Cardiovascular System Practice Questions

Q: At rest, what amount of oxygen is transferred from blood to tissues?

250 ml/min. ### Explanation The correct answer is **A. 250 ml/min**. **1. Why it is correct:** Oxygen consumption ($VO_2$) at rest is determined by the difference between the arterial oxygen content and the venous oxygen content, multiplied by the Cardiac Output (Fick’s Principle). * **Arterial $O_2$ content ($CaO_2$):** ~20 ml/dL * **Mixed Venous $O_2$ content ($CvO_2$):** ~15 ml/dL * **Arteriovenous $O_2$ difference:** 5 ml/dL (meaning 5 ml of $O_2$ is extracted for every 100 ml of blood). * **Resting Cardiac Output:** ~5 L/min (50 dL/min). * **Calculation:** $50 \text{ dL/min} \times 5 \text{ ml/dL} = \mathbf{250 \text{ ml/min}}$. **2. Why the other options are incorrect:** * **B (500 ml/min):** This value is too high for resting conditions but may be seen during light-to-moderate physical activity. * **C & D (750–1000 ml/min):** These values represent oxygen delivery ($DO_2$) rather than consumption. Total oxygen delivery to tissues at rest is approximately 1000 ml/min ($20 \text{ ml/dL} \times 50 \text{ dL/min}$). Only about 25% of this delivered oxygen is actually extracted by tissues at rest. **3. High-Yield Clinical Pearls for NEET-PG:** * **Utilization Coefficient:** At rest, this is approximately **0.25 (25%)**, meaning tissues extract only a quarter of the available oxygen. During strenuous exercise, this can increase to 75–85%. * **The Heart Exception:** The myocardium is the most efficient extractor; it has a high resting extraction ratio (~70–80%), meaning it cannot significantly increase extraction during stress and must rely on increasing coronary blood flow. * **Basal Metabolic Rate (BMR):** $VO_2$ is a primary surrogate measure for BMR.

Q: Which of the following is NOT a feature of T wave changes in ECG in hypokalemia?

Tall T wave. **Explanation:** In **hypokalemia** (low serum potassium levels), the resting membrane potential of cardiac cells becomes more negative, and the duration of the action potential increases. This primarily affects the repolarization phase (Phase 3), leading to characteristic ECG changes. **Why "Tall T wave" is the correct answer:** Tall, peaked T waves are a hallmark of **Hyperkalemia**, not hypokalemia. In hyperkalemia, the increased extracellular potassium increases the conductance of delayed rectifier K+ channels, shortening the action potential and causing the T wave to become narrow and "tented." Therefore, a tall T wave is NOT a feature of hypokalemia. **Analysis of other options (Features of Hypokalemia):** * **Flat T wave:** As potassium levels drop (typically <3.0 mEq/L), the T wave amplitude decreases, becoming flattened. * **Inverted T wave:** In severe hypokalemia, the T wave may become completely inverted due to altered repolarization sequences. * **Peaked T wave:** While "peaked" usually refers to hyperkalemia, some texts use it to describe the prominent **U waves** seen in hypokalemia, which can sometimes be mistaken for T waves. However, in the context of this question, "Tall/Peaked T waves" as a primary morphology is the classic differentiator for hyperkalemia. **NEET-PG High-Yield Pearls for Hypokalemia ECG:** 1. **ST-segment depression:** Often the first sign. 2. **Decreased T wave amplitude:** Flattening or inversion. 3. **Prominent U waves:** The most characteristic sign (best seen in V2-V4). 4. **Apparent Prolonged QU interval:** Often mistaken for a long QT interval because the T and U waves fuse. 5. **Mnemonic:** "Hypo-low" (Low ST, Low T, but High U).

Q: Chemoreceptors operate between which pressure range?

