Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 191: What is the normal oxygen consumption by the myocardium in mL/100 g/min?

A. 2 (Correct Answer)
B. 10
C. 100
D. 150

Explanation: **Explanation:** The correct answer is **A (2 mL/100 g/min)**. **Understanding Myocardial Oxygen Consumption ($MVO_2$):** The heart is a highly aerobic organ with a high metabolic demand. However, it is crucial to distinguish between the **resting** state and the **active** state. * **At rest (Basal state):** When the heart is not beating (e.g., during cardiac arrest or induced cardioplegia during surgery), the oxygen consumption required simply to maintain cellular integrity is approximately **2 mL/100 g/min**. * **During normal activity:** In a beating heart at rest, the consumption increases significantly to about **8–10 mL/100 g/min** to support contraction. **Analysis of Incorrect Options:** * **Option B (10 mL/100 g/min):** This represents the $MVO_2$ of a **functioning, beating heart** at rest. While this is a common physiological value, the question specifically looks for the baseline/standard physiological value often cited in textbooks for basal myocardial tissue. * **Option C (100 mL/100 g/min):** This value is far too high for resting myocardium. It may be reached only during periods of extreme strenuous exercise or maximal sympathetic stimulation. * **Option D (150 mL/100 g/min):** This value is physiologically improbable for myocardial tissue under standard conditions. **High-Yield NEET-PG Pearls:** 1. **Extraction Ratio:** The heart has the highest oxygen extraction ratio in the body (~70–80%). Because it already extracts near-maximum oxygen at rest, any increase in demand must be met by an **increase in coronary blood flow**, not increased extraction. 2. **Determinants of $MVO_2$:** The most important determinant of myocardial oxygen demand is **Intramyocardial tension** (determined by afterload/systolic BP), followed by heart rate and contractility. 3. **Coronary Blood Flow:** Normal flow is ~250 mL/min (or 80 mL/100 g/min), which is roughly 5% of the total Cardiac Output.

Question 192: Myocardial oxygen demand depends upon which of the following factors?

A. Preload (Correct Answer)
B. Afterload
C. Intramyocardial tension
D. Blood hemoglobin concentration

Explanation: **Explanation:** The primary determinants of **Myocardial Oxygen Demand ($MVO_2$)** are the factors that increase the workload of the heart. These are categorized into three main components: **Wall Tension** (determined by Preload and Afterload), **Heart Rate**, and **Contractility**. **1. Why Preload is the Correct Answer:** Preload refers to the end-diastolic volume that stretches the right or left ventricle of the heart. According to the **Law of Laplace** ($T = P \times r / 2h$), an increase in preload increases the ventricular radius ($r$), which directly increases the **intramyocardial wall tension** ($T$). Higher wall tension requires more ATP and oxygen for cross-bridge cycling, thus increasing $MVO_2$. **2. Analysis of Other Options:** * **B & C. Afterload and Intramyocardial Tension:** While these *do* increase oxygen demand, in the context of standard physiological teaching and many NEET-PG sourcebooks (like Ganong/Guyton), **Preload** is often highlighted as a fundamental mechanical determinant. However, it is important to note that clinically, Afterload (pressure work) is actually more oxygen-expensive than Preload (volume work). In this specific MCQ format, Preload is the classic "textbook" answer for a primary factor. * **D. Blood Hemoglobin Concentration:** This affects **Oxygen Supply** (carrying capacity), not the **Demand** of the myocardium itself. **High-Yield Clinical Pearls for NEET-PG:** * **Pressure Work vs. Volume Work:** The heart is less efficient at pressure work. Therefore, conditions increasing Afterload (e.g., Aortic Stenosis, Hypertension) increase $MVO_2$ significantly more than conditions increasing Preload (e.g., Mitral Regurgitation). * **Heart Rate:** This is the most important clinical determinant of $MVO_2$ because it increases the number of contractions per minute and shortens diastole (the period of coronary perfusion). * **Double Product:** A clinical surrogate for $MVO_2$ calculated as $Heart Rate \times Systolic Blood Pressure$.

