Which of the following best describes hypoxic pulmonary vasoconstriction?
What physiological mechanism leads to an increase in cardiac output?
What is the minimum fluid urine output for neutral solute balance?
Fever increases water loss by how much for each degree Celsius increase in body temperature?
Which of the following statements about breathing is incorrect?
Which of the following stimuli is primarily responsible for triggering the Bezold-Jarisch reflex?
What is the air remaining in the lung after normal expiration?
Maximum voluntary ventilation is:
Which equation is used to calculate physiological dead space?
In a healthy person, arterial baroreceptor activity is seen at what stage of the cardiac cycle?
NEET-PG 2013 - Physiology NEET-PG Practice Questions and MCQs
Question 11: Which of the following best describes hypoxic pulmonary vasoconstriction?
- A. Reversible pulmonary vasoconstriction due to hypoxia (Correct Answer)
- B. Irreversible pulmonary vasoconstriction due to hypoxia
- C. Redirects blood to well-ventilated areas
- D. Occurs immediately in response to hypoxia
Explanation: ***Reversible pulmonary vasoconstriction due to hypoxia*** - Hypoxic pulmonary vasoconstriction (HPV) is a physiological response in which **pulmonary arterioles constrict** in areas of the lung with low oxygen levels. - This mechanism is **reversible**, meaning that when oxygen levels improve, the constricted vessels will dilate again. - The underlying mechanism involves hypoxia-induced inhibition of voltage-gated K⁺ channels in pulmonary arterial smooth muscle, leading to membrane depolarization, Ca²⁺ influx, and smooth muscle contraction. *Irreversible pulmonary vasoconstriction due to hypoxia* - This statement is incorrect because HPV is fundamentally a **reversible process**, designed to adapt to transient changes in alveolar oxygen. - Irreversible vasoconstriction typically occurs in chronic hypoxia, leading to **pulmonary hypertension** and structural remodeling (vascular remodeling with medial hypertrophy), which is a pathological state rather than the acute physiological response of HPV. *Redirects blood to well-ventilated areas* - While this is the **physiological purpose** and overall effect of hypoxic pulmonary vasoconstriction, it describes the functional outcome rather than what HPV fundamentally is. - The redirection of blood flow is the **consequence** of vasoconstriction in hypoxic areas, which optimizes ventilation-perfusion matching. *Occurs immediately in response to hypoxia* - While HPV does begin rapidly in response to hypoxia (within seconds to minutes), this describes the **timing characteristic** rather than what HPV is. - This statement is also somewhat imprecise, as the response involves intracellular signaling pathways that take time to manifest fully, though the onset is relatively quick compared to other vascular responses.
Question 12: 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 13: What is the minimum fluid urine output for neutral solute balance?
- A. 300 ml
- B. 750 ml
- C. 500 ml
- D. 400 ml (Correct Answer)
Explanation: ***400 ml*** - The kidneys must excrete approximately **600 mOsm of solutes daily** to maintain neutral solute balance. - With a maximum urine concentrating ability of **1200-1400 mOsm/L**, the minimum volume required is calculated as: 600 mOsm ÷ 1400 mOsm/L = **428 ml**. - Therefore, **400 ml** is the conventionally accepted minimum urine output for neutral solute balance. - Below this volume, even with maximal concentration, solute excretion would be inadequate. *300 ml* - **300 ml** would be insufficient to excrete the 600 mOsm daily solute load even at maximal concentration (300 × 1400 = 420 mOsm only). - This volume would lead to accumulation of solutes and **azotemia** (elevated BUN and creatinine). *500 ml* - While **500 ml** would certainly be adequate for solute excretion, it exceeds the calculated minimum of ~428 ml. - The question asks for the *minimum* volume, making **400 ml** the more precise answer according to standard textbooks. *750 ml* - **750 ml** is well above the minimum required for neutral solute balance. - This volume represents normal physiological urine output but is not the minimum threshold for maintaining solute balance.