40-100 mm Hg. **Explanation:** The peripheral chemoreceptors (located in the carotid and aortic bodies) are primarily sensitive to a decrease in $PO_2$, an increase in $PCO_2$, and a decrease in pH. However, they also play a crucial role in blood pressure regulation through the **Chemoreceptor Reflex**. **Why Option B is Correct:** The chemoreceptor reflex is not active under normal physiological blood pressure. It is considered an "emergency mechanism" that kicks in when the Mean Arterial Pressure (MAP) falls below the threshold of the baroreceptor reflex. Specifically, when blood pressure drops below **80–100 mm Hg**, blood flow to the chemoreceptor bodies decreases, leading to local hypoxia and hypercapnia within the receptors. This stimulates the vasomotor center to cause peripheral vasoconstriction, helping to raise blood pressure. The reflex is most potent and active between **40 and 100 mm Hg**. **Analysis of Incorrect Options:** * **Option A:** While chemoreceptors are active below 90 mm Hg, this range is too narrow. They remain functional and vital down to 40 mm Hg. * **Options C & D:** These ranges (70–150 and 70–220 mm Hg) are more characteristic of the **Baroreceptor Reflex**, which is the primary short-term regulator of blood pressure. Baroreceptors are most sensitive at a MAP of around 100 mm Hg and operate between 60 and 180 mm Hg. **High-Yield Clinical Pearls for NEET-PG:** * **Hierarchy of Reflexes:** Baroreceptors are the "First line of defense" (60–180 mm Hg), Chemoreceptors are the "Second line" (40–100 mm Hg), and the **CNS Ischemic Response** is the "Last ditch stand" (active below 60 mm Hg, most powerful below 20 mm Hg). * **Location:** Carotid bodies are located at the bifurcation of the common carotid artery (innervated by Hering’s nerve/CN IX); Aortic bodies are in the aortic arch (innervated by CN X). * **Key Stimulus:** The most potent stimulus for peripheral chemoreceptors is a **decrease in $PO_2$** (hypoxia).

Q: In a patient with a cardiac output of 5 liters/min and a body surface area of 1.7 square meters, what is the cardiac index?

3 L/min/sq. m. ### Explanation **1. Understanding the Correct Answer (A)** The **Cardiac Index (CI)** is a hemodynamic parameter that relates the Cardiac Output (CO) to a patient’s Body Surface Area (BSA). It is a more accurate clinical indicator than cardiac output alone because it accounts for the individual's body size. The formula for Cardiac Index is: $$\text{Cardiac Index (CI)} = \frac{\text{Cardiac Output (CO)}}{\text{Body Surface Area (BSA)}}$$ **Calculation:** * Given CO = 5 L/min * Given BSA = 1.7 m² * $CI = 5 / 1.7 \approx 2.94 \text{ L/min/m}^2$ Rounding to the nearest whole number, the answer is **3 L/min/m²**. **2. Analysis of Incorrect Options** * **Option B (4 L/min/m²):** This value is higher than the calculated result and represents a hyperdynamic state (e.g., sepsis or thyrotoxicosis). * **Option C (5 L/min/m²):** This incorrectly equates the Cardiac Output value with the Cardiac Index, ignoring the BSA correction. * **Option D (2.5 L/min/m²):** While this is within the lower limit of the normal physiological range, it does not match the mathematical calculation provided in the prompt. **3. Clinical Pearls & High-Yield Facts** * **Normal Range:** The normal Cardiac Index is typically **2.5 to 4.0 L/min/m²**. * **Clinical Significance:** A CI below **2.2 L/min/m²** is a diagnostic hallmark of **cardiogenic shock**. * **BSA Calculation:** In clinical practice, BSA is most commonly calculated using the **Mosteller formula** or the **DuBois formula**. * **NEET-PG Tip:** Remember that while CO changes with posture and exercise, the CI allows for a standardized comparison between a small child and a large adult.

Q: Greatest resistance in peripheral blood circulation is due to:

Arterioles. **Explanation:** The correct answer is **Arterioles**. Resistance to blood flow is governed by **Poiseuille’s Law**, which states that resistance is inversely proportional to the fourth power of the radius ($R \propto 1/r^4$). Arterioles are known as the **"resistance vessels"** of the body. While they have a smaller radius than arteries, their primary significance lies in their thick layer of smooth muscle. This allows them to undergo significant changes in diameter (vasoconstriction and vasodilation) under the influence of the sympathetic nervous system and local metabolites. This high muscular-to-lumen ratio creates the greatest drop in mean arterial pressure (approximately 50-60 mmHg) across the entire circulatory circuit. **Why other options are incorrect:** * **Arteries:** These are "conduit vessels" with large radii; therefore, they offer very low resistance to flow. * **Capillaries:** Although an individual capillary has a smaller radius than an arteriole, the **total cross-sectional area** of the capillary bed is massive. Because they are arranged in parallel, the total resistance offered by capillaries is lower than that of arterioles. * **Veins:** These are "capacitance vessels" that hold about 60-70% of the blood volume. They have thin walls and large lumens, offering minimal resistance. **NEET-PG High-Yield Pearls:** * **Site of maximum pressure drop:** Arterioles. * **Site of maximum total cross-sectional area:** Capillaries. * **Site of slowest blood flow velocity:** Capillaries (essential for nutrient exchange). * **Major determinant of Total Peripheral Resistance (TPR):** Arteriolar tone.

