Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 361: A patient has an oxygen consumption of 240 ml/min, a pulmonary vein oxygen concentration of 180 ml/L of blood, and a pulmonary artery oxygen concentration of 160 ml/L of blood. What is the cardiac output in L/min?

A. 8
B. 10
C. 12 (Correct Answer)
D. 14

Explanation: ### Explanation This question is based on the **Fick Principle**, a fundamental concept in cardiovascular physiology used to calculate the blood flow to an organ, most commonly used to determine **Cardiac Output (CO)**. #### 1. Why the Correct Answer (C) is Right The Fick Principle states that the uptake of a substance by an organ per unit time is equal to the arterial concentration of the substance minus the venous concentration, multiplied by the blood flow. For the lungs, this is expressed as: **Cardiac Output (L/min) = Oxygen Consumption ($\text{VO}_2$) / (Arterial $\text{O}_2$ Content - Mixed Venous $\text{O}_2$ Content)** * **Oxygen Consumption ($\text{VO}_2$):** 240 ml/min * **Arterial $\text{O}_2$ Content (Pulmonary Vein):** 180 ml/L * **Mixed Venous $\text{O}_2$ Content (Pulmonary Artery):** 160 ml/L * **Arteriovenous (A-V) $\text{O}_2$ Difference:** $180 - 160 = 20 \text{ ml/L}$ **Calculation:** $\text{CO} = 240 / 20 = \mathbf{12 \text{ L/min}}$ #### 2. Why Other Options are Wrong * **Option A (8 L/min):** This would result if the A-V difference were 30 ml/L ($240/30$). * **Option B (10 L/min):** This would occur if the A-V difference were 24 ml/L ($240/24$). * **Option D (14 L/min):** This does not mathematically align with the provided consumption and concentration values. #### 3. Clinical Pearls & High-Yield Facts * **Mixed Venous Blood:** In clinical practice, mixed venous oxygen is measured from the **Pulmonary Artery** (using a Swan-Ganz catheter) because it contains blood returning from both the superior and inferior vena cava, thoroughly mixed in the right ventricle. * **Standard Values:** In a healthy resting adult, $\text{VO}_2$ is approx. 250 ml/min and CO is approx. 5 L/min, making the normal A-V $\text{O}_2$ difference roughly 50 ml/L (or 5 ml/100ml). * **NEET-PG Tip:** Always check the units. If oxygen concentration is given in "ml per 100ml" (volumes percent), you must multiply by 10 to convert it to "ml per Liter" before using the formula for Cardiac Output in L/min.

Question 362: On an ECG tracing, the presence of right atrial hypertrophy is suggested by which of the following?

A. Tall spiky P wave (Correct Answer)
B. Widened P wave
C. Prolonged P-R interval
D. Increased P-Q segment

Explanation: ### Explanation **1. Why "Tall spiky P wave" is correct:** The P wave on an ECG represents atrial depolarization. In **Right Atrial Hypertrophy (RAH)**, the increased muscle mass of the right atrium generates a higher electrical voltage during the initial phase of atrial activation. This results in a P wave that is **tall and peaked** (amplitude **>2.5 mm** in lead II). This morphology is classically referred to as **"P-pulmonale"** because it is frequently associated with chronic obstructive pulmonary disease (COPD) or pulmonary hypertension. **2. Analysis of Incorrect Options:** * **B. Widened P wave:** This is characteristic of **Left Atrial Hypertrophy (LAH)**. Since the left atrium is the last to depolarize, its enlargement extends the duration of the P wave (>0.12s), often resulting in a notched or "M-shaped" wave known as **P-mitrale**. * **C. Prolonged P-R interval:** This indicates a delay in conduction through the AV node, signifying a **First-degree AV block**, not atrial chamber enlargement. * **D. Increased P-Q segment:** The P-Q (or P-R) segment represents the time between the end of atrial depolarization and the start of ventricular depolarization. Its prolongation is typically seen in conditions affecting the AV node or in **pericarditis** (where PR-segment depression is a key finding). **3. High-Yield Clinical Pearls for NEET-PG:** * **P-pulmonale (RAH):** Tall, peaked P waves in Lead II, III, aVF (>2.5 mm). * **P-mitrale (LAH):** Wide, notched P waves in Lead II (>0.12s) and a deep terminal negative deflection in Lead V1. * **Bi-atrial Enlargement:** Shows features of both (tall and wide P waves). * **Common Causes of RAH:** Pulmonary stenosis, Tricuspid stenosis, and Cor pulmonale.