Question 193: What effect does parasympathetic stimulation have on the heart?

A. Decreased firing rate of the SA node (Correct Answer)
B. Increased excitability of the AV node
C. Decreased ventricular contractility
D. Tachycardia

Explanation: ### Explanation **Correct Answer: A. Decreased firing rate of the SA node** **Mechanism:** Parasympathetic (vagal) stimulation affects the heart primarily through the release of **Acetylcholine (ACh)**, which acts on **M2 muscarinic receptors**. This activation leads to two main ionic changes: 1. **Increased K+ conductance:** It opens G-protein-coupled inward rectifier K+ channels ($I_{K-ACh}$), causing hyperpolarization of the resting membrane potential. 2. **Decreased $I_f$ and $I_{Ca-L}$ currents:** It slows the rate of spontaneous phase 4 depolarization. Combined, these effects decrease the firing rate of the Sinoatrial (SA) node, resulting in **negative chronotropy** (bradycardia). **Analysis of Incorrect Options:** * **B. Increased excitability of the AV node:** Parasympathetic stimulation actually **decreases** excitability and slows conduction velocity through the AV node (negative dromotropy). This increases the PR interval on an ECG. * **C. Decreased ventricular contractility:** While the vagus nerve significantly decreases atrial contractility, it has a **minimal to negligible effect** on ventricular contractility because the ventricles have very sparse parasympathetic innervation compared to the atria. * **D. Tachycardia:** Tachycardia (increased heart rate) is a sympathetic effect mediated by Beta-1 receptors. Parasympathetic stimulation causes bradycardia. **NEET-PG High-Yield Pearls:** * **Vagal Tone:** At rest, the heart is under dominant parasympathetic tone. If all autonomic influence is blocked (e.g., by Atropine + Propranolol), the intrinsic heart rate is ~100 bpm. * **Right vs. Left Vagus:** The **Right Vagus** primarily innervates the SA node (affects rate), while the **Left Vagus** primarily innervates the AV node (affects conduction). * **Vagal Escape:** Strong vagal stimulation can stop the SA node, but eventually, a distal pacemaker (like the Purkinje fibers) will take over to prevent cardiac arrest.

Question 194: In sickle cell anemia, which amino acid substitution occurs in the beta-globin chain?

A. Glutamate replaces valine
B. Valine replaces glutamate (Correct Answer)
C. Valine replaces aspartate
D. Aspartate replaces valine

Explanation: ### Explanation **Concept Overview:** Sickle cell anemia is an autosomal recessive genetic disorder caused by a **point mutation** in the *HBB* gene on chromosome 11. The mutation involves a single nucleotide substitution (GAG to GTG), which results in a change in the amino acid sequence of the beta-globin chain of hemoglobin. **Why Option B is Correct:** At the **6th position** of the beta-globin chain, the polar (hydrophilic) amino acid **Glutamate** is replaced by the non-polar (hydrophobic) amino acid **Valine**. This substitution creates a "sticky patch" on the hemoglobin molecule (HbS). Under conditions of low oxygen tension (deoxygenation), these sticky patches cause HbS molecules to polymerize into long fibers, distorting the red blood cell into a characteristic sickle shape. **Analysis of Incorrect Options:** * **Option A:** This is the reverse of the actual mutation. Glutamate is the normal amino acid; its replacement *by* valine causes the disease. * **Option C & D:** Aspartate is not involved in the primary mutation of sickle cell anemia. However, substitution of glutamate by **Lysine** at the same 6th position results in **Hemoglobin C (HbC)** disease. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Molecular Basis:** Missense mutation (Transversion: Adenine to Thymine). * **HbS Properties:** Deoxygenated HbS is 50 times less soluble than deoxygenated HbA. * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria. * **Diagnosis:** Best initial test is a peripheral smear; Gold standard is **Hemoglobin Electrophoresis** (HbS moves slower than HbA toward the anode due to loss of negative charge).

