Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 571: Which one of the following neurotransmitters functions to increase cardiac output?

A. GABA
B. Serotonin
C. Norepinephrine (Correct Answer)
D. Glutamate

Explanation: **Explanation:** **Norepinephrine (Option C)** is the correct answer because it is the primary neurotransmitter of the postganglionic sympathetic nervous system. It increases cardiac output through its action on **$\beta_1$-adrenergic receptors** located in the myocardium and the sinoatrial (SA) node. This binding triggers a G-protein-mediated increase in cAMP, leading to: 1. **Positive Inotropy:** Increased force of contraction. 2. **Positive Chronotropy:** Increased heart rate. Since Cardiac Output (CO) = Stroke Volume (SV) × Heart Rate (HR), both mechanisms contribute to a significant rise in CO. **Analysis of Incorrect Options:** * **GABA (Option A):** The primary inhibitory neurotransmitter in the CNS. While it can influence central blood pressure regulation, it does not have a direct peripheral stimulatory effect on cardiac output. * **Serotonin (Option B):** Primarily involved in mood regulation and platelet aggregation. While it has complex effects on the vasculature (vasoconstriction/dilation), it is not the physiological mediator for increasing cardiac output. * **Glutamate (Option D):** The major excitatory neurotransmitter in the CNS. It is involved in synaptic plasticity and memory but does not act directly on the heart to increase output. **High-Yield Clinical Pearls for NEET-PG:** * **Receptor Specificity:** While Norepinephrine acts on $\alpha_1$ and $\beta_1$ receptors, **Epinephrine** (from the adrenal medulla) acts on $\alpha_1, \beta_1,$ and $\beta_2$. * **Parasympathetic Control:** Acetylcholine (ACh) acts on **$M_2$ receptors** to decrease heart rate (negative chronotropy), thereby decreasing cardiac output. * **Bowditch Effect:** An increase in heart rate leads to an increase in the force of contraction due to the inability of Na+/K+ ATPase to keep up, leading to higher intracellular calcium.

Question 572: Thrombin activity is inhibited by which of the following?

A. Chymotrypsin
B. Heparin (Correct Answer)
C. Alpha 2 antitrypsin
D. Alpha 2 macroglobulin

Explanation: ### Explanation **Correct Option: B (Heparin)** Thrombin (Factor IIa) is a central protease in the coagulation cascade. Its activity is primarily regulated by **Antithrombin III (AT-III)**, a naturally occurring plasma protein. Heparin acts as an indirect anticoagulant by binding to AT-III, inducing a conformational change that increases AT-III’s affinity for Thrombin by nearly **1,000-fold**. This complex rapidly inactivates Thrombin and other serine proteases (Factors IXa, Xa, XIa, and XIIa), effectively halting clot formation. **Analysis of Incorrect Options:** * **A. Chymotrypsin:** This is a digestive proteolytic enzyme produced in the pancreas. It functions in the small intestine to break down proteins into peptides and has no physiological role in inhibiting the blood coagulation cascade. * **C. Alpha 2 antitrypsin:** While it is a serine protease inhibitor (Serpin), its primary clinical role is inhibiting **neutrophil elastase** in the lungs. Deficiency leads to emphysema and liver cirrhosis, not coagulation disorders. * **D. Alpha 2 macroglobulin:** This is a broad-spectrum protease inhibitor that can neutralize various enzymes, including thrombin, but its role is minor compared to the potent AT-III/Heparin mechanism. It serves more as a secondary backup inhibitor. **High-Yield NEET-PG Pearls:** * **Mechanism of Action:** Heparin does not dissolve existing clots; it prevents the formation and extension of new clots. * **Monitoring:** The efficacy of Unfractionated Heparin (UFH) is monitored using **aPTT** (Intrinsic pathway). * **Antidote:** The specific antagonist for Heparin overdose is **Protamine Sulfate** (derived from salmon sperm). * **LMWH vs. UFH:** Low Molecular Weight Heparin (e.g., Enoxaparin) has a higher affinity for inhibiting **Factor Xa** than Thrombin (IIa).

Question 573: All of the following statements regarding the heart are true EXCEPT?

