Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 611: Following birth, which of the following changes occur in the fetus?

A. The foramen ovale closes, allowing the entire blood volume to circulate into the pulmonary vein.
B. Lung fluid is forced out of the fetal alveoli with the first few breaths. (Correct Answer)
C. The ductus arteriosus closes.
D. The umbilical vein and artery remain functional for several weeks.

Explanation: **Explanation:** At birth, the transition from intrauterine to extrauterine life involves immediate physiological adaptations. The correct answer is **B** because, during the first few breaths, the high pressure generated by the neonate forces fetal lung fluid out of the alveoli into the lymphatic and pulmonary circulation. This process is aided by the "vaginal squeeze" during delivery and the activation of sodium channels (ENaC) that switch the lung from a fluid-secreting to a fluid-absorbing organ. **Analysis of Options:** * **Option A is incorrect:** While the foramen ovale closes due to increased left atrial pressure, it does not direct blood into the pulmonary vein; rather, it ensures blood flows from the right atrium to the right ventricle and then to the lungs. * **Option C is incorrect:** Although the ductus arteriosus eventually closes, it does not happen immediately "following birth" in a functional sense for all infants; functional closure occurs within 10–15 hours, and anatomical closure takes weeks. Option B is the more immediate and fundamental physiological change. * **Option D is incorrect:** The umbilical vessels constrict almost immediately after birth due to thermal and mechanical stimuli and the rise in oxygen tension, becoming non-functional remnants (e.g., ligamentum teres). **High-Yield NEET-PG Pearls:** * **Transient Tachypnea of the Newborn (TTN):** Caused by delayed clearance of fetal lung fluid, commonly seen in Cesarean sections where the "vaginal squeeze" is absent. * **First Breath Stimuli:** Triggered by hypoxia, hypercapnia, and tactile cooling. * **Closure Sequence:** Foramen Ovale (functional closure is immediate due to $\uparrow$ Left Atrial pressure); Ductus Arteriosus (mediated by $\uparrow$ $O_2$ and $\downarrow$ Prostaglandin E2).

Question 612: Rate of impulse generation is maximum in which part of the cardiac conduction system?

A. SA node (Correct Answer)
B. AV node
C. Bundle of His
D. Purkinje system

Explanation: **Explanation:** The correct answer is **A. SA node**. The cardiac conduction system consists of specialized cells capable of **autorhythmicity** (spontaneous depolarization). The part of the heart with the highest intrinsic rate of impulse generation acts as the **dominant pacemaker**, as it reaches the threshold for an action potential first and resets the other potential pacemakers. 1. **SA Node (Sinoatrial Node):** Located in the right atrium, it has the highest firing rate, typically **60–100 beats per minute (bpm)**. It is known as the "Primary Pacemaker" of the heart because its rapid rate suppresses the slower intrinsic rhythms of distal structures (a phenomenon called overdrive suppression). 2. **AV Node (Atrioventricular Node):** This acts as the "Secondary Pacemaker." Its intrinsic rate is slower, approximately **40–60 bpm**. It also provides the critical "AV nodal delay" to allow for ventricular filling. 3. **Bundle of His:** This has an even slower intrinsic rate of about **30–40 bpm**. 4. **Purkinje System:** These fibers have the slowest rate of impulse generation (**15–30 bpm**) but possess the **fastest conduction velocity** (approx. 4 m/s) to ensure near-simultaneous ventricular contraction. **High-Yield NEET-PG Pearls:** * **Hierarchy of Pacemakers:** SA Node > AV Node > Bundle of His > Purkinje fibers. * **Conduction Velocity Order:** Purkinje fibers (Fastest) > Atria > Ventricles > AV Node (Slowest). * **Clinical Note:** If the SA node fails, the AV node takes over (Junctional rhythm). If both fail, a ventricular escape rhythm (Idioventricular rhythm) from the Purkinje system may occur, which is usually insufficient to maintain adequate cardiac output.

Question 613: Coronary blood flow is increased by all of the following except?

