Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 71: Coronary blood flow is maximum during which phase of the cardiac cycle?

A. Isovolumic relaxation phase (Correct Answer)
B. Isovolumic contraction phase
C. Ejection phase
D. Isovolumic contraction phase

Explanation: **Explanation:** The correct answer is **Isovolumic relaxation phase**. **1. Why Isovolumic Relaxation is Correct:** Coronary blood flow, particularly to the **Left Ventricle (LV)**, is unique because it occurs primarily during **diastole**. During systole, the contracting myocardium compresses the intramyocardial blood vessels (extravascular compression), significantly increasing resistance and reducing flow. * The **Isovolumic Relaxation Phase** marks the beginning of diastole. At this point, the aortic pressure is high (following the closure of the aortic valve), while the ventricular muscle begins to relax, removing the mechanical compression on the coronary arteries. This combination of high perfusion pressure and low resistance allows for the **maximum peak** of coronary blood flow. **2. Why the Incorrect Options are Wrong:** * **Isovolumic Contraction Phase:** This is the period of highest intramyocardial pressure. The sudden contraction of the ventricle squeezes the coronary vessels, causing a sharp **decrease** (and sometimes a brief reversal) in flow. * **Ejection Phase:** Although aortic pressure is high, the ventricular muscle remains contracted. While some flow occurs due to high driving pressure, it is significantly less than during diastole because of ongoing mechanical compression. **3. NEET-PG High-Yield Pearls:** * **Phasic Flow:** Left coronary flow is maximum in early diastole; Right coronary flow is more uniform because the right ventricular pressure is lower and does not fully occlude its vessels during systole. * **Subendocardium:** This layer is most vulnerable to ischemia because it experiences the greatest compressive force during systole. * **Heart Rate:** Tachycardia reduces the duration of diastole more than systole, which can compromise coronary perfusion. * **Extraction:** The heart has the highest oxygen extraction ratio (70-80%) in the body; therefore, increased oxygen demand must be met by increasing flow, not extraction.

Question 72: A 60-year-old patient with a 25-year history of hypertension underwent renal artery Doppler, which showed narrowing and turbulence in the right renal artery. If the diameter of the lumen is reduced by 50%, by how much will the blood flow be reduced?

A. 1/4th
B. 1/8th
C. 1/16th (Correct Answer)
D. 1/32nd

Explanation: ### Explanation The correct answer is **1/16th**. #### 1. Underlying Medical Concept: Poiseuille’s Law The relationship between blood vessel dimensions and blood flow is governed by **Poiseuille’s Law**. According to this principle, the flow rate ($Q$) is directly proportional to the fourth power of the radius ($r^4$) of the vessel: $$Q \propto r^4$$ In this scenario, the diameter (and thus the radius) is reduced by 50%, meaning the new radius is **1/2** of the original. To find the change in flow, we apply the fourth-power rule: $$(1/2)^4 = 1/2 \times 1/2 \times 1/2 \times 1/2 = \mathbf{1/16}$$ Therefore, reducing the lumen diameter by half results in the blood flow decreasing to **1/16th** of its original value. #### 2. Analysis of Incorrect Options * **A. 1/4th:** This would be the reduction if flow were proportional to the square of the radius ($r^2$), which applies to surface area, not flow. * **B. 1/8th:** This would be the reduction if flow were proportional to the cube of the radius ($r^3$). * **D. 1/32nd:** This would be the reduction if flow were proportional to the fifth power ($r^5$). #### 3. Clinical Pearls & High-Yield Facts for NEET-PG * **Resistance ($R$):** Resistance is inversely proportional to the fourth power of the radius ($R \propto 1/r^4$). If the radius is halved, resistance increases **16-fold**. * **Arterioles:** These are the primary "resistance vessels" of the body because small changes in their diameter (via sympathetic tone) lead to massive changes in total peripheral resistance (TPR) and blood pressure. * **Turbulence:** The question mentions turbulence, which occurs when the **Reynolds number** exceeds 2000. Narrowing (stenosis) increases velocity, which promotes turbulent flow, clinically heard as a **bruit** (e.g., renal artery bruit in hypertension).