A. The nerve fibers that conduct the pain associated with ischemia accompany the sympathetic nerve supply to the heart.
B. The sinoatrial node receives a nerve supply and thus initiates the heart beat autonomously. (Correct Answer)
C. The atrioventricular node is located in the interatrial septum.
D. The diaphragmatic surface of the heart is composed primarily of the left ventricular wall.

Explanation: ### Explanation **1. Why Option B is the Correct (False) Statement:** The statement is a contradiction in terms. While the Sinoatrial (SA) node does receive a rich nerve supply (autonomic nervous system), it **does not** require this nerve supply to initiate the heartbeat. The SA node possesses **intrinsic automaticity** (pacemaker activity) due to the presence of "funny" sodium channels ($I_f$). If all nerves to the heart are severed (as in a heart transplant), the heart will continue to beat autonomously, albeit at a higher intrinsic rate (approx. 100 bpm) because the inhibitory vagal tone is removed. **2. Analysis of Other Options:** * **Option A:** True. Cardiac pain (angina) fibers follow the sympathetic pathways back to the T1–T5 spinal segments. This explains **referred pain** to the chest, left arm, and jaw. * **Option C:** True. The AV node is located in the **Koch’s Triangle** within the posteroinferior part of the interatrial septum, near the opening of the coronary sinus. * **Option D:** True. The diaphragmatic (inferior) surface of the heart sits on the central tendon of the diaphragm and is formed by both ventricles, but primarily by the **left ventricle** (two-thirds). **3. NEET-PG High-Yield Pearls:** * **Pacemaker Hierarchy:** SA Node (60–100 bpm) > AV Node (40–60 bpm) > Purkinje Fibers (20–40 bpm). * **Blood Supply:** The SA node is supplied by the SA nodal artery, which arises from the **Right Coronary Artery (RCA)** in 60% of individuals. * **Vagal Effect:** Right vagus primarily influences the SA node (rate), while the left vagus primarily influences the AV node (conduction). * **Transplanted Heart:** Because it is denervated, it shows a higher resting heart rate and lacks the immediate "fight or flight" tachycardic response.

Question 574: During exercise, what happens to blood flow to the brain?

A. Decreased
B. Increased
C. Unaltered (Correct Answer)
D. Initially increased and then decreases

Explanation: **Explanation:** The correct answer is **Unaltered (C)**. **1. Why it is correct:** During exercise, there is a massive increase in total cardiac output to meet the metabolic demands of skeletal muscles. However, the brain requires a constant, steady supply of oxygen and glucose to maintain consciousness and neurological function. This is achieved through **Cerebral Autoregulation**. Despite fluctuations in systemic blood pressure and the redistribution of blood to muscles and skin, the cerebral blood flow (CBF) remains remarkably constant at approximately **750 mL/min (or 15% of resting cardiac output)**. While the *percentage* of total cardiac output directed to the brain decreases, the *absolute volume* of blood flow remains unchanged. **2. Why other options are incorrect:** * **A. Decreased:** The brain is a vital organ; any significant decrease in flow would lead to syncope (fainting). Autoregulation prevents this during the sympathetic surge of exercise. * **B. Increased:** While blood flow to the heart and skeletal muscles increases significantly, the rigid cranium and autoregulatory mechanisms (myogenic and metabolic) prevent hyperperfusion of the brain. * **D. Initially increased and then decreases:** There is no physiological basis for this biphasic response in a healthy individual during standard exercise. **3. NEET-PG High-Yield Pearls:** * **Most sensitive factor for CBF:** Arterial $PCO_2$. Hypercapnia causes vasodilation (increasing flow), while hypocapnia (hyperventilation) causes vasoconstriction. * **Autoregulation Range:** Cerebral blood flow remains constant between a Mean Arterial Pressure (MAP) of **60 to 140 mmHg**. * **Organ with the highest blood flow per 100g tissue:** Carotid bodies (followed by the Kidneys). * **Organ with the highest oxygen extraction ($A-V O_2$ difference):** The Heart.

Question 575: What is the normal CO2 content in arterial blood?