A. Beta adrenergic blockade (Correct Answer)
B. A decrease in arterial PO2
C. An increase in arterial PCO2
D. Vagal stimulation

Explanation: **Explanation:** The coronary circulation is primarily regulated by **local metabolic demand** rather than neural control. The correct answer is **Beta-adrenergic blockade**, as it is the only factor listed that **decreases** coronary blood flow. **1. Why Beta-adrenergic blockade is correct:** Beta-1 receptors are located on the myocardium. Blocking them (using Beta-blockers) leads to a decrease in heart rate (negative chronotropy) and contractility (negative inotropy). This reduces the myocardial oxygen demand ($MVO_2$). Since coronary blood flow is tightly coupled to metabolic demand, a decrease in $MVO_2$ results in secondary vasoconstriction and reduced coronary flow. **2. Why the other options are incorrect:** * **Decrease in arterial $PO_2$ (Hypoxia):** This is the most potent stimulator of coronary vasodilation. Hypoxia leads to the release of **adenosine**, which causes profound vasodilation to increase oxygen delivery. * **Increase in arterial $PCO_2$ (Hypercapnia):** Increased $CO_2$ and the resulting acidosis act as local metabolic vasodilators, increasing blood flow to wash out metabolic byproducts. * **Vagal Stimulation:** While the direct effect of Acetylcholine on coronary vessels is mild vasodilation, the primary reason this increases flow in a physiological context is by increasing the **diastolic filling time** (via bradycardia). Since the left ventricle is perfused almost entirely during diastole, a longer diastolic phase allows for increased coronary perfusion. **High-Yield NEET-PG Pearls:** * **Adenosine** is the most important local metabolic regulator of coronary blood flow. * **Lactate, Potassium ions, and Nitric Oxide** also act as coronary vasodilators. * The **Left Ventricle** receives its blood supply primarily during **diastole**, whereas the Right Ventricle receives flow during both systole and diastole. * **Coronary Steal Phenomenon:** Potent vasodilators (like Dipyridamole) can divert blood away from ischemic zones toward non-ischemic zones.

Question 614: A sudden increase in total peripheral resistance has all of the following effects EXCEPT:

A. Increase the diastolic blood pressure
B. Reduce the stroke volume
C. Increase the mean arterial blood pressure
D. Increase cardiac output (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Increase cardiac output.** **1. Why Option D is correct (The Concept):** Total Peripheral Resistance (TPR) is the resistance against which the left ventricle must pump to eject blood (Afterload). According to the formula **Mean Arterial Pressure (MAP) = Cardiac Output (CO) × TPR**, if TPR increases suddenly, the heart must work against a higher resistance. This increase in afterload leads to a **decrease in Stroke Volume (SV)**, which subsequently **decreases Cardiac Output**. Therefore, a sudden increase in TPR cannot increase cardiac output; it typically reduces it or keeps it stable at the cost of increased myocardial oxygen demand. **2. Why the other options are incorrect:** * **A. Increase the diastolic blood pressure:** Diastolic blood pressure is primarily determined by the tone of the arterioles (TPR). When TPR increases, the rate at which blood leaves the arterial system during diastole decreases, leading to a higher diastolic pressure. * **B. Reduce the stroke volume:** As mentioned, TPR is a major component of afterload. An increase in afterload increases the end-systolic volume, thereby reducing the stroke volume. * **C. Increase the mean arterial blood pressure:** Since MAP is directly proportional to TPR (MAP = CO × TPR), an increase in resistance leads to an immediate rise in arterial pressure. **3. NEET-PG High-Yield Pearls:** * **Afterload vs. CO:** In a healthy heart, the Frank-Starling mechanism may partially compensate, but in a failing heart, an increase in TPR significantly drops CO. * **Determinants of BP:** Systolic BP is primarily determined by **Stroke Volume** and aortic compliance, while Diastolic BP is primarily determined by **TPR** and heart rate. * **Vessel Type:** The **Arterioles** are the primary site of TPR in the systemic circulation.

Question 615: Which of the following physiological changes can occur when there is vagal stimulation of the heart?

