NEET-PG 2013 - Physiology Practice Questions

Q: Which equation is used to calculate physiological dead space?

Bohr equation. ***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.

Q: Central chemoreceptors are most sensitive to which of the following changes in blood?

PCO2. ***PCO2*** - Central chemoreceptors, located in the **medulla oblongata**, are exquisitely sensitive to changes in the **partial pressure of carbon dioxide (PCO2)** in the arterial blood. - An increase in blood PCO2 readily crosses the **blood-brain barrier** to the cerebrospinal fluid (CSF), where it is converted to carbonic acid and then to H+ and HCO3-. The resulting **drop in CSF pH** directly stimulates these chemoreceptors, leading to increased ventilation. *PO2* - While **peripheral chemoreceptors** (carotid and aortic bodies) are sensitive to changes in **PO2**, particularly when it drops significantly (below 60 mmHg), central chemoreceptors are not. - The primary role of central chemoreceptors is to monitor and respond to changes in CO2 and pH, rather than oxygen levels. *pH* - Central chemoreceptors are indirectly sensitive to **pH changes** in the cerebrospinal fluid (CSF), which result from blood PCO2 changes. - However, they are not directly or primarily sensitive to changes in **blood pH** because hydrogen ions do not readily cross the blood-brain barrier. *HCO3-* - Bicarbonate ions (**HCO3-**) are important in buffering pH, but central chemoreceptors do not directly sense bicarbonate levels. - Changes in HCO3- indirectly affect pH, and it is the resultant **H+ concentration** in the CSF, derived from CO2, that primarily stimulates central chemoreceptors.

Q: Which of the following best describes hypoxic pulmonary vasoconstriction?

Reversible pulmonary vasoconstriction due to hypoxia. ***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.

Q: Which of the following stimuli is primarily responsible for triggering the Bezold-Jarisch reflex?

Activation of cardiac C-fiber afferents. ***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.

Q: P wave is due to:

Atrial depolarization. **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.

Q: Deoxygenated blood is not seen in which of the following?

Pulmonary vein. ***Pulmonary vein*** - The pulmonary veins carry **oxygenated blood** from the lungs back to the left atrium of the heart. - Their primary function is to transport blood that has undergone **gas exchange** in the lungs, making it rich in oxygen. *Pulmonary artery* - The pulmonary artery carries **deoxygenated blood** from the right ventricle of the heart to the lungs. - This is an exception to the general rule that arteries carry oxygenated blood, as its purpose is to deliver blood for **oxygenation**. *Right atrium* - The right atrium receives **deoxygenated blood** from the systemic circulation via the superior and inferior vena cava. - It acts as a collecting chamber for blood that has supplied oxygen to the body's tissues before it is pumped to the lungs. *Umbilical artery* - The umbilical arteries carry **deoxygenated blood** and waste products from the fetus to the placenta. - In fetal circulation, these arteries are responsible for removing metabolic wastes and carbon dioxide from the fetal circulation.

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

Increased myocardial contractility. ***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.

Q: Slowest blood flow is seen in?

Capillaries. ***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**.

Q: In a healthy person, arterial baroreceptor activity is seen at what stage of the cardiac cycle?

Both. ***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 1

Which equation is used to calculate physiological dead space?

Accepted Answer

Bohr equation

Answer

Dalton's law

Answer

Charles's law

Answer

Boyle's law

Question 2

Central chemoreceptors are most sensitive to which of the following changes in blood?

Accepted Answer

PCO2

Answer

PO2

Answer

HCO3-

Answer

pH

Question 3

Which of the following best describes hypoxic pulmonary vasoconstriction?

Accepted Answer

Reversible pulmonary vasoconstriction due to hypoxia

Answer

Irreversible pulmonary vasoconstriction due to hypoxia

Answer

Redirects blood to well-ventilated areas

Answer

Occurs immediately in response to hypoxia

Question 4

Which of the following stimuli is primarily responsible for triggering the Bezold-Jarisch reflex?

Accepted Answer

Activation of cardiac C-fiber afferents

Answer

Parasympathetic withdrawal

Answer

Decreased venous return

Answer

Increased sympathetic stimulation

Question 5

Which of the following statements about breathing is incorrect?

Accepted Answer

Normal breathing occurs when transpulmonary pressure is 5-8 cm H2O

Answer

Inspiration is an active process

Answer

Expiration during quiet breathing is passive

Answer

Compliance is influenced by multiple factors including surfactant.

Question 6

P wave is due to:

Accepted Answer

Atrial depolarization

Answer

Atrial repolarization

Answer

Ventricular depolarization

Answer

Ventricular repolarization

Question 7

Deoxygenated blood is not seen in which of the following?

Accepted Answer

Pulmonary vein

Answer

Pulmonary artery

Answer

Umbilical artery

Answer

Right atrium

Question 8

What physiological mechanism leads to an increase in cardiac output?

Accepted Answer

Increased myocardial contractility

Answer

Inhalation

Answer

Increased parasympathetic activity

Answer

Transitioning from a supine to a standing position

Question 9

Slowest blood flow is seen in?

Accepted Answer

Capillaries

Answer

Arteriole

Answer

Veins

Answer

Venules

Question 10

In a healthy person, arterial baroreceptor activity is seen at what stage of the cardiac cycle?

Accepted Answer

Both

Answer

None of the options

Answer

Diastole

Answer

Systole

NEET-PG 2013 — Physiology