Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 351: Which one of the following is released by blood platelets during hemorrhage to produce vasoconstriction?

A. Serotonin (Correct Answer)
B. Histamine
C. Thrombosthenin
D. Bradykinin

Explanation: **Explanation:** The correct answer is **Serotonin (5-Hydroxytryptamine)**. **1. Why Serotonin is Correct:** During a hemorrhage, platelets adhere to the damaged vascular endothelium and undergo activation. As part of the "release reaction," platelets degranulate, releasing substances stored in their **dense granules (delta granules)**. Serotonin is a potent **vasoconstrictor** released during this process. Its primary physiological role in hemostasis is to cause local narrowing of the blood vessel, which reduces blood flow to the site of injury, thereby minimizing blood loss and facilitating the formation of a stable platelet plug. **2. Why the Other Options are Incorrect:** * **Histamine:** Primarily released by mast cells and basophils during allergic reactions. It is a potent **vasodilator** (except in the lungs) and increases capillary permeability, which is the opposite of the required response during hemorrhage. * **Thrombosthenin:** This is a contractile protein (similar to actomyosin) found within platelets. Its role is **clot retraction** (shrinking the clot), not vasoconstriction of the blood vessel itself. * **Bradykinin:** A powerful **vasodilator** formed from kininogens. It also increases vascular permeability and mediates pain; it does not assist in the initial vasoconstrictive phase of hemostasis. **3. NEET-PG High-Yield Pearls:** * **Platelet Granules:** * **Alpha granules:** Contain Fibrinogen, vWF, and Platelet-Derived Growth Factor (PDGF). * **Dense (Delta) granules:** Contain **S**erotonin, **A**DP, **C**alcium (the "SAC" mnemonic). * **Thromboxane A2 (TXA2):** Another critical vasoconstrictor and platelet aggregator synthesized by platelets via the COX-1 pathway. * **Initial Response to Injury:** The very first response to vascular injury is **transient neurogenic vasoconstriction**, followed quickly by myogenic and chemical (Serotonin/TXA2) vasoconstriction.

Question 352: Which of the following is an endothelial smooth muscle relaxing factor?

A. Nitric oxide (Correct Answer)
B. Angiotensin
C. Dopamine
D. Vasopressin

Explanation: **Explanation:** The vascular endothelium plays a critical role in regulating vascular tone by secreting paracrine substances. **Nitric Oxide (NO)**, formerly known as **Endothelium-Derived Relaxing Factor (EDRF)**, is the primary mediator of vasodilation produced by endothelial cells. 1. **Why Nitric Oxide is Correct:** NO is synthesized from **L-arginine** by the enzyme endothelial Nitric Oxide Synthase (eNOS). Once released, it diffuses into the adjacent vascular smooth muscle cells and activates **soluble Guanylyl Cyclase (sGC)**. This increases intracellular **cGMP** levels, which leads to protein kinase G activation, sequestration of calcium, and subsequent smooth muscle relaxation (vasodilation). 2. **Why the Other Options are Incorrect:** * **Angiotensin (specifically Angiotensin II):** A potent **vasoconstrictor** produced via the Renin-Angiotensin-Aldosterone System (RAAS). It acts on $AT_1$ receptors to increase blood pressure. * **Dopamine:** While it has complex effects, at high doses it acts on $\alpha_1$ receptors causing **vasoconstriction**. At low doses, it causes vasodilation in renal/mesenteric beds, but it is a catecholamine/neurotransmitter, not a primary "endothelial relaxing factor." * **Vasopressin (ADH):** As the name suggests, it is a powerful **vasoconstrictor** (acting on $V_1$ receptors) released from the posterior pituitary to increase peripheral resistance. **High-Yield Clinical Pearls for NEET-PG:** * **Other Endothelial Vasodilators:** Prostacyclin ($PGI_2$) and Endothelium-derived hyperpolarizing factor (EDHF). * **Endothelial Vasoconstrictors:** **Endothelin-1** (the most potent endogenous vasoconstrictor), Thromboxane $A_2$. * **Pharmacology Link:** Nitroglycerin and Nitroprusside work by releasing NO, mimicking the endogenous EDRF mechanism to treat angina and hypertensive emergencies.

