Cardiovascular System — MCQs

A 25-year-old man is participating in a clinical study to determine the cardiovascular response to physical exercise. Basal measurements are: Respiratory rate: 15/min, Blood pressure: 120/80 mmHg, Cardiac output: 5 L/min, Heart rate: 50/min. What is his stroke volume during resting conditions (in mL/min)?

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 171: Which of the following represents the largest percentage of blood volume?

A. Arteries
B. Arterioles
C. Venules and Veins (Correct Answer)
D. Capillaries

Explanation: ### Explanation The distribution of blood volume within the cardiovascular system is a high-yield concept in hemodynamics. The correct answer is **Venules and Veins** because they act as the primary **capacitance vessels** of the body. #### 1. Why Venules and Veins are Correct Approximately **64% to 70%** of the total blood volume resides in the systemic venous system at any given time. This is due to their high **compliance** (distensibility); their walls are thinner and more elastic than arteries, allowing them to hold large volumes of blood at low pressures. This reservoir function is crucial for maintaining cardiac output via venous return. #### 2. Why Other Options are Incorrect * **Arteries (A):** These contain only about **13-15%** of the blood volume. They are "stress vessels" designed to withstand high pressure rather than store volume. * **Arterioles (B):** These contain a very small percentage (approx. **2-3%**). Their primary role is providing **peripheral resistance** to regulate blood pressure, not storage. * **Capillaries (D):** Despite having the largest total cross-sectional area, they contain only about **5%** of the blood volume. Their thin walls are optimized for rapid nutrient and gas exchange. #### 3. NEET-PG High-Yield Pearls * **Capacitance vs. Resistance:** Veins = Capacitance vessels (Volume); Arterioles = Resistance vessels (Pressure). * **Velocity of Flow:** Blood flow is **slowest in the capillaries** (due to the highest total cross-sectional area), which allows time for exchange. * **Clinical Correlation:** In states of hemorrhage, sympathetic stimulation causes **venoconstriction**, shifting blood from the venous reservoir into the arterial circulation to maintain mean arterial pressure.

Question 172: All of the following favor filtration at the arteriolar end of the capillary bed, EXCEPT:

A. Increase in hydrostatic pressure of capillaries
B. Hydrostatic pressure gradient more than osmotic pressure gradient
C. Positive net filtration pressure (PFP)
D. Negative net filtration pressure (NFP) (Correct Answer)

Explanation: ### Explanation The movement of fluid across a capillary membrane is governed by **Starling’s Forces**. The Net Filtration Pressure (NFP) determines whether fluid moves out of the capillary (filtration) or into the capillary (reabsorption). The formula for NFP is: **NFP = (Pc - Pi) - (πc - πi)** *(Where P = Hydrostatic Pressure, π = Oncotic/Osmotic Pressure, c = capillary, i = interstitial fluid)* **1. Why Option D is the Correct Answer:** A **Negative NFP** indicates that the forces favoring reabsorption (primarily capillary oncotic pressure) exceed the forces favoring filtration. This occurs at the **venular end** of the capillary, where fluid is pulled back into the vessel. Therefore, it does **not** favor filtration. **2. Analysis of Incorrect Options:** * **Option A:** An **increase in capillary hydrostatic pressure (Pc)** directly increases the outward force, pushing fluid into the interstitium, thus favoring filtration. * **Option B:** Filtration occurs when the **hydrostatic pressure gradient (Pc - Pi)** is greater than the **osmotic pressure gradient (πc - πi)**. This is the physiological state at the arteriolar end. * **Option C:** A **Positive NFP** means the sum of outward forces is greater than inward forces, which is the definition of filtration. **3. NEET-PG High-Yield Pearls:** * **Arteriolar End:** Hydrostatic pressure (~35 mmHg) > Oncotic pressure (~25 mmHg) → **Filtration.** * **Venular End:** Hydrostatic pressure (~15 mmHg) < Oncotic pressure (~25 mmHg) → **Reabsorption.** * **Edema Pathophysiology:** Edema is caused by factors favoring filtration: increased Pc (Heart Failure), decreased πc (Nephrotic syndrome/Hypoproteinemia), or increased capillary permeability (Inflammation). * **Lymphatics:** Not all filtered fluid is reabsorbed at the venular end; the excess (~2-4 L/day) is returned to circulation via the lymphatic system.