Question 73: Peripheral resistance is maximum in which of the following?

A. Aorta
B. Artery
C. Arteriole (Correct Answer)
D. Vein

Explanation: ### Explanation The correct answer is **Arteriole**. **1. Why Arterioles have the maximum resistance:** According to **Poiseuille’s Law**, resistance ($R$) is inversely proportional to the fourth power of the radius ($r^4$). While capillaries have a smaller individual radius, **arterioles** are considered the primary "resistance vessels" of the systemic circulation for two reasons: * **Muscular Wall:** Arterioles possess a thick layer of smooth muscle in their tunica media, allowing them to constrict or dilate significantly under sympathetic and local metabolic control. * **Pressure Drop:** The greatest drop in mean arterial pressure (MAP) occurs as blood passes through the arterioles (from ~90 mmHg to ~35 mmHg). This steep pressure gradient is the physiological hallmark of high resistance. **2. Why other options are incorrect:** * **Aorta:** As the largest vessel, it has a massive radius. Since resistance is inversely proportional to radius, the aorta offers minimal resistance to flow. * **Artery:** Large and medium-sized arteries act as conduits. While they have some resistance, it is significantly lower than that of the terminal arterioles. * **Vein:** Veins are "capacitance vessels." They are highly distensible, have large lumens, and operate under low pressure, offering very low resistance. **3. NEET-PG High-Yield Pearls:** * **Total Peripheral Resistance (TPR):** Arterioles contribute approximately 50-70% of the total TPR. * **Capillaries vs. Arterioles:** Although an individual capillary is narrower than an arteriole, the **total cross-sectional area** of the capillary bed is much larger. This results in a lower resistance and the slowest velocity of blood flow (ideal for nutrient exchange). * **Sympathetic Control:** Arterioles are the main site of action for norepinephrine on $\alpha_1$ receptors to regulate systemic blood pressure.

Question 74: All of the following are actions of Cytotoxic T-cells except?

A. They destroy translated and other foreign cells.
B. They display the glycoprotein CD8.
C. They can phagocytose cells. (Correct Answer)
D. They act by using perforins and produce lymphotoxins.

Explanation: ### Explanation **Correct Answer: C. They can phagocytose cells.** **1. Why Option C is Correct (The Underlying Concept):** Cytotoxic T-cells (CD8+ T-cells) are the "assassins" of the adaptive immune system, but they are **not phagocytes**. Phagocytosis—the process of engulfing and digesting large particles or whole cells—is a characteristic function of the innate immune system, primarily performed by **neutrophils, macrophages, and dendritic cells**. Cytotoxic T-cells eliminate target cells by inducing **apoptosis** (programmed cell death) through chemical signaling and membrane disruption, rather than ingestion. **2. Analysis of Incorrect Options:** * **Option A:** Cytotoxic T-cells are specialized to recognize and destroy virally infected cells, transplanted foreign tissues (allografts), and tumor cells. They recognize these via MHC Class I molecules. * **Option B:** CD8 is a transmembrane glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). It specifically binds to the **MHC Class I** molecule, ensuring the T-cell attacks the correct target. * **Option D:** Upon activation, these cells release **perforins** (which create holes in the target cell membrane) and **granzymes/lymphotoxins** (which enter the cell to trigger the caspase cascade), leading to DNA fragmentation and cell death. **3. NEET-PG High-Yield Pearls:** * **MHC Restriction:** Remember the "Rule of 8": **CD8** cells bind **MHC I** (8x1=8), while **CD4** cells bind **MHC II** (4x2=8). * **Fas Ligand:** Besides perforins, CD8+ cells can induce apoptosis by binding their **Fas ligand** to the **Fas receptor** on a target cell. * **Cytokine Production:** While their primary role is killing, activated CD8+ cells also secrete **IFN-γ**, which inhibits viral replication and activates macrophages.