Question 353: Afterload is best determined by?

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

Explanation: **Explanation:** **Afterload** is defined as the "load" or resistance against which the heart must pump to eject blood during systole. In the left ventricle, the primary determinant of afterload is the **Total Peripheral Resistance (TPR)** or Systemic Vascular Resistance. 1. **Why Peripheral Resistance is Correct:** According to Laplace’s Law and basic hemodynamics, the ventricle must generate enough pressure to overcome the resistance offered by the systemic arterioles. When peripheral resistance increases (e.g., via vasoconstriction), the afterload increases, requiring the heart to work harder to open the aortic valve and eject blood. 2. **Why Other Options are Incorrect:** * **End-Diastolic Volume (EDV):** This represents **Preload**, which is the degree of stretch on the ventricular myocardium at the end of diastole (Frank-Starling Law). * **End-Systolic Volume (ESV):** This is the volume of blood remaining in the ventricle after contraction. While afterload affects ESV (high afterload increases ESV), it does not *determine* it. * **Compliance:** This refers to the distensibility of the vessels or heart chambers. While aortic compliance affects systolic blood pressure, it is not the primary clinical determinant of afterload compared to resistance. **High-Yield Clinical Pearls for NEET-PG:** * **Preload** is clinically measured by Pulmonary Capillary Wedge Pressure (PCWP) for the left heart and Central Venous Pressure (CVP) for the right heart. * **Afterload** is clinically represented by Mean Arterial Pressure (MAP) or Systemic Vascular Resistance (SVR). * **Conditions increasing Afterload:** Hypertension, Aortic Stenosis (where the valve itself provides resistance), and Polycythemia (increased viscosity). * **Effect of Afterload:** An acute increase in afterload leads to a **decrease in Stroke Volume** and an increase in myocardial oxygen demand.

Question 354: Cardiac contractility is inhibited by which of the following factors?

A. Digitalis use
B. Respiratory acidosis
C. Metabolic alkalosis
D. Decreased phosphate (↓PO4) (Correct Answer)

Explanation: **Explanation:** **Correct Option: D. Decreased phosphate (↓PO4)** Cardiac contractility (inotropy) is heavily dependent on the availability of **Adenosine Triphosphate (ATP)**. Phosphorus is a critical structural component of ATP and Creatine Phosphate. In states of severe hypophosphatemia, there is a depletion of intracellular ATP stores, which impairs the cross-bridge cycling between actin and myosin filaments. This leads to myocardial depression and decreased contractility, which can clinically manifest as heart failure or difficulty weaning from a ventilator. **Analysis of Incorrect Options:** * **A. Digitalis use:** Digitalis (Digoxin) **increases** contractility. It inhibits the Na+/K+ ATPase pump, leading to an increase in intracellular Na+, which subsequently slows the Na+/Ca2+ exchanger. This results in higher intracellular Ca2+ levels, enhancing inotropy. * **B. Respiratory acidosis:** While acute acidosis (low pH) generally depresses the myocardium by competing with Ca2+ for binding sites on Troponin C, **Respiratory Acidosis** specifically (increased CO2) often triggers a compensatory **sympathetic surge**. This catecholamine release typically maintains or increases contractility in vivo, making it a less definitive inhibitor than electrolyte depletion. * **C. Metabolic alkalosis:** Alkalosis generally does not inhibit contractility; in fact, an increase in pH can slightly increase the calcium sensitivity of myofilaments. **High-Yield NEET-PG Pearls:** * **Positive Inotropes:** Catecholamines (via β1 receptors), Digoxin, Hypercalcemia, and Caffeine. * **Negative Inotropes:** Hypocalcemia, Hyperkalemia, β-blockers, Calcium channel blockers, and severe Hypophosphatemia. * **Bowditch Effect:** An increase in heart rate increases the force of contraction (Treppe phenomenon) due to the accumulation of intracellular calcium.