A. Increased R-R interval (Correct Answer)
B. Increased heart rate
C. Increased cardiac output
D. Increased force of contraction

Explanation: **Explanation:** Vagal stimulation involves the release of **Acetylcholine (ACh)** from the parasympathetic nerve endings (Vagus nerve). ACh binds to **M2 muscarinic receptors** in the Sinoatrial (SA) node, leading to the opening of K+ channels (GIRK channels) and inhibition of Adenylyl Cyclase. This causes hyperpolarization and a decreased rate of phase 4 spontaneous depolarization, resulting in **Bradycardia** (decreased heart rate). 1. **Why Option A is Correct:** The **R-R interval** on an ECG represents the time between two successive heartbeats. Since vagal stimulation slows the heart rate, the time between beats increases, leading to an **increased R-R interval**. 2. **Why Options B & C are Incorrect:** Vagal stimulation decreases the heart rate (Negative Chronotropy). Since Cardiac Output (CO) is the product of Heart Rate and Stroke Volume ($CO = HR \times SV$), a decrease in heart rate typically leads to a **decreased cardiac output**. 3. **Why Option D is Incorrect:** Vagal fibers primarily innervate the SA and AV nodes with sparse innervation to the ventricles. While it has a weak negative inotropic effect on the atria, it does **not increase** the force of contraction. **NEET-PG High-Yield Pearls:** * **Vagal Tone:** At rest, the heart is under constant inhibitory influence of the vagus nerve. If the vagus is blocked (e.g., by Atropine), the resting heart rate increases to the intrinsic rate of the SA node (~100 bpm). * **Vagal Escape:** If the vagus is stimulated intensely, the heart may stop, but it eventually starts beating again at a slow rate due to the emergence of a ventricular pacemaker. * **AV Node:** Vagal stimulation also decreases conduction velocity through the AV node (Negative Dromotropy), which can increase the **P-R interval**.

Question 616: Peripheral vascular resistance is best represented by which of the following parameters?

A. Mean arterial pressure, which is responsible for blood flow to organs.
B. Diastolic blood pressure, as it decreases progressively towards the mid-thoracic aorta. (Correct Answer)
C. Pulse pressure, as it relates to stroke volume and aortic compliance.
D. Systolic pressure, as it increases in the descending aorta.

Explanation: **Explanation:** Peripheral Vascular Resistance (PVR) is the resistance offered by the systemic vasculature to the flow of blood, primarily determined by the diameter of arterioles (the "resistance vessels"). **Why Option B is Correct:** Diastolic Blood Pressure (DBP) is the best clinical indicator of PVR. During diastole, the heart is not ejecting blood; therefore, the pressure maintained in the arterial system depends entirely on the elastic recoil of the large arteries and the resistance to outflow provided by the peripheral arterioles. A decrease in DBP as blood moves toward the periphery (and mid-thoracic aorta) reflects the continuous dissipation of energy against vascular resistance. If PVR increases (e.g., via vasoconstriction), DBP rises because blood leaves the arterial tree more slowly. **Analysis of Incorrect Options:** * **Option A:** Mean Arterial Pressure (MAP) represents the average perfusion pressure throughout the cardiac cycle. While it is calculated using DBP ($MAP = DBP + 1/3 Pulse Pressure$), it is a measure of tissue perfusion rather than a specific index of resistance. * **Option C:** Pulse Pressure (Systolic – Diastolic) is primarily determined by **Stroke Volume** and **Aortic Compliance**. It is not a direct measure of PVR. * **Option D:** Systolic Blood Pressure (SBP) is mainly influenced by the force of ventricular contraction and the stroke volume. While SBP may increase in peripheral arteries due to "pressure wave reflection," it does not represent resistance. **High-Yield NEET-PG Pearls:** * **Primary Site of PVR:** The **Arterioles** (due to their small lumen and thick muscular walls). * **Poiseuille’s Law:** Resistance is inversely proportional to the fourth power of the radius ($R \propto 1/r^4$). Small changes in vessel diameter cause massive changes in PVR. * **Clinical Correlation:** In essential hypertension, the primary hemodynamic abnormality is an increase in PVR, which is clinically manifested as an elevated Diastolic Blood Pressure.