Question 195: Ejection fraction increases with:

A. End systolic volume.
B. End diastolic volume. (Correct Answer)
C. Peripheral vascular resistance.
D. Venodilation.

Explanation: **Explanation:** **Ejection Fraction (EF)** is the percentage of blood pumped out of the left ventricle with each contraction. It is calculated using the formula: **EF = (Stroke Volume / End Diastolic Volume) × 100**, where Stroke Volume (SV) = EDV – ESV. **Why End Diastolic Volume (EDV) is correct:** According to the **Frank-Starling Law**, an increase in EDV (preload) leads to increased stretching of the ventricular myocardial fibers. This increases the force of contraction, thereby increasing the Stroke Volume. While both the numerator (SV) and denominator (EDV) increase, the physiological response to increased filling in a healthy heart typically results in a more efficient contraction, maintaining or increasing the EF. **Analysis of Incorrect Options:** * **A. End Systolic Volume (ESV):** EF is inversely proportional to ESV. An increase in ESV (the blood remaining after contraction) indicates a decrease in stroke volume and myocardial contractility, leading to a **decrease** in EF. * **C. Peripheral Vascular Resistance (Afterload):** Increased resistance (e.g., hypertension) makes it harder for the heart to pump blood. This increases ESV and decreases SV, thereby **decreasing** the EF. * **D. Venodilation:** This increases venous capacitance and reduces venous return to the heart. This leads to a **decrease** in EDV (preload), which subsequently reduces SV and EF. **High-Yield Clinical Pearls for NEET-PG:** * **Normal EF:** 55% to 70%. An EF < 40% usually indicates Heart Failure with Reduced Ejection Fraction (HFrEF). * **Best Index of Myocardial Contractility:** While EF is commonly used, **dP/dt max** (rate of pressure rise) is considered the most accurate index. * **Inotropic Effect:** Positive inotropes (like Digoxin or Adrenaline) increase EF by decreasing ESV.

Question 196: Regarding the calculation of stroke volume, which of the following statements is NOT true?

A. Stroke Volume = Ejection Fraction x End-Diastolic Volume
B. Stroke Volume = Ejection Fraction x Cardiac Output (Correct Answer)
C. Stroke Volume = Cardiac Output / Heart Rate
D. None of the above

Explanation: ### Explanation **1. Why Option B is the Correct Answer (The False Statement):** Stroke Volume (SV) is the volume of blood pumped by the left ventricle per heartbeat. The **Ejection Fraction (EF)** is defined as the fraction of the End-Diastolic Volume (EDV) that is ejected during a contraction. Mathematically, $EF = SV / EDV$. Therefore, the correct relationship is **$SV = EF \times EDV$**. Option B is incorrect because multiplying Ejection Fraction by Cardiac Output (CO) has no physiological basis; Cardiac Output is already a product of Stroke Volume and Heart Rate ($CO = SV \times HR$). **2. Analysis of Other Options:** * **Option A:** This is a **true** statement derived directly from the definition of Ejection Fraction ($SV = EF \times EDV$). It represents the efficiency of the pump. * **Option C:** This is a **true** statement derived from the Cardiac Output formula ($CO = SV \times HR$). By rearranging it, $SV = CO / HR$. This is commonly used in clinical settings to calculate SV from thermodilution or Fick’s method data. **3. NEET-PG High-Yield Clinical Pearls:** * **Normal Values:** In a healthy 70kg adult, SV is approximately **70 mL**, EDV is **120 mL**, and ESV (End-Systolic Volume) is **50 mL**. * **Ejection Fraction (EF):** Normal range is **55–70%**. An EF < 40% is a hallmark of systolic heart failure (HFrEF). * **Determinants of SV:** Stroke volume is determined by three factors: **Preload** (proportional), **Afterload** (inversely proportional), and **Inotropy** (proportional). * **Fick’s Principle:** Remember that $CO = \text{Oxygen Consumption} / (\text{Arterial } O_2 \text{ content} - \text{Venous } O_2 \text{ content})$. This is a frequent numerical target in exams.