Question 355: Conduction velocity is maximum in which of the following cardiac structures?

A. Bundle of His
B. Purkinje System (Correct Answer)
C. Ventricular muscles
D. Atrial pathway

Explanation: The conduction velocity in the heart varies significantly across different tissues to ensure coordinated contraction. The correct answer is the **Purkinje System**. ### 1. Why the Purkinje System is Correct The Purkinje fibers possess the **fastest conduction velocity** in the heart (approximately **1.5 to 4.0 m/s**). This high speed is attributed to: * **Large fiber diameter:** Larger cells offer less internal resistance to current flow. * **High density of Gap Junctions:** These allow for rapid ion transfer between cells. * **High density of fast Na+ channels:** Facilitating a rapid Phase 0 of the action potential. * **Significance:** This rapid conduction ensures that the entire ventricular myocardium is depolarized almost simultaneously, allowing for a powerful, synchronized contraction (systole). ### 2. Why Other Options are Incorrect * **Bundle of His (approx. 1.0 m/s):** While fast, it serves as the narrow "bridge" between the atria and ventricles and is slower than its distal branches (Purkinje fibers). * **Atrial pathway (approx. 1.0 m/s):** Internodal pathways conduct faster than the general atrial muscle but are significantly slower than the specialized ventricular system. * **Ventricular muscles (approx. 0.3 – 0.5 m/s):** These are designed for contraction rather than rapid signal transmission, hence their slower velocity compared to specialized conductive tissues. ### 3. High-Yield Facts for NEET-PG * **Slowest Conduction:** The **AV Node** (approx. 0.01 – 0.05 m/s). This "AV nodal delay" allows the ventricles to fill with blood before contraction. * **Hierarchy of Velocity (Fastest to Slowest):** **P**urkinje > **A**tria > **V**entricles > **AV** Node (Mnemonic: **He** **P**urks **A**t **V**ery **AV**ery slow speeds). * **Hierarchy of Pacemaker Rate (Automaticity):** SA Node (70-80 bpm) > AV Node (40-60 bpm) > Purkinje fibers (15-40 bpm). Do not confuse *velocity* with *firing rate*.

Question 356: All of the following statements are false about stroke volume EXCEPT:

A. Determined by afterload
B. Determined by preload (pre-diastolic volume)
C. Decreases with an increase in heart rate (Correct Answer)
D. Increases with an increase in heart rate

Explanation: **Explanation:** Stroke Volume (SV) is the volume of blood pumped by the left ventricle per beat. It is determined by three primary factors: **Preload** (End-Diastolic Volume), **Afterload** (Systemic Vascular Resistance), and **Contractility**. **Why Option C is Correct:** The relationship between heart rate (HR) and SV is inverse at high physiological ranges. As heart rate increases significantly, the duration of **diastole** (specifically the rapid filling phase) decreases. Since the heart has less time to fill with blood, the End-Diastolic Volume (EDV) drops. According to the Frank-Starling Law, a lower EDV leads to a lower Stroke Volume. Therefore, a marked increase in heart rate leads to a decrease in SV. **Analysis of Incorrect Options:** * **Option A & B:** These statements are technically incomplete or incorrectly phrased in the context of "All are false EXCEPT." While SV is *influenced* by afterload and preload, Option B uses the incorrect term "pre-diastolic volume" (the correct term is **End-Diastolic Volume**). * **Option D:** This is the opposite of the physiological reality. While Cardiac Output (CO = SV × HR) may initially increase with HR, the SV itself does not increase; it eventually plateaus and then declines due to shortened filling time. **High-Yield Clinical Pearls for NEET-PG:** * **Frank-Starling Law:** Stroke volume increases in response to an increase in the volume of blood filling the heart (EDV), stretching the myocardial fibers to an optimal length. * **Cardiac Output:** In a healthy individual, CO is maintained during moderate exercise because the increase in HR compensates for the slight fall in SV. * **Afterload:** An increase in afterload (e.g., hypertension) *decreases* stroke volume. * **Contractility:** Positive inotropes (like Digoxin or Adrenaline) increase SV without changing the EDV.

