Slowest blood flow is seen in?
P wave is due to:
In a healthy person, arterial baroreceptor activity is seen at what stage of the cardiac cycle?
By what percentage can cardiac output increase in a healthy adult during intense physical activity compared to resting levels?
What is the critical closing pressure in the context of capillary physiology?
All are true about baroreceptors, except?
What is the normal mean velocity of blood flow in the aorta?
Duration of maximum contraction depends upon?
What does spermiogenesis refer to?
Which of the following does not stimulate growth hormone (GH) release?
NEET-PG 2013 - Physiology NEET-PG Practice Questions and MCQs
Question 31: 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 32: P wave is due to:
- A. Atrial depolarization (Correct Answer)
- B. Atrial repolarization
- C. Ventricular depolarization
- D. Ventricular repolarization
Explanation: **Atrial depolarization** - The **P wave** on an electrocardiogram (ECG) represents the electrical activity associated with the **depolarization of the atria**. - This depolarization leads to **atrial contraction**, pushing blood into the ventricles. *Atrial repolarization* - **Atrial repolarization** also occurs but is usually hidden within the **QRS complex** and thus not separately visible as a distinct wave on a standard ECG. - While it's an electrical event, it does not produce the P wave. *Ventricular depolarization* - **Ventricular depolarization** is represented by the **QRS complex** on an ECG. - This electrical activity leads to **ventricular contraction**, pumping blood out of the heart. *Ventricular repolarization* - **Ventricular repolarization** is represented by the **T wave** on an ECG. - This process allows the ventricles to relax and refill with blood.
Question 33: In a healthy person, arterial baroreceptor activity is seen at what stage of the cardiac cycle?
- A. None of the options
- B. Diastole
- C. Systole
- D. Both (Correct Answer)
Explanation: ***Both*** - Baroreceptors respond to changes in **arterial pressure**, which fluctuates throughout both systole and diastole. - The baroreflex mechanism is continuously active, monitoring and adjusting blood pressure through changes in **heart rate**, **contractility**, and **vascular resistance** during both phases of the cardiac cycle. *Systole* - While baroreceptors are active during systole due to the **rise in arterial pressure**, they are not exclusively active during this phase. - Their primary role is to detect and respond to the **peak pressure** changes that occur during **ejection**, but their activity extends beyond this. *Diastole* - Baroreceptors continue to fire during diastole, albeit at a lower rate, as blood pressure falls; however, their activity is not limited to this phase alone. - They monitor the **decline in pressure** to help regulate the overall mean arterial pressure, not just the trough. *None of the options* - This option is incorrect because arterial baroreceptors are indeed active and crucial for blood pressure regulation throughout the entire cardiac cycle, encompassing both systole and diastole. - Their continuous monitoring is essential for maintaining **hemodynamic stability**.
Question 34: By what percentage can cardiac output increase in a healthy adult during intense physical activity compared to resting levels?
- A. 300 - 400 % (Correct Answer)
- B. 0 - 50 %
- C. 50 - 100 %
- D. 100 - 200 %
Explanation: ***300 - 400 %*** - In a healthy adult, **cardiac output** can increase remarkably during intense physical activity. - The heart can increase its output by **3 to 4 times** (or 300-400%) above resting levels during peak exertion. - At rest, cardiac output is approximately **5 L/min**, but during maximal exercise, it can reach **20-25 L/min** in well-conditioned individuals. - This represents the heart's **reserve capacity** to meet increased metabolic demands during exercise. *0 - 50 %* - This range represents a very **limited increase** in cardiac output and would be indicative of significant underlying cardiac impairment or **heart failure**. - A healthy individual would experience a much greater increase in cardiac output during intense activity than this small percentage. *50 - 100 %* - This range also suggests a **suboptimal cardiac response** for a healthy adult undergoing intense physical activity. - While some increase is present, it does not reflect the full capacity of a healthy cardiovascular system to adapt to extreme demands. *100 - 200 %* - While a 100-200% increase is substantial, it still **underestimates the maximal capacity** achievable in a healthy, well-conditioned individual during intense physical exertion. - The heart has a greater capacity for increasing its output to meet metabolic demands during peak exercise.
Question 35: What is the critical closing pressure in the context of capillary physiology?
- A. Arterial pressure minus venous pressure
- B. Capillary pressure minus venous pressure
- C. Pressure below which capillaries close (Correct Answer)
- D. None of the options
Explanation: ***Pressure below which capillaries close*** - The **critical closing pressure** is the lowest pressure at which blood can flow through a capillary. - When the luminal pressure falls below this threshold, the capillary collapses due to **extrinsic tissue pressure** and intrinsic vascular tone. *Arterial pressure minus venous pressure* - This calculation represents the **arteriovenous pressure gradient**, which drives blood flow through a vascular bed. - It does not directly define the point at which capillaries collapse. *Capillary pressure minus venous pressure* - This difference primarily influences filtration and reabsorption of fluids across the capillary wall. - It is not directly related to the **critical closing pressure** of the capillaries. *None of the options* - This is incorrect as one of the provided options accurately defines the **critical closing pressure**.
Question 36: All are true about baroreceptors, except?
