Cardiovascular System Practice Questions

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

SA node. **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.

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

Increase cardiac output. ### 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.

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

Increased R-R interval. **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**.

Q: Prepotential of the SA node is due to all except:

Fast sodium channels opening. 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.

Q: When does the first heart sound occur?

Isovolumetric ventricular systole.. 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.

Q: What is the mean pulmonary artery pressure?

15 mm Hg. **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.

Question 1

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

Accepted Answer

Lung fluid is forced out of the fetal alveoli with the first few breaths.

Answer

The foramen ovale closes, allowing the entire blood volume to circulate into the pulmonary vein.

Answer

The ductus arteriosus closes.

Answer

The umbilical vein and artery remain functional for several weeks.

Question 2

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

Accepted Answer

SA node

Answer

AV node

Answer

Bundle of His

Answer

Purkinje system

Question 3

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

Accepted Answer

Beta adrenergic blockade

Answer

A decrease in arterial PO2

Answer

An increase in arterial PCO2

Answer

Vagal stimulation

Question 4

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

Accepted Answer

Increase cardiac output

Answer

Increase the diastolic blood pressure

Answer

Reduce the stroke volume

Answer

Increase the mean arterial blood pressure

Question 5

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

Accepted Answer

Increased R-R interval

Answer

Increased heart rate

Answer

Increased cardiac output

Answer

Increased force of contraction

Question 6

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

Accepted Answer

Diastolic blood pressure, as it decreases progressively towards the mid-thoracic aorta.

Answer

Mean arterial pressure, which is responsible for blood flow to organs.

Answer

Pulse pressure, as it relates to stroke volume and aortic compliance.

Answer

Systolic pressure, as it increases in the descending aorta.

Question 7

Prepotential of the SA node is due to all except:

Accepted Answer

Fast sodium channels opening

Answer

Ca2+ spark

Answer

K+ decay

Answer

Transient Ca channel opening

Question 8

When does the first heart sound occur?

Accepted Answer

Isovolumetric ventricular systole.

Answer

Isovolumetric ventricular diastole.

Answer

Protodiastole.

Answer

Ventricular ejection.

Question 9

The SA node acts as the pacemaker of the heart because?

Accepted Answer

Its resting excitability is the highest among all cardiac tissues.

Answer

It is the only excitable node in the heart.

Answer

It is the only node sensitive to vagal stimulation.

Answer

It is the largest pacemaker tissue in the heart.

Question 10

What is the mean pulmonary artery pressure?

Accepted Answer

15 mm Hg

Answer

10 mm Hg

Answer

20 mm Hg

Answer

25 mm Hg

Cardiovascular System — MCQs

Cardiovascular System — MCQs

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