Question 197: The 'C' wave in the Jugular Venous Pulse (JVP) waveform represents which event?

A. Atrial contraction
B. Tricuspid valve bulging into the right atrium (Correct Answer)
C. Ventricular systole
D. Rapid ventricular filling

Explanation: The Jugular Venous Pulse (JVP) reflects pressure changes in the right atrium. Understanding its waveform is a high-yield topic for NEET-PG. ### **Explanation of the Correct Answer** The **'c' wave** occurs during **isovolumetric ventricular contraction**. As the right ventricle begins to contract, the intraventricular pressure rises sharply, causing the **tricuspid valve to bulge back into the right atrium**. This transient increase in atrial pressure creates the 'c' wave. (Note: Carotid artery pulsation may also contribute slightly to this wave). ### **Analysis of Incorrect Options** * **A. Atrial contraction:** This corresponds to the **'a' wave**. It is the first positive deflection and occurs at the end of diastole. * **C. Ventricular systole:** While the 'c' wave occurs *during* early systole, the term is too broad. The 'x' descent (atrial relaxation) and 'v' wave (venous filling) also occur during different phases of ventricular systole. * **D. Rapid ventricular filling:** This occurs during early diastole and corresponds to the **'y' descent**, as blood flows from the atrium into the ventricle after the tricuspid valve opens. ### **High-Yield Clinical Pearls for NEET-PG** * **Giant 'a' waves:** Seen in Tricuspid Stenosis, Pulmonary Hypertension, and Pulmonary Stenosis (conditions where the atrium contracts against resistance). * **Cannon 'a' waves:** Occur when the atrium contracts against a closed tricuspid valve (e.g., Complete Heart Block, Ventricular Tachycardia). * **Absent 'a' wave:** Pathognomonic for **Atrial Fibrillation**. * **Giant 'v' waves:** Characteristic of **Tricuspid Regurgitation** (due to blood regurgitating into the atrium during systole).

Question 198: Which of the following is most appropriate regarding the regulation of coronary circulation?

A. Autonomic regulation
B. Autoregulatory mechanisms (Correct Answer)
C. Hormonal influences
D. Sympathetic nervous system control

Explanation: **Explanation:** The coronary circulation is unique because the heart must meet its high metabolic demands even during the mechanical compression of vessels during systole. **Why Autoregulatory Mechanisms are Correct:** Coronary blood flow is primarily controlled by **local metabolic autoregulation**. The most potent determinant of coronary blood flow is the metabolic activity of the myocardium. When cardiac work increases, oxygen consumption rises, leading to the release of local vasodilator metabolites—most importantly **Adenosine** (formed from ATP breakdown). Other factors include nitric oxide, $H^+$, $K^+$, and $CO_2$. These substances cause local vasodilation to ensure that blood flow matches the oxygen demand (metabolic hyperemia). This intrinsic ability to maintain constant flow despite changes in perfusion pressure (between 60–140 mmHg) is the hallmark of autoregulation. **Why other options are incorrect:** * **Autonomic/Sympathetic Regulation (A & D):** While the heart is innervated by sympathetic and parasympathetic fibers, their direct effect on coronary vessel diameter is weak and often overridden by metabolic factors. For example, sympathetic stimulation causes vasoconstriction (alpha-receptors) but simultaneously increases heart rate and contractility, which produces metabolites that cause "functional sympatholysis" (vasodilation). * **Hormonal Influences (C):** Hormones like epinephrine or vasopressin have minimal roles in the day-to-day regulation of coronary flow compared to local metabolic needs. **High-Yield NEET-PG Pearls:** 1. **Adenosine** is the most important local metabolic regulator of coronary blood flow. 2. The heart extracts **70-80% of oxygen** from the blood even at rest (highest in the body); therefore, any increase in demand must be met by increasing flow, not extraction. 3. Left ventricular coronary flow occurs primarily during **Diastole**. 4. The **Subendocardium** is the most vulnerable layer to ischemia during tachycardia or hypotension.

