid spread of electrical impulses through the ventricles, leading to their contraction. *Atrial depolarization* - **Atrial depolarization** is represented by the **P wave** on the ECG. - This wave indicates the electrical activation of the atria, which precedes atrial contraction. *AV Conduction* - **AV conduction** time is primarily represented by the **PR interval** on the ECG. - The PR interval measures the time from the beginning of atrial depolarization to the beginning of ventricular depolarization, encompassing the delay at the AV node.

Question 845: 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.

Question 846: 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 847: 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 848: Which of the following statements is true regarding the Bezold-Jarisch reflex?

A. Hypertension
B. Tachycardia
C. Hyperpnea
D. Hypotension (Correct Answer)

Explanation: ***Hypotension*** - The Bezold-Jarisch reflex is a **cardioinhibitory reflex** that is typically activated by strong ventricular contraction or noxious stimuli, leading to a triad of **bradycardia**, **peripheral vasodilation**, and subsequent **hypotension**. - This reflex is thought to be a protective mechanism to prevent excessive cardiac work or to trigger a "fainting" response to remove the body from danger. *Hypertension* - The Bezold-Jarisch reflex primarily causes a **decrease in blood pressure**, making hypertension an incorrect outcome. - Its activation directly opposes the mechanisms that would lead to increased blood pressure. *Tachycardia* - A key component of the Bezold-Jarisch reflex is **bradycardia** (slowing of the heart rate), not tachycardia. - This reflex is mediated by the vagus nerve, which primarily exerts inhibitory control over heart rate. *Hyperpnea* - The Bezold-Jarisch reflex primarily impacts **cardiovascular function** and does not directly cause hyperpnea (increased rate and depth of breathing). - While other reflexes can affect respiration, this particular reflex is not known for its respiratory effects.

Question 849: Aortic valve closure corresponds to the beginning of which phase of the cardiac cycle?

A. Systole
B. Parasystole
C. Isovolumetric contraction
D. Isovolumetric relaxation (Correct Answer)

Explanation: ***Isovolumetric relaxation*** - **Aortic valve closure** marks the end of **ventricular ejection** and the beginning of **isovolumetric relaxation** as both the aortic and mitral valves are closed, and ventricular pressure drops without a change in volume. - This phase is vital for the heart to relax and prepare for filling, corresponding to the **second heart sound (S2)**. *Systole* - **Systole** refers to the **contraction phase** of the heart, encompassing both isovolumetric contraction and ventricular ejection. - Aortic valve closure signifies the end of the **ejection phase** of systole, not its beginning. *Parasystole* - **Parasystole** is an **arrhythmia** where an ectopic pacemaker competes with the normal sinus rhythm, leading to independent atrial or ventricular contractions. - It is a **pathological condition** and not a normal phase of the cardiac cycle. *Isovolumetric contraction* - **Isovolumetric contraction** occurs after the **mitral valve closes** and before the aortic valve opens, causing pressure to build in the ventricle. - This phase precedes **ventricular ejection** and is initiated by mitral valve closure, not aortic valve closure.

Question 850: 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.

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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