Question 75: Vasodilation is caused by all of the following EXCEPT:

A. Prostacyclin
B. Thromboxane A2 (Correct Answer)
C. PGE1
D. PGD2

Explanation: **Explanation:** The regulation of vascular tone is governed by various local mediators, primarily derivatives of arachidonic acid known as eicosanoids. **Why Thromboxane A2 (TXA2) is the correct answer:** Thromboxane A2 is a potent **vasoconstrictor** and a stimulator of platelet aggregation. It is synthesized by platelets via the cyclooxygenase (COX) pathway. Its primary physiological role is to facilitate hemostasis by narrowing blood vessels and promoting clot formation. Because it causes vasoconstriction rather than vasodilation, it is the correct "EXCEPT" choice. **Analysis of Incorrect Options:** * **Prostacyclin (PGI2):** Produced by vascular endothelial cells, it is a powerful **vasodilator** and the physiological antagonist to TXA2. It inhibits platelet aggregation. * **PGE1 (Alprostadil):** A known **vasodilator**. Clinically, it is used to maintain the patency of the ductus arteriosus in neonates with congenital heart defects. * **PGD2:** Primarily produced by mast cells, it acts as a **vasodilator** in most vascular beds (though it can cause bronchoconstriction). **High-Yield NEET-PG Pearls:** * **The PGI2 : TXA2 Balance:** Normal vascular health depends on the balance between Prostacyclin (vasodilator/anti-aggregant) and Thromboxane (vasoconstrictor/pro-aggregant). * **Aspirin:** At low doses, it irreversibly inhibits COX-1 in platelets, reducing TXA2 levels, which is why it is used as an anti-platelet agent. * **Other Vasodilators to remember:** Nitric Oxide (most potent endogenous vasodilator), Bradykinin, Histamine, and VIP (Vasoactive Intestinal Peptide). * **Other Vasoconstrictors to remember:** Endothelin-1 (most potent endogenous vasoconstrictor), Angiotensin II, and Norepinephrine.

Question 76: What is the cause of the dicrotic notch?

A. Passive filling of blood in ventricles
B. Rapid ejection phase
C. Peripheral resistance (Correct Answer)
D. Isovolumic contraction

Explanation: **Explanation:** The **dicrotic notch** (also known as the incisura) is a small downward deflection observed in the arterial pressure waveform, occurring immediately after the closure of the aortic valve. **Why Peripheral Resistance is Correct:** The dicrotic notch marks the end of systole and the beginning of diastole. When the aortic valve closes, the elastic recoil of the aorta pushes blood forward toward the periphery. However, because of **peripheral resistance** in the systemic circulation, blood encounters opposition. This resistance causes a momentary "backflow" or rebound of blood against the closed aortic valve, creating a transient increase in pressure. This pressure rebound is what forms the dicrotic notch and the subsequent dicrotic wave. **Why Other Options are Incorrect:** * **A. Passive filling of blood in ventricles:** This occurs during early diastole (the "v" wave in JVP) and relates to ventricular volume, not the arterial pressure waveform. * **B. Rapid ejection phase:** This occurs during early systole and corresponds to the steep rise (anacrotic limb) of the arterial pulse, not the notch. * **D. Isovolumic contraction:** This is the phase where all valves are closed and ventricular pressure rises without volume change; it precedes the opening of the aortic valve. **High-Yield NEET-PG Pearls:** * **Dicrotic Notch vs. Dicrotic Wave:** The *notch* is the dip caused by aortic valve closure; the *wave* is the subsequent rise due to elastic recoil. * **Clinical Correlation:** A **Dicrotic Pulse** (two peaks per heartbeat) is classically seen in conditions with low cardiac output and high peripheral resistance, such as **dilated cardiomyopathy** or severe heart failure. * **Anacrotic Notch:** Unlike the dicrotic notch, an anacrotic notch is seen on the ascending limb and is characteristic of **Aortic Stenosis**.

Question 77: A patient has an aerial blood pressure of 90 mm Hg and a cardiac output of 5.4 L/min. What is the peripheral resistance?