Question 173: In an isolated muscle piece fiber, repolarization proceeds from which layer to which layer?

A. Epicardium to endocardium
B. Endocardium to epicardium (Correct Answer)
C. Left to right
D. Right to left

Explanation: ### Explanation The direction of electrical activity in the heart depends on whether we are looking at an **intact heart in situ** or an **isolated muscle fiber**. **1. Why Option B is Correct:** In an **isolated muscle fiber** (or a simple strip of cardiac muscle), the sequence of repolarization follows the sequence of depolarization. The first cell to depolarize is the first to recover. Since depolarization spreads from the endocardium to the epicardium, repolarization naturally follows the same path: **Endocardium to Epicardium**. In this scenario, the action potential duration (APD) is uniform across all cells. **2. Why Other Options are Incorrect:** * **Option A (Epicardium to Endocardium):** This is the direction of repolarization in the **intact living heart**. In a whole heart, the epicardial cells have a shorter APD due to a higher density of transient outward potassium currents ($I_{to}$). Additionally, high sub-endocardial pressure during systole delays endocardial recovery. Thus, the epicardium repolarizes first. * **Options C & D:** These refer to lateral conduction across the septum or walls, which is relevant for depolarization (e.g., septal depolarization from left to right), but not the standard transmural sequence of repolarization. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of Opposites":** In an isolated fiber, the T-wave would be inverted (opposite to QRS) because repolarization moves in the same direction as depolarization. * **The Normal ECG:** In a healthy intact heart, the T-wave is **upright** (concordant with QRS) because repolarization moves in the **opposite direction** (Epicardium $\rightarrow$ Endocardium) to depolarization. * **Ventricular Gradient:** The difference in APD between the endocardium and epicardium is what prevents T-wave inversion in a normal ECG.

Question 174: A left shift in the Arneth index indicates which of the following?

A. Anemia
B. Neutrophilia (Correct Answer)
C. Splenomegaly
D. Hyperactive bone marrow

Explanation: **Explanation:** The **Arneth Index** (or Arneth count) is a classification of neutrophils based on the number of lobes in their nuclei. Normally, a mature neutrophil has 2 to 5 lobes. 1. **Why Neutrophilia is correct:** A **"Shift to the Left"** occurs when the bone marrow releases immature neutrophils (band cells or those with fewer than 3 lobes) into the circulation to meet an increased demand, typically during acute bacterial infections or inflammation. This rapid release results in a higher percentage of young cells, leading to **neutrophilia**. 2. **Why other options are incorrect:** * **Anemia:** Relates to red blood cell deficiency, not leukocyte nuclear morphology. * **Splenomegaly:** While the spleen filters blood, it does not directly dictate the nuclear lobulation of circulating neutrophils. * **Hyperactive bone marrow:** While a left shift implies increased production, the term is too broad. Specifically, a left shift is a hallmark of the *myeloid* response (neutrophilia), whereas "hyperactive marrow" could refer to erythroid or megakaryocytic hyperplasia. **High-Yield Clinical Pearls for NEET-PG:** * **Shift to the Right:** An increase in mature neutrophils with more than 5 lobes (hypersegmented neutrophils). This is a classic finding in **Megaloblastic Anemia** (Vitamin B12 or Folate deficiency). * **Normal Arneth Count:** * Stage I (1 lobe): 5% * Stage II (2 lobes): 35% * Stage III (3 lobes): 41% (Most common) * Stage IV (4 lobes): 17% * Stage V (5 lobes): 2% * **Cooke’s Criterion:** A similar classification used to identify megaloblastic changes.

Question 175: Fetal hemoglobin contains which of the following globin chains?