Question 617: Prepotential of the SA node is due to all except:

A. Ca2+ spark
B. Fast sodium channels opening (Correct Answer)
C. K+ decay
D. Transient Ca channel opening

Explanation: The **prepotential** (or pacemaker potential) is the slow, spontaneous depolarization of the SA node that occurs during Phase 4 of the action potential. This phase is responsible for the heart's automaticity. ### Why "Fast Sodium Channels Opening" is Correct Fast sodium channels ($I_{Na}$) are responsible for the rapid depolarization (Phase 0) in **ventricular and atrial myocytes**. However, in the SA node and AV node, these channels are **absent or permanently inactivated** due to the relatively less negative resting membrane potential (approx. -60 mV). Therefore, fast sodium channels play no role in the SA node's prepotential or its action potential. ### Explanation of Other Options * **K+ decay (Option C):** This is the initial trigger for the prepotential. As the cell repolarizes, potassium efflux decreases, reducing the outward positive charge and allowing the membrane potential to drift upward. * **Transient Ca2+ channel opening (Option D):** T-type (Transient) calcium channels open toward the end of the prepotential, providing the final push to reach the threshold. * **Ca2+ spark (Option A):** Localized releases of calcium from the sarcoplasmic reticulum (via ryanodine receptors) occur during late diastole. This "calcium clock" mechanism activates the Na+-Ca2+ exchanger (NCX), contributing to the depolarizing current. ### High-Yield NEET-PG Pearls * **Funny Current ($I_f$):** The prepotential is primarily initiated by $I_f$, a slow inward sodium current through HCN channels. * **Phase 0 in SA Node:** Unlike myocytes, the upstroke in the SA node is due to **L-type (Long-lasting) Ca2+ channels**, not fast sodium channels. * **Parasympathetic Effect:** Acetylcholine increases $K^+$ conductance and decreases $I_f$, hyperpolarizing the cell and slowing the heart rate. * **Sympathetic Effect:** Norepinephrine increases $I_f$ and $Ca^{2+}$ currents, increasing the slope of the prepotential and heart rate.

Question 618: When does the first heart sound occur?

A. Isovolumetric ventricular systole. (Correct Answer)
B. Isovolumetric ventricular diastole.
C. Protodiastole.
D. Ventricular ejection.

Explanation: The first heart sound (**S1**) is produced primarily by the closure of the Atrioventricular (AV) valves (Mitral and Tricuspid) at the onset of ventricular systole. ### Why the Correct Answer is Right: **Isovolumetric Ventricular Systole** is the phase of the cardiac cycle that begins immediately after the AV valves close. As the ventricles begin to contract, the intraventricular pressure rises sharply, exceeding atrial pressure and forcing the AV valves shut. This sudden cessation of blood flow and the resulting vibrations of the valves and ventricular walls produce S1. It marks the beginning of systole and coincides with the **R-wave** on an ECG and the upstroke of the carotid pulse. ### Why the Other Options are Wrong: * **B. Isovolumetric Ventricular Diastole:** This phase occurs at the beginning of diastole, immediately following the closure of the Semilunar valves (Aortic and Pulmonary), which produces the **second heart sound (S2)**. * **C. Protodiastole:** This is the very brief initial phase of diastole before the semilunar valves close. It is not associated with a specific heart sound. * **D. Ventricular Ejection:** This occurs after isovolumetric contraction when ventricular pressure exceeds aortic/pulmonary pressure, forcing the semilunar valves open. This phase is normally silent. ### NEET-PG High-Yield Pearls: * **S1 Components:** M1 (Mitral closure) occurs slightly before T1 (Tricuspid closure). * **Best Heard:** S1 is loudest at the **apex** (mitral area). * **Frequency:** S1 is lower in pitch and longer in duration ("Lubb") compared to S2 ("Dupp"). * **Clinical Correlation:** A loud S1 is classically seen in **Mitral Stenosis**, while a soft S1 is seen in **Mitral Regurgitation** or heart failure.

Question 619: The SA node acts as the pacemaker of the heart because?