A. 1 R (Correct Answer)
B. 2 R
C. 4 R
D. 6 R

Explanation: ### Explanation **1. Why Option A is Correct:** The relationship between blood pressure, cardiac output, and peripheral resistance is governed by the hemodynamic version of Ohm’s Law: **Mean Arterial Pressure (MAP) = Cardiac Output (CO) × Total Peripheral Resistance (TPR)** To find the peripheral resistance (TPR), we rearrange the formula: **TPR = MAP / CO** Plugging in the values provided: * MAP = 90 mm Hg * CO = 5.4 L/min * TPR = 90 / 5.4 = **16.66 mmHg/L/min** In physiological units, **1 Peripheral Resistance Unit (PRU or 'R')** is defined as the resistance when a pressure difference of 1 mm Hg causes a flow of 1 ml/sec. However, in clinical shorthand for exams, 1 R is often used to represent the standard normal resistance (approx. 90/5.4 ≈ 16.6 units). Since the calculation yields exactly the baseline ratio of 16.6, the value is expressed as **1 R**. **2. Why Other Options are Incorrect:** * **Option B (2 R):** This would require a resistance of ~33 units. This occurs in systemic vasoconstriction (e.g., compensatory stage of shock). * **Option C (4 R):** This represents a four-fold increase in resistance, seen in severe hypertensive crises or extreme vasospasm. * **Option D (6 R):** This is an abnormally high resistance, not consistent with the provided physiological parameters of 90 mmHg and 5.4 L/min. **3. Clinical Pearls & High-Yield Facts:** * **The "R" Unit:** In the systemic circulation, normal TPR is about 1 PRU. In the **pulmonary circulation**, the resistance is much lower, approximately **0.12 PRU** (about 1/7th of systemic resistance). * **Poiseuille’s Law:** Resistance is inversely proportional to the **fourth power of the radius ($r^4$)**. This makes the **arterioles** the primary "resistance vessels" of the body, as small changes in their diameter lead to massive changes in TPR. * **Viscosity:** While vessel diameter is the most potent regulator, an increase in hematocrit (Polycythemia) increases blood viscosity, thereby increasing TPR.

Question 78: A patient suffers a severe hemorrhage resulting in a lowered mean arterial pressure. Which of the following would be elevated above normal levels?

A. Splanchnic blood flow
B. Cardiopulmonary receptor activity
C. Right ventricular end-diastolic volume
D. Heart rate (Correct Answer)

Explanation: **Explanation:** The physiological response to hemorrhage is driven by the **Baroreceptor Reflex**, which aims to restore mean arterial pressure (MAP) and maintain perfusion to vital organs. **Why Heart Rate is Elevated:** When hemorrhage occurs, the decrease in blood volume leads to a drop in venous return and cardiac output, causing a fall in MAP. This decrease in pressure is sensed by high-pressure baroreceptors (located in the carotid sinus and aortic arch). The reduced stretch on these receptors decreases their firing rate to the medulla, leading to: 1. **Increased Sympathetic outflow:** Stimulates the SA node to increase **Heart Rate** (tachycardia) and increases myocardial contractility. 2. **Decreased Parasympathetic (Vagal) tone:** Further contributes to the rise in heart rate. **Analysis of Incorrect Options:** * **A. Splanchnic blood flow:** This **decreases**. Sympathetic activation causes vasoconstriction of peripheral and visceral (splanchnic) beds to divert blood toward the brain and heart. * **B. Cardiopulmonary receptor activity:** These are "low-pressure" receptors in the atria and pulmonary vessels. In hemorrhage, decreased blood volume reduces the stretch on these receptors, thereby **decreasing** their firing rate. * **C. Right ventricular end-diastolic volume (RVEDV):** This **decreases**. Hemorrhage reduces total blood volume and venous return (preload), leading to lower filling volumes in the ventricles. **High-Yield Clinical Pearls for NEET-PG:** * **Tachycardia** is often the earliest clinical sign of compensatory shock. * **The Bainbridge Reflex** (which increases HR due to increased atrial stretch) is the *opposite* of what happens here; in hemorrhage, the Baroreceptor reflex dominates. * **Formula to remember:** $MAP = CO \times TPR$. To compensate for low $CO$ (due to low stroke volume), the body must increase $HR$ and $TPR$ (Total Peripheral Resistance).