A. alpha 2, beta 2
B. alpha 2, delta 2
C. alpha 2, gamma 2 (Correct Answer)
D. None of the above

Explanation: **Explanation:** **1. Why Option C is Correct:** Fetal hemoglobin (**HbF**) is the primary oxygen-transport protein in the human fetus during the last seven months of development in utero. It is composed of two **alpha (α)** chains and two **gamma (γ)** chains (**α₂γ₂**). The presence of gamma chains is physiologically significant because they lack the binding site for 2,3-Bisphosphoglycerate (2,3-BPG). This results in HbF having a **higher affinity for oxygen** than adult hemoglobin (HbA), allowing the fetus to effectively extract oxygen from maternal blood across the placenta. **2. Why Other Options are Incorrect:** * **Option A (α₂β₂):** This represents **Hemoglobin A (HbA)**, the major form of adult hemoglobin (approx. 97%). It replaces HbF shortly after birth. * **Option B (α₂δ₂):** This represents **Hemoglobin A₂ (HbA₂)**, a minor component of adult hemoglobin (approx. 2-3%). Elevated levels are often seen in Beta-thalassemia trait. **3. NEET-PG High-Yield Clinical Pearls:** * **HbF Switch:** Synthesis of HbF begins to decline at 30 weeks of gestation, and by 6 months of age, it is largely replaced by HbA. * **Oxygen Dissociation Curve:** Due to its high oxygen affinity, the curve for HbF is **shifted to the left** compared to HbA. * **Sickle Cell Anemia:** Hydroxyurea is used in treatment because it increases the production of HbF, which inhibits the polymerization of HbS. * **Embryonic Hemoglobins:** Before HbF, the embryo produces Gower 1 (ζ₂ε₂), Gower 2 (α₂ε₂), and Portland (ζ₂γ₂).

Question 176: The pressure-volume curve is shifted to the left in which of the following conditions?

A. Mitral regurgitation
B. Aortic regurgitation
C. Mitral stenosis
D. Aortic stenosis (Correct Answer)

Explanation: **Explanation:** The **Pressure-Volume (PV) loop** represents the relationship between left ventricular (LV) pressure and volume during a single cardiac cycle. A **shift to the left** indicates a decrease in ventricular volumes (specifically End-Diastolic Volume and End-Systolic Volume). **Why Aortic Stenosis is Correct:** In **Aortic Stenosis (AS)**, the left ventricle must overcome high resistance to eject blood through a narrowed valve. This leads to **concentric LV hypertrophy**, which reduces ventricular compliance and decreases the internal chamber size. Consequently, the PV loop shifts to the left and upward (due to significantly higher systolic pressures). **Analysis of Incorrect Options:** * **Mitral Regurgitation (MR):** This causes volume overload. The LV receives blood from both the pulmonary veins and the regurgitant volume from the previous cycle, leading to eccentric hypertrophy and a **shift to the right**. * **Aortic Regurgitation (AR):** This is the classic example of extreme volume overload. The LV must accommodate the stroke volume plus the blood leaking back from the aorta, causing massive dilation (bovine heart) and a significant **shift to the right**. * **Mitral Stenosis (MS):** In MS, LV filling is impaired. While this reduces volumes, it does not typically cause the compensatory structural "shift" seen in pressure-overload states like AS. The loop simply becomes smaller. **High-Yield NEET-PG Pearls:** * **Shift to the Right:** Seen in Volume Overload (AR, MR, Dilated Cardiomyopathy). * **Shift to the Left:** Seen in Pressure Overload/Reduced Compliance (AS, Hypertension, Hypertrophic Cardiomyopathy). * **Width of the Loop:** Represents Stroke Volume (SV). SV is increased in AR and decreased in Heart Failure. * **Area of the Loop:** Represents Stroke Work. This is highest in Aortic Stenosis due to the extreme pressures required for ejection.

Question 177: What reflex is responsible for tachycardia during right atrial distension?