Question 363: Which of the following segments of the circulatory system has the highest velocity of blood flow?

A. Aorta (Correct Answer)
B. Arteries
C. Capillaries
D. Venules

Explanation: ### Explanation The velocity of blood flow is governed by the principle of continuity, which states that velocity ($v$) is inversely proportional to the total cross-sectional area ($A$) of the vascular segment ($v = Q/A$, where $Q$ is blood flow/cardiac output). **1. Why Aorta is Correct:** The **Aorta** has the smallest total cross-sectional area (approximately 3–5 cm²) in the entire circulatory system. Since the entire cardiac output must pass through this single vessel, the blood must travel at its maximum speed to maintain flow. The mean velocity in the aorta is roughly **20–40 cm/s**. **2. Why Other Options are Incorrect:** * **Arteries:** While individual arteries are smaller than the aorta, their *total* cross-sectional area is larger due to branching, which leads to a decrease in velocity compared to the aorta. * **Capillaries:** These have the **lowest velocity** of blood flow (approx. 0.03 cm/s). Although an individual capillary is tiny, the collective network of billions of capillaries has the largest total cross-sectional area (approx. 1000 times that of the aorta). This slow flow is physiologically essential to allow adequate time for nutrient and gas exchange. * **Venules:** As blood moves from capillaries into venules and then veins, the total cross-sectional area begins to decrease again, causing the velocity to increase slightly compared to capillaries, but it never reaches the peak velocity seen in the aorta. **High-Yield Clinical Pearls for NEET-PG:** * **Highest Velocity:** Aorta. * **Lowest Velocity:** Capillaries (facilitates exchange). * **Largest Total Cross-Sectional Area:** Capillaries. * **Smallest Total Cross-Sectional Area:** Aorta. * **Maximum Peripheral Resistance:** Arterioles (the "stopcocks" of circulation). * **Highest Blood Volume (Reservoir):** Veins/Venules (~64% of total blood).

Question 364: Baroreceptor stimulation would result in what change?

A. Decreased vagal activity
B. Increased heart rate
C. Decreased sympathetic discharge to heart (Correct Answer)
D. Increased vasomotor tone

Explanation: ### Explanation **Concept:** The baroreceptor reflex is the body's primary mechanism for short-term blood pressure (BP) regulation. Baroreceptors are stretch receptors located in the **carotid sinus** (via Glossopharyngeal nerve) and **aortic arch** (via Vagus nerve). When BP rises, these receptors are stretched, increasing their firing rate to the **Nucleus Tractus Solitarius (NTS)** in the medulla. **Why Option C is Correct:** Stimulation of the NTS leads to two simultaneous responses: 1. **Inhibition of the Vasomotor Center:** This reduces sympathetic outflow to the heart and peripheral blood vessels. 2. **Stimulation of the Cardioinhibitory Center (Nucleus Ambiguus):** This increases parasympathetic (vagal) outflow. Therefore, **decreased sympathetic discharge** to the heart reduces heart rate and contractility, helping to lower BP back to normal. **Why Other Options are Incorrect:** * **A & B:** Baroreceptor stimulation *increases* vagal activity and *decreases* heart rate (bradycardia) to compensate for high BP. * **D:** It leads to a *decrease* in vasomotor tone (vasodilation) by inhibiting sympathetic vasoconstrictor nerves, thereby reducing peripheral resistance. --- ### High-Yield NEET-PG Pearls * **Location:** Carotid sinus baroreceptors are more sensitive than aortic arch receptors; they respond to both increases and decreases in BP, whereas aortic receptors primarily respond to increases. * **Carotid Sinus Massage:** Clinically used to terminate Paroxysmal Supraventricular Tachycardia (PSVT) because it mimics high BP, triggering the reflex to increase vagal tone and slow the heart rate. * **Denervation:** If baroreceptor nerves are cut, the brain perceives "low BP," leading to persistent sympathetic overactivity and hypertension.