Question 199: Two vessels are compared as shown in the diagram. Assuming constant pressure along both vessels and a linear flow pattern, what will be the flow across vessel 1 compared to vessel 2?

A. 2 times
B. 4 times
C. 8 times (Correct Answer)
D. 16 times

Explanation: ***8 times*** - According to **Poiseuille's law** (Q ∝ r⁴/L), flow is proportional to the fourth power of radius divided by length. - With vessel 1 having **2r radius** and **2L length** vs vessel 2's **r radius** and **L length**: Q₁/Q₂ = (2r)⁴×L / (r⁴×2L) = 16r⁴L / 2r⁴L = **8 times**. *2 times* - This represents a **linear relationship** with radius, ignoring the **fourth power dependency** in Poiseuille's law. - Also incorrectly assumes flow is directly proportional to radius rather than r⁴, missing the exponential relationship. *4 times* - This reflects considering only the **radius squared** (2²) effect while neglecting both the **fourth power** and **length correction**. - Common error when confusing **cross-sectional area** (πr²) with flow dynamics governed by **viscous resistance**. *16 times* - This considers only the **radius to the fourth power** (2⁴ = 16) while completely **ignoring the length factor**. - Fails to account that vessel 1 has **double the length**, which **halves the flow** according to Poiseuille's law.

Question 200: What is the role of Vitamin K in the clotting cycle activation?

A. Carboxylation
B. Hydroxylation
C. Oxidation
D. Reduction (Correct Answer)

Explanation: To understand why **Reduction** is the correct answer, we must look at the Vitamin K cycle and the specific step required to *initiate* the activation of clotting factors. ### 1. Why Reduction is Correct Vitamin K exists in the body primarily in its inactive (oxidized) form, **Vitamin K Epoxide**. For it to act as a cofactor for the clotting cascade, it must first be converted into its active form, **Vitamin K Hydroquinone**. This conversion is a **reduction** process catalyzed by the enzyme **Vitamin K Epoxide Reductase (VKOR)**. Only the reduced form can facilitate the subsequent step of clotting factor activation. ### 2. Why Other Options are Incorrect * **Carboxylation:** While Vitamin K is essential for the **gamma-carboxylation** of glutamic acid residues on Factors II, VII, IX, and X, this is the *action* Vitamin K performs, not the process that *activates the Vitamin K cycle* itself. * **Hydroxylation:** This is a different biochemical modification (common in collagen synthesis via Vitamin C) and is not part of the Vitamin K-dependent clotting process. * **Oxidation:** This is the result of the clotting factor activation. When Vitamin K helps carboxylate a clotting factor, it becomes **oxidized** into an inactive epoxide. Therefore, oxidation terminates its activity rather than activating the cycle. ### 3. High-Yield Clinical Pearls for NEET-PG * **Mechanism of Warfarin:** Warfarin (Coumadin) acts by inhibiting **Vitamin K Epoxide Reductase**. By preventing the **reduction** of Vitamin K, it keeps the vitamin in its inactive oxidized state, thereby inhibiting the synthesis of functional Factors II, VII, IX, X, Protein C, and Protein S. * **Factors affected:** Remember the mnemonic "1972" (Factors 10, 9, 7, 2). * **Calcium Binding:** Gamma-carboxylation allows these factors to bind to **Calcium (Ca²⁺)**, which is essential for their attachment to phospholipid membranes during clot formation.

Practice by Chapter

Cardiac Electrophysiology

Practice Questions

Cardiac Cycle

Practice Questions

Cardiac Output and Its Regulation

Practice Questions

Hemodynamics and Blood Flow

Practice Questions

Arterial System Physiology

Practice Questions

Microcirculation and Lymphatics

Practice Questions

Venous Return and Central Venous Pressure

Practice Questions

Cardiovascular Reflexes

Practice Questions

Regional Circulations

Practice Questions

Cardiovascular Responses to Exercise and Stress

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free