A. Bezold Jarisch reflex
B. Bain-bridge reflex (Correct Answer)
C. Cushing reflex
D. J reflex

Explanation: ### Explanation The correct answer is the **Bainbridge reflex** (also known as the atrial reflex). **1. Why Bainbridge Reflex is Correct:** The Bainbridge reflex is an autonomic reflex that occurs in response to an increase in venous return. When the **right atrium is distended** (due to increased blood volume), stretch receptors located in the junction of the vena cavae and the right atrium are stimulated. These receptors send afferent signals via the **vagus nerve** to the medulla. The efferent response results in an **increase in heart rate (tachycardia)** by increasing sympathetic activity and inhibiting parasympathetic tone to the SA node. This reflex helps prevent blood from pooling in the venous system. **2. Why Other Options are Incorrect:** * **Bezold-Jarisch Reflex:** This is a "cardio-inhibitory" reflex. It involves chemoreceptors and mechanoreceptors in the ventricles that respond to noxious stimuli or ischemia, leading to a triad of **bradycardia, hypotension, and apnea**. * **Cushing Reflex:** This is a physiological response to **increased intracranial pressure (ICP)**. It presents as a triad of hypertension, bradycardia, and irregular respiration. * **J Reflex (Juxtacapillary Reflex):** These receptors are located in the alveolar walls near pulmonary capillaries. They are stimulated by pulmonary congestion or edema, leading to **rapid shallow breathing (tachypnea)**, bradycardia, and hypotension. **3. High-Yield Clinical Pearls for NEET-PG:** * **Bainbridge vs. Baroreceptor Reflex:** These two often work in opposition. While the Bainbridge reflex increases HR in response to high volume, the Baroreceptor reflex decreases HR in response to high pressure. The final heart rate depends on the net effect of both. * **Reverse Bainbridge:** A decrease in right atrial pressure leads to a decrease in heart rate (seen during hemorrhage). * **Sinus Arrhythmia:** The Bainbridge reflex is partially responsible for the increase in heart rate during inspiration (as inspiration increases venous return).

Question 178: What is the reception for the Bezold-Jarisch reflex?

A. Stretch receptors
B. Mechanical receptors
C. Terminal ends of C fibers (Correct Answer)
D. All of the above

Explanation: The **Bezold-Jarisch reflex** is a cardio-inhibitory reflex characterized by a triad of **bradycardia, hypotension, and apnea**. ### 1. Why "Terminal ends of C fibers" is correct: The receptors for this reflex are located primarily in the **ventricular myocardium** (specifically the inferoposterior wall of the left ventricle). These receptors are the **unmyelinated vagal afferent C-fibers**. When these terminal ends are stimulated by chemical substances (like veratridine, nicotine, or capsaicin) or mechanical triggers (like severe underfilling of the ventricles), they send signals via the vagus nerve to the nucleus tractus solitarius (NTS), leading to increased parasympathetic and decreased sympathetic output. ### 2. Why other options are incorrect: * **A & B (Stretch/Mechanical receptors):** While the reflex can be triggered by mechanical stimuli (e.g., during profound hypovolemia or myocardial infarction), the *specific histological structure* responsible for the reception is the C-fiber ending. "Stretch receptors" usually refers to high-pressure baroreceptors in the carotid sinus or low-pressure receptors in the atria (Bainbridge reflex), which are distinct from the C-fiber mediated Bezold-Jarisch pathway. * **D (All of the above):** This is incorrect because the reflex is specifically defined by its mediation through chemosensitive and mechanosensitive C-fibers, rather than generalized stretch or mechanical receptors found elsewhere in the CVS. ### 3. High-Yield Clinical Pearls for NEET-PG: * **The Triad:** Bradycardia + Hypotension + Apnea (or shallow breathing). * **Clinical Trigger:** It is often seen in **Inferior Wall MI** (due to the high density of C-fibers in the inferoposterior wall) and during **spinal anesthesia** (due to decreased venous return). * **Contrast with Bainbridge Reflex:** While the Bainbridge reflex causes tachycardia in response to increased atrial filling, the Bezold-Jarisch reflex causes bradycardia, often in response to empty, hypercontractile ventricles. * **Chemical Stimuli:** Veratrum alkaloids are the classic experimental triggers.

Question 179: Vagal stimulation decreases heart rate by?