A. 19 mL/dL
B. 29 mL/dL
C. 36 mL/dL
D. 49 mL/dL (Correct Answer)

Explanation: **Explanation:** The correct answer is **49 mL/dL** (Option D). In cardiovascular physiology, it is essential to distinguish between the partial pressure of a gas ($PCO_2$) and its actual volume content in the blood. **1. Why 49 mL/dL is Correct:** In a healthy adult, arterial blood typically contains approximately **48–50 mL of $CO_2$ per 100 mL (dL)** of blood. This $CO_2$ exists in three forms: dissolved in plasma (approx. 5%), carbamino compounds bound to hemoglobin (approx. 5%), and the majority as bicarbonate ions ($HCO_3^-$) (approx. 90%). When blood passes through systemic tissues, it picks up an additional 4 mL/dL, bringing the **venous $CO_2$ content to approximately 52–54 mL/dL**. **2. Analysis of Incorrect Options:** * **Option A (19 mL/dL):** This value represents the approximate **Oxygen ($O_2$) content** in arterial blood (Normal: 19–20 mL/dL). * **Option B (29 mL/dL):** This is a distractor; however, 24–30 mEq/L is the normal range for plasma bicarbonate concentration, not total gas content. * **Option C (36 mL/dL):** This value does not correspond to standard physiological gas constants in arterial blood. **3. High-Yield NEET-PG Pearls:** * **Arterial-Venous (A-V) Difference:** The A-V difference for $CO_2$ is small (**4 mL/dL**) compared to $O_2$ (**5 mL/dL**). * **Haldane Effect:** Deoxygenation of blood increases its ability to carry $CO_2$. This is crucial in the tissues. * **Chloride Shift (Hamburger Phenomenon):** To maintain electrical neutrality, $Cl^-$ enters RBCs as $HCO_3^-$ leaves them in systemic capillaries. * **Partial Pressures:** Do not confuse *content* with *pressure*. Normal arterial $PCO_2$ is **40 mmHg**, and venous $PCO_2$ is **46 mmHg**.

Question 576: Thrombomodulin-thrombin complex prevents clotting because it:

A. Inhibits prothrombin activator
B. Activates anti-thrombin III
C. Activates protein C, which inactivates activated factors V and VIII, and also removes thrombin (Correct Answer)
D. Activates heparin

Explanation: ### Explanation **1. Why Option C is Correct:** Thrombomodulin is a glycoprotein receptor expressed on the surface of intact vascular endothelial cells. Its primary role is to convert thrombin from a **pro-coagulant** enzyme into an **anti-coagulant** enzyme. When thrombin binds to thrombomodulin, it undergoes a conformational change. This **Thrombomodulin-Thrombin complex** then activates **Protein C** (to Activated Protein C or APC). APC, along with its cofactor Protein S, proteolytically inactivates **Factors Va and VIIIa**, which are essential rate-limiting cofactors in the clotting cascade. Furthermore, by binding to thrombomodulin, thrombin is "sequestered" and removed from the circulation, preventing it from converting fibrinogen to fibrin or activating platelets. **2. Why Other Options are Incorrect:** * **Option A:** Prothrombin activator (Factor Xa, Va, Ca²⁺, and phospholipids) is inhibited primarily by the Tissue Factor Pathway Inhibitor (TFPI), not directly by the thrombomodulin complex. * **Option B:** Antithrombin III (AT-III) is a natural anticoagulant that is activated by **Heparin** or heparin-like molecules on the endothelial surface, not by thrombomodulin. * **Option D:** Heparin is not "activated" by this complex; rather, heparin acts as a catalyst to increase the activity of Antithrombin III. **3. NEET-PG High-Yield Pearls:** * **Protein C & S Deficiency:** Leads to a hypercoagulable state and is a classic cause of **Warfarin-induced skin necrosis** (due to the short half-life of Protein C). * **Factor V Leiden:** The most common inherited hypercoagulability disorder; it involves a mutation where Factor V is resistant to inactivation by Activated Protein C. * **Endothelial Surface:** It is naturally thromboresistant due to three main factors: Glycocalyx (repels platelets), Thrombomodulin (activates Protein C), and Heparin-sulfate (activates AT-III).