A. It is the only excitable node in the heart.
B. Its resting excitability is the highest among all cardiac tissues. (Correct Answer)
C. It is the only node sensitive to vagal stimulation.
D. It is the largest pacemaker tissue in the heart.

Explanation: **Explanation:** The SA (Sinoatrial) node is designated as the **"Primary Pacemaker"** of the heart due to the principle of **overdrive suppression**. While multiple tissues in the heart possess intrinsic rhythmicity (automaticity), the SA node has the **highest rate of spontaneous diastolic depolarization** (Phase 4). In physiological conditions, the SA node fires at a rate of 60–100 bpm, which is faster than the AV node (40–60 bpm) or the Purkinje fibers (20–40 bpm). Because it reaches the threshold potential first, it triggers an action potential that spreads through the myocardium and resets other potential pacemakers before they can fire spontaneously. Thus, its "highest excitability" or intrinsic firing rate allows it to control the cardiac rhythm. **Analysis of Incorrect Options:** * **Option A:** Incorrect. Other tissues like the AV node, Bundle of His, and Purkinje fibers are also excitable and possess automaticity (latent pacemakers). * **Option C:** Incorrect. Both the SA node and the AV node are richly supplied by the vagus nerve. Vagal stimulation slows the heart rate (negative chronotropy) and decreases AV conduction velocity (negative dromotropy). * **Option D:** Incorrect. The size of the tissue does not determine its pacemaking priority; the rate of Phase 4 depolarization does. **High-Yield NEET-PG Pearls:** * **Location:** The SA node is located at the junction of the superior vena cava and the right atrium (subepicardial). * **Ionic Basis:** The "pacemaker potential" (Phase 4) is primarily due to **Funny currents ($I_f$)** through HCN channels (sodium influx) and T-type calcium channels. * **Blood Supply:** In 60% of individuals, the SA nodal artery arises from the **Right Coronary Artery (RCA)**. * **Ectopic Pacemaker:** If the SA node fails, the AV node typically takes over, resulting in a "nodal rhythm."

Question 620: What is the mean pulmonary artery pressure?

A. 10 mm Hg
B. 15 mm Hg (Correct Answer)
C. 20 mm Hg
D. 25 mm Hg

Explanation: **Explanation:** The pulmonary circulation is a **low-pressure, low-resistance system** compared to the systemic circulation. The normal pressures in the pulmonary artery are approximately **25 mm Hg (systolic)** and **8 mm Hg (diastolic)**. The **Mean Pulmonary Artery Pressure (mPAP)** is calculated using the formula: $mPAP = Diastolic + 1/3 (Systolic – Diastolic)$ $mPAP = 8 + 1/3 (25 – 8) = 8 + 5.6 = 13.6 \text{ mm Hg}$ In clinical practice and standard physiological texts (like Guyton), the normal mPAP is rounded to **15 mm Hg**. **Analysis of Options:** * **Option A (10 mm Hg):** This is too low for a mean pressure; however, it is closer to the normal **Left Atrial Pressure (LAP)**, which averages around 5–10 mm Hg. * **Option B (15 mm Hg):** **Correct.** This represents the physiological average for a healthy adult at rest. * **Option C (20 mm Hg):** This is the upper limit of normal. Pressures between 20–24 mm Hg are considered "borderline." * **Option D (25 mm Hg):** This is the normal **Systolic** Pulmonary Artery Pressure. If the *mean* pressure exceeds 25 mm Hg at rest, it is the diagnostic threshold for **Pulmonary Hypertension**. **High-Yield NEET-PG Pearls:** 1. **Pulmonary Hypertension Definition:** mPAP >20 mm Hg (Updated guidelines) or >25 mm Hg (Classic definition). 2. **Driving Pressure:** The pressure gradient required to move blood through the lungs is only ~10 mm Hg (mPAP minus Left Atrial Pressure). 3. **Vascular Resistance:** Pulmonary vascular resistance is roughly **1/10th** of systemic vascular resistance. 4. **Hypoxic Vasoconstriction:** Unlike systemic vessels, pulmonary arterioles **constrict** in response to low $O_2$ to shunt blood to better-ventilated areas.

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