Question 365: Which of the following is not a measure of stroke volume?

A. Left ventricular end-diastolic volume minus left ventricular end-systolic volume
B. Ejection fraction times left ventricular end-diastolic volume
C. Ejection fraction times cardiac output (Correct Answer)
D. Cardiac output divided by heart rate

Explanation: **Explanation:** **Stroke Volume (SV)** is the volume of blood pumped by the left ventricle per heartbeat. To identify the incorrect measure, we must evaluate the mathematical relationships between cardiac parameters. **1. Why Option C is the Correct Answer (The Incorrect Measure):** Stroke volume is a component of Cardiac Output (CO), not a product of it. The formula for Cardiac Output is $CO = SV \times HR$. Therefore, multiplying Ejection Fraction (EF) by Cardiac Output does not yield a physiologically recognized volume. This option is mathematically and conceptually incorrect. **2. Analysis of Incorrect Options (Correct Measures of SV):** * **Option A (LVEDV – LVESV):** This is the standard definition of SV. It represents the difference between the volume of blood in the ventricle at the end of filling (diastole) and the volume remaining after contraction (systole). * **Option B (EF × LVEDV):** Ejection Fraction is defined as the fraction of blood ejected from the LVEDV ($EF = SV / LVEDV$). Rearranging this formula gives $SV = EF \times LVEDV$. * **Option D (CO / HR):** Since $Cardiac\ Output = Stroke\ Volume \times Heart\ Rate$, dividing CO by HR mathematically isolates the Stroke Volume. **Clinical Pearls for NEET-PG:** * **Normal Values:** Average SV is **70 mL**; average LVEDV is **120 mL**; average LVESV is **50 mL**. * **Ejection Fraction:** Normal range is **55–65%**. It is the most common clinical index of left ventricular systolic function. * **Determinants of SV:** SV is regulated by **Preload** (proportional via Frank-Starling Law), **Afterload** (inversely proportional), and **Inotropy** (contractility). * **Pulse Pressure:** In clinical practice, pulse pressure (Systolic BP - Diastolic BP) is often used as a surrogate indicator of stroke volume.

Question 366: Isovolumetric relaxation proceeds during which phase of the cardiac cycle?

A. Ventricular ejection
B. Ventricular relaxation (Correct Answer)
C. Atrial contraction
D. Atrial relaxation

Explanation: **Explanation:** **Isovolumetric relaxation (IVR)** is a crucial phase of the cardiac cycle that occurs during **early ventricular diastole (ventricular relaxation)**. It begins immediately after the closure of the semilunar valves (Aortic and Pulmonary) and ends when the Atrioventricular (AV) valves open. 1. **Why Option B is correct:** During this phase, the ventricles begin to relax, causing intraventricular pressure to drop rapidly. However, because the ventricular pressure is still higher than atrial pressure but lower than arterial pressure, **all four valves are closed**. Since no blood enters or leaves the ventricles, the volume remains constant (isovolumetric) while the pressure falls. 2. **Why other options are incorrect:** * **Ventricular ejection:** This is a phase of ventricular *systole* where the semilunar valves are open and blood is pumped out. * **Atrial contraction:** This occurs at the end of ventricular diastole (the "atrial kick") to complete ventricular filling. * **Atrial relaxation:** This occurs simultaneously with ventricular contraction (systole) and is not the primary driver of IVR. **High-Yield NEET-PG Pearls:** * **Duration:** IVR is the second shortest phase of the cardiac cycle (~0.06 - 0.08s). * **Heart Sounds:** The **Second Heart Sound (S2)** marks the *beginning* of this phase (closure of semilunar valves). * **Pressure Dynamics:** This phase shows the steepest decline in ventricular pressure on a Wiggers diagram. * **Volume:** The volume of blood remaining in the ventricle during this phase is the **End-Systolic Volume (ESV)**, typically ~50-60 mL.

Question 367: What does the inward flow of Na+ in the heart lead to?

