Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 871: When blood pressure falls below 40 mm Hg, which mechanism of regulation is working?

A. CNS ischemic reflex (Correct Answer)
B. Chemoreceptor response
C. Baroreceptor response
D. None of the options

Explanation: ***CNS ischemic reflex*** - The **CNS ischemic reflex** is activated when blood pressure falls below 60 mmHg, with maximal activation below 40 mmHg, indicating severe ischemia in the brain's vasomotor center. - This reflex elicits an intense **sympathetic vasoconstriction** and cardiac stimulation to prioritize blood flow to the brain even at the expense of other organs. *Chemoreceptor response* - The chemoreceptor reflex is primarily activated by a decrease in **arterial pO2**, an increase in **pCO2**, or a decrease in **pH**. - While it can increase blood pressure, it is not the primary or most profound regulatory mechanism specifically triggered by extremely low blood pressure (below 40 mmHg) to prevent brain ischemia. *Baroreceptor response* - **Baroreceptors** are most sensitive to changes in blood pressure within the normal to moderately hypotensive range (e.g., 60-180 mmHg). - At very low pressures (below 40-50 mmHg), baroreceptors become **less sensitive** or "saturated," and their effectiveness in raising blood pressure significantly diminishes. *None of the options* - This option is incorrect because the **CNS ischemic reflex** specifically functions as a powerful, last-ditch mechanism to maintain cerebral blood flow during severe hypotension which is a life saving reflex during conditions like hemorrhage.

Question 872: What physiological mechanism leads to an increase in cardiac output?

A. Inhalation
B. Increased myocardial contractility (Correct Answer)
C. Increased parasympathetic activity
D. Transitioning from a supine to a standing position

Explanation: ***Increased myocardial contractility*** - **Increased myocardial contractility** directly leads to a greater **stroke volume** (the amount of blood pumped with each beat), thus increasing cardiac output (Cardiac Output = Stroke Volume × Heart Rate). - This can be stimulated by factors such as **sympathetic nervous system activation** or positive inotropic agents. *Inhalation* - While inhalation can temporarily affect venous return and intrathoracic pressure, it does not directly or consistently lead to a sustained increase in **cardiac output**. - Its primary effect is on **respiration**, not cardiac performance. *Increased parasympathetic activity* - Increased parasympathetic activity, primarily via the **vagus nerve**, acts to **decrease heart rate** and myocardial contractility. - This effect would typically **reduce cardiac output**, not increase it. *Transitioning from a supine to a standing position* - Transitioning to a standing position usually causes a **temporary decrease in venous return** and a brief drop in cardiac output as blood pools in the lower extremities. - The body then compensates by increasing heart rate and peripheral vascular resistance to maintain blood pressure, but the initial effect on cardiac output is generally a transient decrease.

Question 873: Slowest blood flow is seen in?

A. Arteriole
B. Veins
C. Capillaries (Correct Answer)
D. Venules

Explanation: ***Capillaries*** - Blood flow is slowest in capillaries due to their **large total cross-sectional area**, allowing sufficient time for efficient **exchange of nutrients, gases, and waste products** between blood and tissues. - Despite their individual small diameter, the combined area of millions of capillaries significantly reduces the overall velocity of blood flow. *Arteriole* - **Arterioles** are designed to **regulate blood flow** into capillary beds by constricting and dilating, but blood velocity is still relatively high compared to capillaries. - While smaller than arteries, the **cross-sectional area** of individual arterioles does not collectively exceed that of the major arteries enough to cause the slowest flow rate in the circulatory system. *Veins* - Blood flow in **veins** is generally faster than in capillaries, and is aided by muscle pumps and valves, as they collect blood from the capillary beds. - Although veins have a larger total capacity than arteries, the **velocity of blood flow increases** as blood returns to the heart through progressively larger vessels. *Venules* - **Venules** collect blood from capillaries and begin the return journey to the heart, with blood flow velocity starting to increase as they merge into larger veins. - While slightly faster than in capillaries, the flow in venules is still relatively slow compared to larger veins and arteries, but not the slowest in the system due to their **collecting function and relatively small combined cross-sectional area compared to the entire capillary network**.

Question 874: What is the normal mean velocity of blood flow in the aorta?

A. 100-150 cm/sec
B. 200-250 cm/sec
C. 250-300 cm/sec
D. 40-50 cm/sec (Correct Answer)

Explanation: ***40-50 cm/sec*** - This range represents the **normal mean velocity** of blood flow in the **aorta**, reflecting efficient cardiac output and systemic circulation. - Blood flow velocity can vary slightly based on factors like age, cardiac health, and physical activity, but this range is a common physiological benchmark. *100-150 cm/sec* - This velocity is significantly **higher** than normal for mean aortic flow and would typically indicate a state of **hyperdynamic circulation** or specific pathological conditions. - Such elevated velocities might be seen in conditions like severe **aortic stenosis**, where the heart works harder to push blood through a narrowed valve. *200-250 cm/sec* - This range is **pathologically high** for mean aortic blood flow and is not compatible with normal physiological function. - Velocities in this range would strongly suggest a severe **cardiovascular abnormality**, such as critical **aortic stenosis** or a significant **arteriovenous shunt**. *250-300 cm/sec* - This velocity is **extremely high** and far exceeds any normal or even most pathological mean aortic flow rates found in humans. - Such high velocities would likely be associated with a highly turbulent and severely compromised cardiovascular system, potentially leading to **acute circulatory failure**.