Question 577: Which of the following causes vasoconstriction in all vascular beds?

A. PGE2
B. PGF2α
C. PGI2
D. TXA2 (Correct Answer)

Explanation: **Explanation** The correct answer is **TXA2 (Thromboxane A2)**. **Why TXA2 is correct:** Thromboxane A2 is a potent eicosanoid synthesized primarily by platelets via the cyclooxygenase (COX) pathway. Its primary physiological roles are **platelet aggregation** and **potent vasoconstriction**. Unlike other prostaglandins which may have varying effects depending on the tissue or receptor subtype, TXA2 acts via the TP receptor to cause generalized vasoconstriction across all vascular beds, including the renal, coronary, and pulmonary circulations. **Why the other options are incorrect:** * **PGE2 (Prostaglandin E2):** This is primarily a **vasodilator** in most vascular beds. While it can cause constriction in specific areas (like the ductus arteriosus or certain segments of the renal vasculature under specific conditions), it is generally known for its role in inflammatory vasodilation and maintaining the patency of the ductus arteriosus. * **PGF2α (Prostaglandin F2 alpha):** While PGF2α is a vasoconstrictor (especially in the pulmonary and uterine vessels), it is primarily known for its potent **smooth muscle contracting** effects on the uterus and bronchi. It does not exert a universal vasoconstrictive effect across all vascular beds as consistently as TXA2. * **PGI2 (Prostacyclin):** Produced by vascular endothelial cells, PGI2 is the functional antagonist to TXA2. It is a potent **vasodilator** and inhibitor of platelet aggregation. **High-Yield NEET-PG Pearls:** * **Aspirin's Mechanism:** Low-dose aspirin irreversibly inhibits COX-1, shifting the balance in favor of **PGI2 (vasodilator)** over **TXA2 (vasoconstrictor)**, which explains its cardioprotective effect. * **Vasoactive Mnemonic:** Remember **"I"** for **I**nhibition (PGI2 inhibits aggregation/constriction) and **"TX"** for **T**hrombus (TXA2 promotes thrombus/constriction). * **Ductus Arteriosus:** PGE2 keeps it open; NSAIDs (Indomethacin) close it by inhibiting PGE2 synthesis.

Question 578: Nitric oxide (NO) is secreted by which of the following?

A. Endothelium (Correct Answer)
B. Ectoderm
C. Endoderm
D. Bones

Explanation: **Explanation:** **Nitric Oxide (NO)**, formerly known as **Endothelium-Derived Relaxing Factor (EDRF)**, is a potent vasodilator synthesized primarily in the **vascular endothelium**. 1. **Why Endothelium is Correct:** In the vascular system, the enzyme **Endothelial Nitric Oxide Synthase (eNOS)** acts on the amino acid **L-arginine** to produce NO. Once released, NO diffuses into the underlying vascular smooth muscle cells, where it activates **guanylyl cyclase**. This increases intracellular **cGMP**, leading to smooth muscle relaxation and vasodilation. This process is crucial for regulating blood pressure and regional blood flow. 2. **Why Other Options are Incorrect:** * **Ectoderm and Endoderm:** These are primary germ layers formed during embryogenesis. While they give rise to various tissues (e.g., Ectoderm forms the nervous system; Endoderm forms the GI tract lining), they are not direct "secretory sites" for NO in the context of cardiovascular physiology. * **Bones:** Bone tissue provides structural support and mineral storage. While some NO production occurs in bone cells (osteoblasts/osteoclasts) for remodeling, it is not the primary or classic physiological source associated with systemic NO secretion. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor:** L-arginine is the essential amino acid required for NO synthesis. * **Isoforms of NOS:** There are three types: **nNOS** (Neuronal), **iNOS** (Inducible/Macrophages), and **eNOS** (Endothelial). * **Mechanism:** NO → ↑ cGMP → Protein Kinase G → Dephosphorylation of Myosin Light Chain → **Vasodilation**. * **Clinical Link:** Nitroglycerin works by being converted into NO, providing rapid relief in angina pectoris. * **Septic Shock:** Overproduction of NO by **iNOS** (induced by cytokines) leads to the massive peripheral vasodilation seen in sepsis.