A. Plateau phase
B. Action potential (Correct Answer)
C. Repolarization
D. No change

Explanation: **Explanation:** The correct answer is **Action Potential** (specifically the rapid depolarization phase). In cardiac physiology, the initiation of an action potential in both ventricular myocytes and the conduction system (Purkinje fibers) is driven by the rapid influx of sodium ions ($Na^+$). When the cell membrane reaches a threshold potential, **voltage-gated fast $Na^+$ channels** open, leading to a massive inward current ($I_{Na}$). This rapid influx causes the membrane potential to shift from a negative resting state (approx. -90mV) to a positive value (approx. +20mV), characterizing **Phase 0** of the cardiac action potential. **Analysis of Incorrect Options:** * **A. Plateau phase:** This is **Phase 2** of the action potential. It is primarily maintained by the inward flow of **Calcium ($Ca^{2+}$)** through L-type channels, balanced by an outward flow of Potassium ($K^+$). * **C. Repolarization:** This occurs during **Phases 1, 2, and 3**. It is primarily driven by the **outward flow of Potassium ($K^+$)** ions, which restores the negative resting membrane potential. * **D. No change:** Inward $Na^+$ flow causes a significant electrical shift (depolarization), which is the fundamental basis of cardiac excitability. **High-Yield Clinical Pearls for NEET-PG:** * **Phase 0 (Depolarization):** Driven by $Na^+$ influx in myocytes, but by **$Ca^{2+}$ influx** in the SA/AV nodes (slow response tissues). * **Class I Antiarrhythmics:** These drugs (e.g., Lidocaine, Flecainide) work specifically by blocking these fast $Na^+$ channels, thereby slowing the rate of Phase 0 depolarization. * **Tetrodotoxin:** A potent toxin that inhibits these voltage-gated $Na^+$ channels, preventing action potential generation.

Question 368: Most Baroreceptors and chemoreceptors are situated in which anatomical location?

A. Infundibulum of the Right Ventricle
B. Crista terminalis
C. Left auricle (Correct Answer)
D. Infundibulum of the Left Ventricle

Explanation: ### Explanation **Correct Answer: C. Left auricle** **1. Why the Left Auricle is Correct:** While the most famous baroreceptors and chemoreceptors are located in the **carotid sinus** and **aortic arch**, the heart itself contains intrinsic receptors. Within the heart, the highest density of these receptors is found in the **atria**, specifically the **Left Auricle (Left Atrial Appendage)**. * **Baroreceptors (Low-pressure receptors):** These are stretch receptors located in the atrial walls. When blood volume increases, the stretching of the left auricle triggers the **Bainbridge reflex** (increasing heart rate) and inhibits ADH release to promote diuresis. * **Chemoreceptors:** Small clusters of chemoreceptive cells (similar to aortic bodies) are found in the subendocardial tissue of the left auricle, sensitive to changes in blood pH and oxygen tension. **2. Analysis of Incorrect Options:** * **A & D. Infundibulum (Right/Left Ventricle):** The infundibulum (outflow tract) contains fewer sensory receptors compared to the atria. Ventricular receptors (like those involved in the Bezold-Jarisch reflex) are primarily located in the inferoposterior wall of the left ventricle, not the infundibulum. * **B. Crista terminalis:** This is a vertical ridge in the right atrium that separates the smooth and rough portions. While it is a landmark for the SA node, it is not the primary site for baro- or chemoreceptor concentration. **3. NEET-PG High-Yield Pearls:** * **Bainbridge Reflex:** Atrial stretch → Increased Heart Rate (to prevent venous congestion). * **ANP Secretion:** The atrial myocytes (especially in the auricles) secrete **Atrial Natriuretic Peptide (ANP)** in response to stretch, promoting sodium and water excretion. * **Location Summary:** * High-pressure baroreceptors: Carotid Sinus (CN IX) and Aortic Arch (CN X). * Low-pressure baroreceptors: Atria (especially the junction of vena cava/pulmonary veins and the auricles).

Question 369: Which of the following is NOT a Vitamin K dependent clotting factor?