Question 357: All of the following statements are false EXCEPT:

A. The right ventricle does more work than the left ventricle.
B. The left ventricle does more work than the right ventricle. (Correct Answer)
C. Both ventricles do equal work.
D. Maximum ventricular filling occurs during isovolumetric contraction.

Explanation: ### Explanation **Concept: Ventricular Work and Hemodynamics** The work performed by the heart is primarily determined by the **Stroke Work**, which is the product of the Stroke Volume and the Mean Arterial Pressure (Work = Pressure × Volume). **1. Why Option B is Correct:** While both the right and left ventricles pump the same volume of blood (Stroke Volume) to maintain circulatory balance, the **Left Ventricle (LV)** must overcome a much higher systemic resistance compared to the pulmonary resistance faced by the **Right Ventricle (RV)**. * Mean Systemic Arterial Pressure: ~90–100 mmHg * Mean Pulmonary Arterial Pressure: ~15 mmHg Because the LV pumps the same volume against a pressure that is roughly 5–7 times higher, its total work output is significantly greater. This is also why the LV wall is 3 times thicker than the RV wall. **2. Why Other Options are Incorrect:** * **Options A & C:** These are incorrect because they ignore the pressure component of the work equation. Equal volume does not mean equal work when the afterload (pressure) differs so vastly between the systemic and pulmonary circuits. * **Option D:** During **Isovolumetric Contraction**, all valves (AV and Semilunar) are closed. There is no change in volume; therefore, no filling occurs. Maximum ventricular filling actually occurs during the **First Rapid Filling Phase** (early diastole). **NEET-PG High-Yield Pearls:** * **External Work (Stroke Work):** Represents only about 10% of total cardiac energy consumption. * **Internal Work:** The majority of cardiac energy (90%) is used for **isovolumetric contraction** (tension development). * **Law of LaPlace:** Explains why a dilated heart (increased radius) requires more wall tension (and thus more work/oxygen) to generate the same pressure. * **Oxygen Consumption:** The LV has a much higher myocardial oxygen demand ($MVO_2$) than the RV due to this increased workload.

Question 358: What is the typical amount of coronary blood flow per minute?

A. 225 ml
B. 250 ml (Correct Answer)
C. 50 ml
D. 300 ml

Explanation: **Explanation:** The correct answer is **250 ml/min**. In a healthy adult at rest, the coronary blood flow averages approximately **225 to 250 ml/min**. This represents about **4% to 5% of the total cardiac output** (assuming a standard cardiac output of 5 L/min). The myocardium has a high metabolic demand and the highest oxygen extraction ratio in the body (70-80%), necessitating a consistent and robust blood supply even at rest. **Analysis of Options:** * **Option B (250 ml):** This is the standard physiological value cited in major textbooks (like Guyton and Ganong). It reflects the flow required to meet the resting myocardial oxygen consumption ($MVO_2$). * **Option A (225 ml):** While this is within the physiological range, 250 ml is the more frequently tested "classic" value in medical examinations. * **Option C (50 ml):** This value is far too low for the entire heart; however, it is approximately the coronary flow per 100g of heart tissue per minute (the specific flow is ~70-90 ml/min/100g). * **Option D (300 ml):** This value exceeds the typical resting flow, though coronary flow can increase 4 to 5-fold during heavy exercise due to metabolic vasodilation. **High-Yield Facts for NEET-PG:** * **Phasic Flow:** Unlike other organs, the left ventricle receives the majority of its blood flow during **diastole**. During systole, intramyocardial pressure compresses the subendocardial vessels, significantly reducing flow. * **Oxygen Extraction:** The heart cannot increase oxygen extraction much further during stress; therefore, any increase in oxygen demand must be met by a proportional **increase in coronary blood flow** (primarily via adenosine-mediated vasodilation). * **Control:** The most important local metabolic factor controlling coronary blood flow is **Adenosine**.