- A. Stimulated when BP decreases (Correct Answer)
- B. Stimulation causes increased vagal discharge
- C. Stimulate nucleus ambiguus
- D. Afferents are through sino-aortic nerves
Explanation: ***Stimulated when BP decreases*** - Baroreceptors are **stretch receptors** located in the walls of the carotid sinus and aortic arch. - They are stimulated by an **increase in blood pressure (BP)**, which causes stretching of the arterial walls, not by a decrease. *Afferents are through sino-aortic nerves* - This statement is **true**. Afferent impulses from the carotid sinus baroreceptors travel via the **glossopharyngeal nerve (IX)**, and those from the aortic arch baroreceptors travel via the **vagus nerve (X)**. - These nerves collectively form the **sino-aortic nerves** that relay information to the brainstem. *Stimulation causes increased vagal discharge* - This statement is **true**. When baroreceptors are stimulated by **increased BP**, they send signals to the cardiovascular center in the medulla. - This leads to increased **parasympathetic (vagal) outflow** to the heart, causing a decrease in heart rate and contractility, and inhibition of sympathetic outflow. *Stimulate nucleus ambiguus* - This statement is **true**. The **nucleus ambiguus** is a brainstem nucleus that contains the cell bodies of preganglionic parasympathetic neurons that contribute to the vagus nerve. - Baroreceptor stimulation leads to activation of the nucleus ambiguus, thereby increasing **vagal output** to the heart.
Question 37: 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 38: Duration of maximum contraction depends upon?
- A. Both
- B. Absolute refractory period (Correct Answer)
- C. None of the two
- D. Relative refractory period
Explanation: ***Absolute refractory period*** - The duration of **maximum (sustained) contraction** in skeletal muscle depends primarily on the **absolute refractory period** - The absolute refractory period (1-2 ms in skeletal muscle) is much **shorter than the contraction duration** (20-200 ms), allowing for **temporal summation** - When stimuli arrive after the refractory period but before complete relaxation, contractions **summate** to produce **tetanus** (sustained maximum contraction) - A shorter refractory period allows **higher frequency stimulation** → more complete summation → stronger and longer sustained contraction - This is why skeletal muscle can achieve **complete tetanus** at stimulation frequencies of 50-100 Hz *Relative refractory period* - While the relative refractory period affects excitability, it is the **absolute refractory period** that sets the fundamental limit on maximum stimulation frequency - The relative refractory period is less critical for determining the duration of maximum contraction *None of the two* - This is incorrect because the refractory period directly determines the **maximum frequency** at which muscle can be stimulated - Higher stimulation frequency (limited by refractory period) → better temporal summation → sustained maximum contraction (tetanus) - The refractory period is the key factor enabling or limiting the duration of maximum contraction *Both* - While both refractory periods influence excitability, the **absolute refractory period** is the primary determinant - It sets the absolute limit on stimulation frequency and thus the ability to achieve and maintain tetanic contraction
Question 39: What does spermiogenesis refer to?
- A. Formation of spermatozoa from spermatogonia
- B. Formation of spermatozoa from spermatids (Correct Answer)
- C. Formation of spermatids from spermatocytes
- D. Formation of secondary spermatocytes from primary spermatocytes
Explanation: ***Formation of spermatozoa from spermatids*** - **Spermiogenesis** is the final stage of spermatogenesis, involving the remarkable transformation of a round **spermatid** into a motile, mature **spermatozoon**. - This process includes crucial morphological changes such as the formation of the **acrosome**, condensation of the nucleus, development of the flagellum, and shedding of excess cytoplasm. *Formation of spermatozoa from spermatogonia* - This describes the entire process of **spermatogenesis**, which begins with **spermatogonia** and encompasses multiple stages including mitosis, meiosis, and spermiogenesis. - While it's the ultimate outcome, it doesn't specifically define the detailed transformation from spermatid to sperm. *Formation of spermatids from spermatocytes* - This stage refers to **meiosis II**, where **secondary spermatocytes** undergo division to produce **spermatids**. - Spermatids are precursors to spermatozoa and still require significant morphological changes to become mature sperm. *Formation of secondary spermatocytes from primary spermatocytes* - This describes **meiosis I**, where a **primary spermatocyte** divides to form two **secondary spermatocytes**. - This step reduces the chromosome number by half but doesn't involve the final morphological changes seen in spermiogenesis.
Question 40: Which of the following does not stimulate growth hormone (GH) release?
- A. Exercise
- B. Free fatty acids (Correct Answer)
- C. Fasting
- D. Stress
Explanation: ***Free fatty acids*** - High levels of **free fatty acids** in the bloodstream inhibit growth hormone (GH) secretion. - This occurs through a **negative feedback loop** at the level of the hypothalamus and pituitary gland. *Fasting* - **Fasting** (especially prolonged) is a potent stimulus for GH release, helping to mobilize fat stores and maintain **glucose homeostasis**. - During fasting, ghrelin levels increase, which further promotes GH secretion. *Exercise* - **Physical exercise** is a well-known physiological stimulus for GH release, contributing to muscle growth and repair. - The intensity and duration of exercise can influence the magnitude of GH secretion. *Stress* - Various forms of **stress**, including physical (e.g., trauma, surgery) and psychological stress, stimulate GH release. - This response is mediated in part by the **sympathetic nervous system** and increased cortisol levels.