Question 875: Which of the following statements about cardiac muscle is true?

A. Cardiac muscle fibers are arranged in sheets.
B. Cardiac muscle cells have centrally located nuclei. (Correct Answer)
C. Cardiac muscle fibers are spindle-shaped.
D. Cardiac muscle lacks gap junctions.

Explanation: ***Cardiac muscle cells have centrally located nuclei.*** - Unlike **skeletal muscle** cells which have multiple, peripherally located nuclei, cardiac muscle cells typically have one or two **centrally located nuclei**. - This is a key distinguishing histological feature when observing cardiac muscle tissue under a microscope. *Cardiac muscle fibers are arranged in sheets.* - While cardiac muscle forms the walls of the heart, its individual fibers (cells) are **branched** and interconnected, not typically described as being arranged in discrete sheets. - The arrangement allows for a **syncytium-like functionality**, enabling coordinated contraction. *Cardiac muscle fibers are spindle-shaped.* - **Spindle-shaped cells** with a single central nucleus are characteristic of **smooth muscle**, not cardiac muscle. - Cardiac muscle cells are branched and generally cylindrical with blunt ends. *Cardiac muscle lacks gap junctions.* - Cardiac muscle cells are abundant in **gap junctions**, which are critical for electrical coupling and synchronous contraction. - These gap junctions are located within **intercalated discs** and allow for rapid propagation of action potentials between cells.

Question 876: Which of the following statements regarding fetal circulation is correct?

A. The presence of fetal hemoglobin shifts the oxyhemoglobin dissociation curve to the right.
B. The foramen ovale closes before birth.
C. PO2 of fetal blood leaving the placenta is higher than maternal mixed venous PO2.
D. The heart of the fetus receives blood with higher oxygen saturation than maternal mixed venous blood. (Correct Answer)

Explanation: ***The heart of the fetus receives blood with higher oxygen saturation than maternal mixed venous blood.*** - The **umbilical vein** carries oxygenated blood from the placenta with an oxygen saturation of approximately **80-85%**, which is higher than maternal mixed venous blood saturation of approximately **75%**. - Through preferential streaming via the **ductus venosus** and **foramen ovale**, a significant portion of this highly oxygenated blood reaches the **left atrium** and **left ventricle**, ensuring that the fetal heart muscle and brain receive blood with relatively high oxygen saturation. - The **coronary arteries** supplying the fetal heart arise from the ascending aorta, which receives this preferentially oxygenated blood, allowing the fetal myocardium to receive blood with higher oxygen saturation than maternal mixed venous blood. *PO2 of fetal blood leaving the placenta is higher than maternal mixed venous PO2.* - This statement is **INCORRECT**. The **PO2 of fetal blood** leaving the placenta (umbilical vein) is approximately **30-35 mmHg**, which is actually **lower** than maternal mixed venous PO2 of approximately **40 mmHg**. - However, despite the lower PO2, fetal blood has adequate oxygen content due to **fetal hemoglobin (HbF)** having higher oxygen affinity and the higher hemoglobin concentration in fetal blood. *The presence of fetal hemoglobin shifts the oxyhemoglobin dissociation curve to the right.* - This statement is **INCORRECT**. **Fetal hemoglobin (HbF)** has a higher affinity for oxygen than adult hemoglobin (HbA), binding oxygen more readily at lower partial pressures. - This results in a **leftward shift** of the oxyhemoglobin dissociation curve, not a rightward shift, facilitating oxygen uptake from maternal blood across the placenta. *The foramen ovale closes before birth.* - This statement is **INCORRECT**. The **foramen ovale** is an opening between the right and left atria that allows oxygenated blood to bypass the pulmonary circulation in utero. - It remains open throughout fetal life and typically closes **shortly after birth** (within hours to days) due to increased left atrial pressure from increased pulmonary blood flow and decreased right atrial pressure.

Question 877: In pregnancy, plasma volume increase is maximum at what gestational age?