A. Decrease in action potential amplitude
B. Decrease in the slope of prepotential (Correct Answer)
C. Increase in repolarization phase duration
D. Decrease in conduction velocity

Explanation: ### Explanation **1. Why the Correct Answer is Right:** Heart rate is primarily determined by the rate of spontaneous depolarization in the SA node. Vagal (parasympathetic) stimulation releases **Acetylcholine (ACh)**, which acts on **M2 receptors** in the SA node. This triggers two main ionic changes: * **Increased $K^+$ conductance:** ACh opens G protein-coupled inward rectifier $K^+$ channels ($K_{ACh}$), causing hyperpolarization of the resting membrane. * **Decreased $I_f$ (Funny current) and $I_{Ca}$ (Calcium current):** This reduces the rate of diastolic depolarization, effectively **decreasing the slope of the prepotential (pacemaker potential)**. Since the slope is flatter, it takes longer for the membrane potential to reach the threshold, thereby decreasing the heart rate (negative chronotropic effect). **2. Why the Other Options are Wrong:** * **Option A:** Action potential amplitude in the SA node is largely determined by the density of Calcium channels. While vagal stimulation slightly reduces $I_{Ca}$, the primary mechanism for rate control is the change in prepotential slope, not the peak amplitude. * **Option C:** Vagal stimulation actually *shortens* the action potential duration in the atria (by increasing $K^+$ exit), though its effect on the ventricular repolarization phase is minimal. * **Option D:** While vagal stimulation does decrease conduction velocity (negative dromotropic effect), especially at the AV node, this describes a delay in the impulse rather than the mechanism for decreasing the *heart rate* itself. **3. Clinical Pearls for NEET-PG:** * **Prepotential (Pacemaker Potential):** Occurs during Phase 4. The three currents involved are $I_f$ (inward $Na^+$), $I_{Ca-T}$ (transient $Ca^{2+}$), and a decrease in $K^+$ efflux. * **Sympathetic Effect:** Increases heart rate by **increasing the slope** of the prepotential via $\beta_1$ receptors (increased cAMP). * **Vagal Tone:** At rest, the heart is under dominant vagal influence. Atropine (a vagolytic) increases heart rate by blocking this tonic inhibition.

Question 180: A 25-year-old man is participating in a clinical study to determine the cardiovascular response to physical exercise. Basal measurements are: Respiratory rate: 15/min, Blood pressure: 120/80 mmHg, Cardiac output: 5 L/min, Heart rate: 50/min. What is his stroke volume during resting conditions (in mL/min)?

A. 50
B. 75
C. 100 (Correct Answer)
D. 125

Explanation: ### Explanation **1. Why the Correct Answer is Right** The question asks for the **Stroke Volume (SV)**, which is the volume of blood pumped by the left ventricle per heartbeat. The relationship between Cardiac Output (CO), Heart Rate (HR), and Stroke Volume is defined by the formula: $$\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$$ Given in the question: * Cardiac Output (CO) = 5 L/min (which is 5000 mL/min) * Heart Rate (HR) = 50 beats/min To find the Stroke Volume: $$\text{SV} = \frac{\text{CO}}{\text{HR}} = \frac{5000 \text{ mL/min}}{50 \text{ beats/min}} = \mathbf{100 \text{ mL/beat}}$$ **2. Why Incorrect Options are Wrong** * **Option A (50):** This would be the result if the Cardiac Output was only 2.5 L/min or if the Heart Rate was 100/min. * **Option B (75):** This is the average stroke volume for a standard 70 kg male with a heart rate of 70-72 bpm. However, this patient has **sinus bradycardia (50 bpm)**, necessitating a higher stroke volume to maintain a normal cardiac output. * **Option D (125):** This value would result if the heart rate were lower (40 bpm) or the cardiac output higher (6.25 L/min). **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Athletic Heart:** A resting heart rate of 50 bpm in a young man often suggests high vagal tone (common in athletes). To maintain a normal CO of 5 L/min, the heart compensates with an increased SV (Stroke Volume). * **Stroke Volume Determinants:** SV is determined by **Preload** (End-diastolic volume), **Afterload** (Systemic vascular resistance), and **Contractility** (Inotropy). * **Ejection Fraction (EF):** Remember that $EF = \frac{SV}{EDV} \times 100$. A normal EF is typically 55-65%. * **Units Matter:** Always convert Liters to Milliliters (1 L = 1000 mL) before calculating to avoid decimal errors.

Practice by Chapter

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