Q: What is the autonomic supply to the SA node?

Sympathetic, excitatory. **Explanation:** The Sinoatrial (SA) node is the primary pacemaker of the heart, located in the right atrium. While it possesses intrinsic automaticity, its firing rate is modulated by the **Autonomic Nervous System (ANS)**. **1. Why Option B is Correct:** The SA node receives a rich supply of sympathetic fibers (primarily via the right stellate ganglion). Sympathetic stimulation releases **Norepinephrine**, which acts on **$\beta_1$ receptors**. This increases the slope of the prepotential (pacemaker potential) by increasing $I_f$ (funny current) and $I_{Ca}$ (calcium current), leading to a faster heart rate (**positive chronotropic effect**). Thus, the sympathetic supply is strictly **excitatory** to the heart's rate. **2. Why Other Options are Incorrect:** * **Option A:** While the parasympathetic system (via the Vagus nerve) does supply the SA node, its effect is **inhibitory** (slowing the heart rate). * **Options C & D:** These are incorrect because the ANS does not have a singular effect. The sympathetic system is excitatory, while the parasympathetic system is inhibitory. They work antagonistically to maintain cardiovascular homeostasis. **Clinical Pearls for NEET-PG:** * **Vagal Tone:** At rest, the heart is under dominant parasympathetic (vagal) influence. This is why the resting heart rate (~70 bpm) is lower than the intrinsic SA node rate (~100 bpm). * **Right vs. Left:** The **Right Vagus** nerve primarily supplies the **SA node** (affects rate), while the **Left Vagus** primarily supplies the **AV node** (affects conduction velocity). * **Neurotransmitters:** Sympathetic = Norepinephrine ($\beta_1$ receptors); Parasympathetic = Acetylcholine ($M_2$ receptors).

Question 1

At rest, what amount of oxygen is transferred from blood to tissues?

Accepted Answer

250 ml/min

Answer

500 ml/min

Answer

750 ml/min

Answer

1000 ml/min

Question 2

Which of the following is NOT a feature of T wave changes in ECG in hypokalemia?

Accepted Answer

Tall T wave

Answer

Inverted T wave

Answer

Flat T wave

Answer

Peaked T wave

Question 3

What causes the normal atrioventricular (AV) delay of 0.1 seconds?

Accepted Answer

Decrease in the number of gap junctions between cardiac cells

Answer

Decrease in the amplitude of cardiac action potential firing

Answer

Electrical resistance offered by myocytes

Answer

Lack of tight junctions between cardiac cells

Question 4

Chemoreceptors operate between which pressure range?

Accepted Answer

40-100 mm Hg

Answer

Below 90 mm Hg

Answer

70-150 mm Hg

Answer

70-220 mm Hg

Question 5

Blood flow through a vessel varies directly with which of the following?

Accepted Answer

Difference of pressure

Answer

Resistance

Answer

Viscosity

Answer

Length of the vessel

Question 6

All of the following statements about the third heart sound (S3) are true, except?

Accepted Answer

Occurs due to rapid ventricular filling

Answer

May be heard in mitral regurgitation

Answer

May be heard in Atrial Septal Defect (ASD)

Answer

May be heard in Ventricular Septal Defect (VSD)

Question 7

In a patient with a cardiac output of 5 liters/min and a body surface area of 1.7 square meters, what is the cardiac index?

Accepted Answer

3 L/min/sq. m

Answer

4 L/min/sq. m

Answer

5 L/min/sq. m

Answer

2.5 L/min/sq. m

Question 8

Greatest resistance in peripheral blood circulation is due to:

Accepted Answer

Arterioles

Answer

Arteries

Answer

Veins

Answer

Capillaries

Question 9

What is the autonomic supply to the SA node?

Accepted Answer

Sympathetic, excitatory

Answer

Parasympathetic, inhibitory

Answer

Both parasympathetic and sympathetic, excitatory

Answer

Both parasympathetic and sympathetic, inhibitory

Question 10

What is the mean circulatory filling pressure?

Accepted Answer

Arterial pressure measured at the point when the heart stops beating.

Answer

The difference between systemic and pulmonary arterial pressure.

Answer

The difference between central venous pressure and central arterial pressure.

Answer

Mean atrial pressure.

Cardiovascular System — MCQs

Cardiovascular System — MCQs

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