A. Factor II
B. Protein C (Correct Answer)
C. Factor X
D. Factor IX

Explanation: **Explanation:** The synthesis of certain coagulation factors in the liver requires **Vitamin K** as a cofactor for the enzyme **gamma-glutamyl carboxylase**. This enzyme adds a carboxyl group to glutamate residues on these proteins, allowing them to bind calcium ions and adhere to phospholipid surfaces—a critical step in the clotting cascade. **Why the answer is Protein C (Note on Question Context):** There appears to be a technical nuance in this question. Traditionally, the **Vitamin K-dependent proteins** include: * **Pro-coagulants:** Factors II (Prothrombin), VII, IX, and X. * **Anti-coagulants:** Protein C, Protein S, and Protein Z. In many standard NEET-PG patterns, if the question asks for "clotting factors" (pro-coagulants) specifically, **Protein C** is the odd one out because it is an **anticoagulant** (it inactivates Factors Va and VIIIa), even though it is Vitamin K-dependent. **Analysis of Options:** * **Factor II (Prothrombin):** A Vitamin K-dependent pro-coagulant. It is the precursor to thrombin. * **Factor IX (Christmas Factor):** A Vitamin K-dependent pro-coagulant involved in the intrinsic pathway. * **Factor X (Stuart-Prower Factor):** A Vitamin K-dependent pro-coagulant that marks the beginning of the common pathway. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mnemonic:** Remember "**2, 7, 9, 10, C, and S**" to recall all Vitamin K-dependent proteins. 2. **Warfarin (Coumadin):** Acts by inhibiting Vitamin K Epoxide Reductase (VKOR). 3. **Warfarin-Induced Skin Necrosis:** Occurs because Protein C has a shorter half-life than the pro-coagulant factors. When starting Warfarin, Protein C levels drop first, creating a transient hypercoagulable state. 4. **PT/INR:** This is the most sensitive lab test to monitor Vitamin K deficiency or Warfarin therapy because **Factor VII** has the shortest half-life among the clotting factors.

Question 370: Sinus bradycardia is seen in:

A. Athletes (Correct Answer)
B. Exercise
C. Thyrotoxicosis
D. Beta adrenoceptor agonist

Explanation: **Explanation:** **Sinus bradycardia** is defined as a heart rate of less than 60 beats per minute originating from the SA node. **Why Athletes is the correct answer:** In highly trained athletes, chronic aerobic conditioning leads to **increased vagal (parasympathetic) tone** and a simultaneous decrease in sympathetic drive at rest. Additionally, exercise induces physiological cardiac hypertrophy, which increases the **stroke volume**. According to the formula *Cardiac Output = Stroke Volume × Heart Rate*, a higher stroke volume allows the heart to maintain the required cardiac output at a lower resting heart rate. This is a physiological adaptation known as "Athletic Heart Syndrome." **Why the other options are incorrect:** * **Exercise:** During exercise, the body’s demand for oxygen increases, leading to sympathetic activation and withdrawal of vagal tone. This results in **sinus tachycardia**, not bradycardia. * **Thyrotoxicosis:** Excess thyroid hormones (T3/T4) increase the expression of beta-1 adrenergic receptors in the heart and have direct chronotropic effects, leading to **tachycardia** and often atrial fibrillation. * **Beta adrenoceptor agonists:** These drugs (e.g., Adrenaline, Isoprenaline) stimulate beta-1 receptors in the SA node, increasing the rate of phase 4 depolarization, which causes **tachycardia**. (Note: Beta-blockers/antagonists cause bradycardia). **High-Yield NEET-PG Pearls:** * **Other causes of Sinus Bradycardia:** Hypothyroidism, Hypothermia, Obstructive Jaundice (bile salts act on the SA node), Raised Intracranial Pressure (Cushing’s reflex), and drugs like Digoxin or Beta-blockers. * **Bainbridge Reflex:** An increase in right atrial pressure leads to an increase in heart rate (tachycardia) to pump the excess blood. * **Oculocardiac Reflex:** Pressure on the eyeball can trigger profound bradycardia via the trigeminal (afferent) and vagus (efferent) nerves.

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