Question 359: Pressure on the carotid sinus causes which of the following?

A. Hyperapnea
B. Reflex bradycardia (Correct Answer)
C. Tachycardia
D. Dyspnea

Explanation: **Explanation:** The **carotid sinus** is a dilated area at the base of the internal carotid artery that functions as a high-pressure **baroreceptor**. When pressure is applied to the carotid sinus (either via increased arterial blood pressure or external massage), the stretch-sensitive receptors are activated. 1. **Mechanism:** Afferent impulses are carried via the **Hering’s nerve** (a branch of the Glossopharyngeal nerve, CN IX) to the Nucleus Tractus Solitarius (NTS) in the medulla. 2. **Response:** The medulla responds by increasing parasympathetic (vagal) outflow and inhibiting sympathetic activity. The increased vagal tone acts on the SA node to decrease the heart rate, resulting in **reflex bradycardia**, and on the peripheral vasculature to cause vasodilation (hypotension). **Analysis of Incorrect Options:** * **Tachycardia (C):** This is the opposite of the baroreceptor reflex. Tachycardia occurs when baroreceptor firing *decreases* (e.g., during hemorrhage or standing up). * **Hyperapnea (A) and Dyspnea (D):** These relate to the **carotid body**, not the carotid sinus. The carotid body contains *chemoreceptors* that respond to hypoxia (low $PaO_2$), hypercapnia, and acidosis to increase the rate and depth of respiration. **Clinical Pearls for NEET-PG:** * **Carotid Sinus Hypersensitivity:** In some elderly individuals, even minor pressure (like a tight collar) can trigger an exaggerated reflex, leading to syncope. * **Carotid Sinus Massage:** A clinical maneuver used to terminate **Paroxysmal Supraventricular Tachycardia (PSVT)** by increasing vagal tone. * **Mnemonic for Innervation:** **S**inus is **N**ine (CN IX), **A**ortic arch is **T**en (CN X). Both carry afferents to the medulla.

Question 360: Which protein is responsible for preventing the overstretching of cardiac muscle?

A. Actinin
B. Nebulin
C. Myomesin
D. Titin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Titin** (also known as connectin). Titin is the largest known protein in the human body and acts as a molecular spring within the sarcomere. **1. Why Titin is Correct:** Titin extends from the Z-disc to the M-line. Its primary physiological role is to provide **passive elasticity** to the cardiac muscle. When the muscle is stretched, the elastic segments of titin (specifically the PEVK domain) unfold, creating a restorative force that prevents overstretching and assists in the elastic recoil during diastole. In the heart, titin is stiffer than in skeletal muscle, which is crucial for the Frank-Starling mechanism and preventing ventricular over-distension. **2. Why the Other Options are Incorrect:** * **Actinin (α-actinin):** This is a structural protein located in the **Z-discs**. Its primary function is to anchor the actin (thin) filaments to the Z-line; it does not provide elasticity. * **Nebulin:** This protein acts as a "molecular ruler" that regulates the length of actin filaments. While prominent in skeletal muscle, it is largely absent or replaced by **nebulette** in cardiac muscle. * **Myomesin:** This protein is found in the **M-line**. It functions to bind titin and myosin together, maintaining the structural integrity of the thick filaments in the center of the sarcomere. **High-Yield Clinical Pearls for NEET-PG:** * **Titin Mutations:** Mutations in the *TTN* gene are the most common genetic cause of **Dilated Cardiomyopathy (DCM)**. * **Frank-Starling Law:** Titin contributes to "length-dependent activation," increasing the sensitivity of troponin C to calcium as the muscle stretches. * **Third Filament:** Titin is often referred to as the "third filament" of the sarcomere (alongside actin and myosin).

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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