Question 79: What is the most potent vasopressor?

A. Angiotensin II (Correct Answer)
B. Renin
C. Aldosterone
D. Cortisol

Explanation: **Explanation:** **Angiotensin II** is the correct answer because it is one of the most powerful endogenous vasoconstrictors known. It acts directly on **AT1 receptors** located on vascular smooth muscle cells, leading to intense systemic vasoconstriction. This increases total peripheral resistance (TPR) and, consequently, arterial blood pressure. It is significantly more potent than norepinephrine in its pressor effects. **Analysis of Incorrect Options:** * **Renin (B):** Renin is a proteolytic enzyme secreted by the juxtaglomerular cells. It does not have any direct vasopressor activity; its role is to catalyze the conversion of Angiotensinogen to Angiotensin I. * **Aldosterone (C):** This is a mineralocorticoid that regulates blood pressure primarily through **volume expansion**. It promotes sodium and water reabsorption in the distal tubules, but it does not cause acute vasoconstriction. * **Cortisol (D):** While cortisol is essential for maintaining vascular tone by increasing the sensitivity of receptors to catecholamines (permissive effect), it is not a direct potent vasopressor. **High-Yield NEET-PG Pearls:** * **Potency Hierarchy:** While Angiotensin II is the most potent *physiological* vasopressor in this list, **Urotensin II** is technically the most potent endogenous vasoconstrictor discovered in humans (though rarely tested). * **RAAS Pathway:** Angiotensin I is converted to Angiotensin II by **ACE** (Angiotensin Converting Enzyme), primarily in the pulmonary capillaries. * **Clinical Link:** ACE inhibitors and ARBs (Angiotensin Receptor Blockers) are first-line antihypertensives because they block the potent pressor effects of Angiotensin II.

Question 80: What is the result of increased preload on cardiac muscle?

A. Lengthening of muscle fibre (Correct Answer)
B. Shortening of muscle fibre
C. No effect
D. Variable effect

Explanation: **Explanation:** The correct answer is **A. Lengthening of muscle fibre.** This question tests the fundamental concept of **Frank-Starling’s Law of the Heart**. Preload is defined as the degree of stretch on the ventricular myocardium at the end of diastole, just before contraction begins. It is clinically represented by the **End-Diastolic Volume (EDV)**. When venous return increases, the ventricles fill with more blood. This volume exerts pressure against the ventricular walls, causing the individual cardiac myocytes to stretch. According to the Frank-Starling mechanism, this **lengthening of the muscle fiber** increases the number of active cross-bridge attachments between actin and myosin filaments, thereby increasing the force of the subsequent contraction (stroke volume). **Analysis of Incorrect Options:** * **B. Shortening of muscle fibre:** Shortening occurs during the systolic phase (contraction), not as a result of preload. Preload is a diastolic phenomenon. * **C & D. No/Variable effect:** These are incorrect because the relationship between filling volume and fiber length is a physiological constant (within limits). Increased volume *always* results in increased fiber length in a healthy heart. **High-Yield NEET-PG Pearls:** * **Preload vs. Afterload:** Preload is "stretch" (EDV); Afterload is "resistance" (Total Peripheral Resistance/MAP). * **Optimal Length:** The heart operates on the ascending limb of the length-tension curve. The optimal sarcomere length for maximal force is approximately **2.2 μm**. * **Clinical Correlation:** In Heart Failure, the heart may overstretch beyond the optimal point, leading to a decrease in contractile force. * **LaPlace’s Law:** Relates to preload by stating that wall tension increases as the radius of the chamber increases (T = P × r / 2h).

Practice by Chapter

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Cardiac Cycle

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Cardiac Output and Its Regulation

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Cardiovascular Reflexes

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Cardiovascular Responses to Exercise and Stress

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