Question 14: Fever increases water loss by how much for each degree Celsius increase in body temperature?
- A. 100 ml/day
- B. 200 ml/day (Correct Answer)
- C. 400 ml/day
- D. 800 ml/day
Explanation: ***200 ml/day*** - For every 1-degree Celsius (or 1.8-degree Fahrenheit) increase in body temperature, there is an approximate **200 ml increase in insensible water loss** per day due to increased metabolism and sweating. - This value highlights the importance of **adequate fluid replacement** in febrile patients to prevent dehydration. *100 ml/day* - This value is **insufficient** to account for the increased insensible fluid losses associated with fever. - Using this estimate could lead to **underestimation of fluid requirements** and potential dehydration in febrile patients. *400 ml/day* - This value is **higher than the typical estimated increase** in water loss per degree Celsius of fever. - While extreme fever might cause higher losses, 200 ml/day is the standard clinical approximation for a 1-degree rise. *800 ml/day* - This value represents a **significant overestimation** of the fluid loss per degree Celsius increase in fever. - Such a high estimate would generally be seen only in very severe conditions or with much larger temperature increases.
Question 15: Which of the following statements about breathing is incorrect?
- A. Inspiration is an active process
- B. Normal breathing occurs when transpulmonary pressure is 5-8 cm H2O (Correct Answer)
- C. Expiration during quiet breathing is passive
- D. Compliance is influenced by multiple factors including surfactant.
Explanation: ***Normal breathing occurs when transpulmonary pressure is 5-8 cm H2O*** - This statement is **incorrect** because it misrepresents transpulmonary pressure during normal breathing. - Normal **transpulmonary pressure** during quiet breathing typically ranges from approximately **3-6 cm H2O** during inspiration, with an average of about **5 cm H2O** at functional residual capacity. - The range "5-8 cm H2O" is too high for normal quiet breathing. While transpulmonary pressure can reach 8 cm H2O during deeper inspiration, stating this as the range for "normal breathing" is inaccurate. - Transpulmonary pressure is the difference between alveolar pressure and pleural pressure (P_L = P_alv - P_pl), which drives lung inflation. *Expiration during quiet breathing is passive* - During quiet breathing, **expiration is a passive process** driven by the **elastic recoil of the lungs** and chest wall. - No active muscular contraction is required for air to leave the lungs during unforced expiration. *Inspiration is an active process* - **Inspiration is an active process** requiring muscular contraction, primarily of the **diaphragm and external intercostal muscles**. - These muscles contract to increase the thoracic volume, which decreases intrapleural and alveolar pressures, drawing air into the lungs. *Compliance is influenced by multiple factors including surfactant* - **Lung compliance**, a measure of the lung's distensibility, is significantly influenced by **surfactant**. - Surfactant reduces **surface tension** in the alveoli, preventing their collapse and increasing compliance.
Question 16: Which of the following stimuli is primarily responsible for triggering the Bezold-Jarisch reflex?
- A. Parasympathetic withdrawal
- B. Decreased venous return
- C. Increased sympathetic stimulation
- D. Activation of cardiac C-fiber afferents (Correct Answer)
Explanation: ***Activation of cardiac C-fiber afferents*** - The **Bezold-Jarisch reflex** is primarily triggered by stimulation of **cardiac mechanoreceptors and chemoreceptors** located in the ventricles, particularly the inferoposterior wall of the left ventricle. - These receptors have **unmyelinated vagal C-fiber afferents** that transmit signals to the medullary cardiovascular centers. - Activation of these afferents leads to the characteristic triad: **bradycardia, hypotension, and vasodilation** via increased parasympathetic activity and withdrawal of sympathetic tone. - Common triggers include vigorous ventricular contraction with decreased filling, certain drugs (veratridine), myocardial ischemia (especially inferior wall MI), and reperfusion. *Decreased venous return* - While **decreased venous return** creates the hemodynamic context (ventricular underfilling) that can lead to vigorous contraction of a relatively empty ventricle, it is not itself the *trigger* of the reflex. - The actual trigger is the activation of the ventricular receptors sensing this abnormal contraction pattern, which then signal via C-fiber afferents. - Decreased venous return alone, without receptor activation, would not produce the reflex. *Parasympathetic withdrawal* - **Parasympathetic withdrawal** would cause tachycardia and is opposite to the Bezold-Jarisch reflex, which involves **increased parasympathetic activity**. - This is a compensatory response seen in other reflexes like the baroreceptor reflex during hypotension. *Increased sympathetic stimulation* - **Increased sympathetic stimulation** produces tachycardia, increased contractility, and vasoconstriction—effects opposite to the Bezold-Jarisch reflex. - The reflex actually causes **sympathetic withdrawal** along with parasympathetic activation.