Question 579: Pacemaker potential occurs due to which of the following?

A. Opening of transient, rapid T-type Ca2+ channels (Correct Answer)
B. Opening of K+ channels
C. Closure of Ca2+ channels
D. Closure of funny Na+ channels

Explanation: **Explanation:** The **Pacemaker Potential** (also known as the prepotential or Phase 4 depolarization) is the slow, spontaneous depolarization of the SA node that brings the membrane potential to the threshold, triggering an action potential. **1. Why Option A is Correct:** The pacemaker potential occurs in three sequential ionic steps: * **Initial Phase:** Triggered by the opening of **"Funny" Na+ channels ($I_f$)**, which allow sodium influx. * **Late Phase:** As the membrane depolarizes, **T-type (Transient) Ca2+ channels** open. These channels provide the final push (calcium influx) to reach the threshold potential (approx. -40 mV). * Once the threshold is reached, **L-type (Long-lasting) Ca2+ channels** open to cause the actual depolarization (Phase 0). Therefore, the opening of T-type channels is a critical component of the pacemaker potential. **2. Why Other Options are Incorrect:** * **Option B:** Opening of K+ channels causes **repolarization** (Phase 3), as K+ leaves the cell, making the interior more negative. * **Option C:** Closure of Ca2+ channels would stop depolarization; it is the *opening* of these channels that drives the potential toward the threshold. * **Option D:** The prepotential begins with the **opening** of funny Na+ channels, not their closure. **Clinical Pearls & High-Yield Facts:** * **SA Node:** The primary pacemaker because it has the steepest prepotential slope. * **Autonomic Influence:** Sympathetic stimulation increases the slope (faster heart rate), while Parasympathetic (Vagus) stimulation decreases the slope and hyperpolarizes the cell (slower heart rate). * **Ivabradine:** A clinical drug that selectively blocks the **$I_f$ (funny) channels**, used to reduce heart rate in angina and heart failure. * **Phase 0 in SA Node:** Unlike ventricular muscle (where Phase 0 is Na+ dependent), the SA node's Phase 0 is **Ca2+ dependent**.

Question 580: The shape of the arterial pulse is influenced by which of the following?

A. Viscosity of blood
B. Velocity of blood
C. Arterial wall elasticity (Correct Answer)
D. Cross-sectional area of the artery

Explanation: **Explanation:** The shape and contour of the arterial pulse are primarily determined by the **elasticity of the arterial walls** (compliance) and the stroke volume. When the left ventricle ejects blood into the aorta, the energy is stored by the expansion of the elastic arterial walls (Potential Energy). During diastole, the elastic recoil (Windkessel effect) converts this into kinetic energy, maintaining continuous blood flow and shaping the pulse wave. A decrease in elasticity, such as in atherosclerosis, leads to a steeper rise and a higher pulse pressure. **Why other options are incorrect:** * **Viscosity of blood (A):** Viscosity primarily affects peripheral resistance and the *rate* of flow (Poiseuille’s Law) rather than the specific morphological shape of the pressure wave. * **Velocity of blood (B):** While velocity relates to flow, the pulse is a *pressure wave* that travels significantly faster (5–12 m/s) than the actual blood flow (0.5 m/s). Velocity does not dictate the contour of the wave. * **Cross-sectional area (D):** This determines the velocity of flow (Inversely proportional) but does not influence the characteristic dicrotic notch or the upstroke of the arterial pulse. **High-Yield Clinical Pearls for NEET-PG:** * **Anacrotic Notch:** Seen in the ascending limb in Aortic Stenosis. * **Dicrotic Notch:** Caused by the closure of the aortic valve; it marks the beginning of diastole. * **Pulsus Bisferiens:** A double-peaked pulse seen in AR + AS or Hypertrophic Obstructive Cardiomyopathy (HOCM). * **Windkessel Effect:** The process where elastic recoil of central arteries maintains diastolic pressure; loss of this effect causes isolated systolic hypertension in the elderly.

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