A. 10 wks
B. 20 wks
C. 25 wks
D. 30 wks (Correct Answer)

Explanation: ***30 wks*** - **Plasma volume** typically reaches its maximum expansion around **30-34 weeks of gestation**, increasing by approximately 40-50% compared to pre-pregnancy levels. - This increase is crucial for supporting the **fetoplacental unit**, enhancing nutrient delivery, and protecting against supine hypotension. *10 wks* - At **10 weeks**, the increase in plasma volume is still modest, with significant expansion primarily occurring in the **second trimester**. - Most of the rapid expansion begins after the **first trimester**, around the 12-week mark. *20 wks* - While plasma volume is significantly increasing by **20 weeks**, it has not yet reached its peak. - The continuous expansion continues through the **third trimester** before stabilizing. *25 wks* - At **25 weeks**, plasma volume is substantially elevated, but the maximum expansion is usually observed a few weeks later. - The peak is generally in the **early third trimester**, around 30-34 weeks.

Question 878: What is the primary neural mechanism responsible for vasoconstriction in the skin?

A. Sympathetic (Correct Answer)
B. Parasympathetic
C. Enteric nervous system
D. Somatic nervous system

Explanation: ***Sympathetic nervous system*** - The **sympathetic nervous system** primarily controls blood vessel tone, including vasoconstriction in the skin, through the release of **norepinephrine**. - **Adrenergic receptors** (alpha-1) on vascular smooth muscle cells respond to norepinephrine, leading to contraction and narrowing of the blood vessels. *Parasympathetic nervous system* - The **parasympathetic nervous system** generally has very limited, if any, direct innervation to cutaneous blood vessels for vasoconstriction; its primary role in the cardiovascular system is to decrease heart rate. - While it can cause vasodilation in some tissues, it does not mediate vasoconstriction in the skin. *Somatic nervous system* - The **somatic nervous system** is responsible for voluntary muscle control and transmitting sensory information, not for regulating **autonomic functions** like skin blood flow. - It innervates **skeletal muscles** and sensory receptors directly, lacking connections to cutaneous blood vessels for vasoconstriction. *Enteric nervous system* - The **enteric nervous system** is a complex network of neurons found within the walls of the **gastrointestinal tract**, where it primarily controls digestion. - It does not play any direct role in regulating vasoconstriction in the skin.

Question 879: Which of the following statements about volume receptors is NOT true?

A. They are located in carotid sinus (Correct Answer)
B. They are low pressure receptors
C. They mediate vasopressin release
D. They provide afferents for thirst control

Explanation: ***They are located in carotid sinus*** - Volume receptors, primarily **atrial stretch receptors** and receptors in the **pulmonary vessels**, are located in the low-pressure areas of the circulation, not the carotid sinus. - The carotid sinus primarily contains **baroreceptors** which detect changes in arterial pressure, not blood volume. *They are low pressure receptors* - This statement is true; volume receptors are indeed **low-pressure receptors** found in the atria and great veins. - They primarily monitor **extracellular fluid volume** and central venous pressure. *They provide afferents for thirst control* - This statement is true; when blood volume decreases, the firing rate of these receptors decreases, signaling the **central nervous system** to stimulate thirst. - This is an important mechanism for regulating **fluid intake** and maintaining hydration. *They mediate vasopressin release* - This statement is true; a decrease in blood volume reduces the afferent signaling from volume receptors, which consequently stimulates the release of **vasopressin (ADH)**. - Vasopressin then increases **water reabsorption** in the kidneys to conserve fluid.

Question 880: Which of the following factors increases stroke volume?

A. Increased end-diastolic and end-systolic volumes
B. Decreased end-diastolic and end-systolic volumes
C. Increased end-diastolic volume and decreased end-systolic volume (Correct Answer)
D. Decreased end-diastolic volume and increased end-systolic volume

Explanation: ***Increased end-diastolic volume and decreased end-systolic volume*** - **Stroke volume (SV)** is calculated as **End-Diastolic Volume (EDV)** minus **End-Systolic Volume (ESV)**. Therefore, increasing the volume before contraction while decreasing the volume after contraction will maximize the ejected blood. - A higher **EDV** signifies greater **preload** (more blood filling the ventricle), and a lower **ESV** indicates more complete ejection of blood, often due to increased **contractility** or decreased **afterload**. *Increased end-diastolic and end-systolic volumes* - While an **increased EDV** would tend to increase stroke volume, an **increased ESV** suggests that the heart is ejecting less blood per beat, which would decrease stroke volume. - The combined effect makes it less likely to unequivocally increase stroke volume, as the increase in ESV might offset or even surpass the effect of increased EDV. *Decreased end-diastolic and end-systolic volumes* - Both a **decreased EDV** (less filling) and a **decreased ESV** (more complete ejection) work against each other in terms of stroke volume calculation. - If **EDV** decreases, there's less blood to eject, and if the decrease in **EDV** is proportionally larger than the decrease in **ESV**, stroke volume will decrease. *Decreased end-diastolic volume and increased end-systolic volume* - A **decreased EDV** means less blood is available for ejection, reducing preload and the amount of blood the heart can pump. - An **increased ESV** means the heart is ejecting less blood with each beat, indicating reduced contractility or increased afterload, both of which would decrease stroke volume.

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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