Question 17: What is the air remaining in the lung after normal expiration?
- A. Tidal Volume (TV)
- B. Residual Volume (RV)
- C. Functional Residual Capacity (FRC) (Correct Answer)
- D. Vital Capacity (VC)
Explanation: ***Functional Residual Capacity (FRC)*** - **FRC** represents the volume of air remaining in the lungs after a **normal expiration**. - It is the sum of the **expiratory reserve volume (ERV)** and the **residual volume (RV)**. *Tidal Volume (TV)* - **TV** is the volume of air inspired or expired with a **normal breath**. - It does not represent the total air remaining in the lungs after expiration. *Residual Volume (RV)* - **RV** is the volume of air remaining in the lungs after a **maximal expiration**. - It is a component of FRC but does not fully describe the air remaining after a *normal* expiration. *Vital Capacity (VC)* - **VC** is the maximum volume of air that can be exhaled after a **maximal inspiration**. - It represents the maximum amount of air that can be exchanged with a single breath, not the air remaining after normal expiration.
Question 18: Maximum voluntary ventilation is:
- A. 25 L/min
- B. 50 L/min
- C. 100 L/min
- D. 150 L/min (Correct Answer)
Explanation: ***150 L/min*** - The **Maximum Voluntary Ventilation (MVV)** represents the largest volume of air that can be breathed in and out using maximal effort over a 10-15 second period. - While it varies among individuals, a typical average value for a healthy adult is approximately **150-170 L/min**. *25 L/min* - This value is significantly lower than the typical MVV; 25 L/min is closer to a normal **resting minute ventilation** (tidal volume multiplied by respiratory rate). - Resting minute ventilation reflects the volume of air exchanged at rest, not the maximum capacity. *50 L/min* - This value is still considerably lower than the average MVV and does not represent the maximum capacity of the respiratory system. - It might be seen in individuals with **severe pulmonary impairment** or at a very high resting metabolic rate. *100 L/min* - While higher than resting values, 100 L/min is generally below the average maximum voluntary ventilation for a healthy adult. - It could represent a MVV in individuals with **mild to moderate respiratory compromise** or less effort during the test.
Question 19: Which equation is used to calculate physiological dead space?
- A. Dalton's law
- B. Bohr equation (Correct Answer)
- C. Charles's law
- D. Boyle's law
Explanation: ***Bohr equation*** - The Bohr equation is used to calculate **physiological dead space**, which is the sum of anatomical dead space and alveolar dead space. - It relates the partial pressure of carbon dioxide in arterial blood to the partial pressure of carbon dioxide in expired air, along with **tidal volume** and expired volume. *Dalton's law* - Dalton's law states that the **total pressure** exerted by a mixture of non-reactive gases is equal to the **sum of the partial pressures** of individual gases. - It is used to calculate partial pressures of gases in a mixture, not dead space. *Charles's law* - Charles's law describes the relationship between the **volume and temperature** of a gas at constant pressure. - It states that the volume of a given mass of gas is directly proportional to its absolute temperature. *Boyle's law* - Boyle's law describes the inverse relationship between the **pressure and volume** of a gas at constant temperature. - It is fundamental to understanding mechanics of breathing, but not dead space calculation.
Question 20: 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**.