What is the Bohr effect in relation to hemoglobin's affinity for oxygen?
Which of the following has the lowest Respiratory Quotient (RQ)?
Nonshivering thermogenesis in adults is due to:
Diurnal variation of ACTH depends on ?
What is the effect of acetylcholine on the Lower Esophageal Sphincter (LES)?
Which of the following is most important in sodium and water retention ?
ANP acts at which site?
What is the normal range of interstitial pressure?
According to some older studies, which sperm chromosome was hypothesized to be associated with faster initial motility?
Which structure of the eye has the maximum refractive power?
NEET-PG 2012 - Physiology NEET-PG Practice Questions and MCQs
Question 71: What is the Bohr effect in relation to hemoglobin's affinity for oxygen?
- A. Decrease in CO2 affinity of hemoglobin when the pH of blood falls
- B. Decrease in O2 affinity of hemoglobin when the pH of blood rises
- C. Decrease in O2 affinity of hemoglobin when the pH of blood falls (Correct Answer)
- D. Decrease in CO2 affinity of hemoglobin when the pH of blood rises
Explanation: ***Decrease in O2 affinity of hemoglobin when the pH of blood falls*** - The **Bohr effect** describes how **hemoglobin's affinity for oxygen decreases** in acidic environments (lower pH), leading to increased oxygen release to tissues. - This physiological response is crucial in active tissues, where increased metabolism produces more **carbon dioxide** and **lactic acid**, lowering the local pH. *Decrease in CO2 affinity of hemoglobin when the pH of blood falls* - This statement incorrectly relates the Bohr effect to **CO2 affinity** and its change with pH in this manner. - The Bohr effect primarily concerns oxygen affinity, not CO2 affinity; CO2 and H+ directly influence oxygen binding. *Decrease in O2 affinity of hemoglobin when the pH of blood rises* - An **increase in pH** (alkaline environment) would, in fact, **increase hemoglobin's affinity for oxygen**, promoting oxygen uptake in the lungs. - This describes the opposite of the Bohr effect, which is about oxygen release in acidic conditions. *Decrease in CO2 affinity of hemoglobin when the pH of blood rises* - While pH changes do affect CO2 transport, this statement does not accurately describe the Bohr effect. - The **Haldane effect** is more relevant to the relationship between oxygenation status and hemoglobin's CO2 affinity.
Question 72: Which of the following has the lowest Respiratory Quotient (RQ)?
- A. Heart
- B. Brain
- C. RBC
- D. Adipose (Correct Answer)
Explanation: ***Adipose*** - **Adipose tissue** primarily metabolizes **fatty acids** for energy, which have the lowest theoretical RQ of approximately **0.7**. - A lower RQ indicates that less carbon dioxide is produced relative to the oxygen consumed during metabolic fuel oxidation. - Among tissues that perform aerobic respiration, adipose tissue has the lowest RQ. *Brain* - The brain primarily uses **glucose** as its energy source under normal conditions, which has an RQ of approximately **1.0**. - During prolonged fasting, the brain can adapt to use **ketone bodies** (RQ ≈ 0.89), but glucose remains the primary fuel. - Higher RQ than adipose tissue. *RBC* - **Red blood cells (RBCs)** lack mitochondria and rely exclusively on **anaerobic glycolysis** for energy, metabolizing glucose to lactate. - RBCs **do not consume oxygen** for energy metabolism and therefore **do not have a meaningful RQ value** (RQ = CO₂ produced / O₂ consumed in aerobic respiration). - This makes RBC an inappropriate answer to a question about "lowest RQ" since RQ is undefined for anaerobic metabolism. *Heart* - The heart is a highly metabolic organ that can utilize various substrates, including **fatty acids**, **glucose**, **lactate**, and **ketone bodies**. - While it has a high capacity for fatty acid oxidation, it also significantly uses glucose and lactate, leading to an overall RQ typically between **0.7-0.9**. - Higher average RQ than adipose tissue due to mixed substrate utilization.
Question 73: Nonshivering thermogenesis in adults is due to:
- A. Muscle metabolism
- B. Thyroid hormone
- C. Noradrenaline
- D. Brown fat between the shoulders (Correct Answer)
Explanation: ***Brown fat between the shoulders*** - In adults, the primary **effector tissue** for **non-shivering thermogenesis** is **brown adipose tissue (BAT)**, with major depots located between the shoulders, around the neck, and along the spine. - **BAT** contains specialized mitochondria with **uncoupling protein 1 (UCP1)** that uncouples oxidative phosphorylation, generating heat instead of ATP. - This is the tissue where non-shivering thermogenesis actually occurs, making it the direct answer to what non-shivering thermogenesis is "due to." *Noradrenaline* - **Noradrenaline** is the key neurotransmitter that **activates brown fat** via **β3-adrenergic receptors** to initiate non-shivering thermogenesis. - While noradrenaline is the **trigger/stimulus**, the actual heat production occurs in brown adipose tissue. - Noradrenaline itself does not produce heat directly; it acts as the signal that activates the thermogenic machinery in BAT. *Thyroid hormone* - **Thyroid hormone** increases **basal metabolic rate** and can potentiate the thermogenic response by upregulating UCP1 expression in brown fat. - Its role is **permissive and long-term** rather than being the immediate effector of acute non-shivering thermogenesis. - It modulates overall cellular metabolism but is not the primary mechanism for rapid heat generation in cold exposure. *Muscle metabolism* - **Muscle contraction** during shivering generates heat through increased ATP hydrolysis, which is **shivering thermogenesis**. - **Non-shivering thermogenesis** specifically refers to heat production **without muscle contraction**, making muscle metabolism the mechanism for shivering, not non-shivering, thermogenesis.
Question 74: Diurnal variation of ACTH depends on ?
- A. Suprachiasmatic nucleus (Correct Answer)
- B. Supraoptic nucleus
- C. Ventrolateral nucleus
- D. Thalamus
Explanation: ***Suprachiasmatic nucleus*** - The **suprachiasmatic nucleus (SCN)** acts as the body's **master circadian clock**, synchronizing various physiological rhythms, including the **diurnal variation of ACTH** secretion. - It receives light input from the **retina** and projects to other brain regions to regulate the timing of hormone release. *Supraoptic nucleus* - The **supraoptic nucleus (SON)** is primarily involved in the production of **vasopressin (ADH)** and **oxytocin**, which are released by the posterior pituitary. - It does not directly control the diurnal rhythm of ACTH. *Ventrolateral nucleus* - The **ventrolateral preoptic area (VLPO)** is a key region for **sleep regulation**, promoting sleep by inhibiting wake-promoting neurotransmitters. - While it contributes to sleep-wake cycles, it is not the primary regulator of ACTH's diurnal variation. *Thalamus* - The **thalamus** is a major relay center for sensory information and plays a role in consciousness, sleep, and alertness. - It does not directly control the **circadian rhythm of ACTH secretion**.
Question 75: What is the effect of acetylcholine on the Lower Esophageal Sphincter (LES)?
- A. Causes contraction (Correct Answer)
- B. Causes relaxation
- C. No effect on LES
- D. Contraction followed by relaxation
Explanation: ***Correct Option: Causes contraction*** - Acetylcholine acts on **M3 muscarinic receptors** on LES smooth muscle cells to cause **contraction** - This is part of the **excitatory cholinergic pathway** that maintains LES tone and prevents gastroesophageal reflux - Acetylcholine is released from **excitatory motor neurons** in the myenteric plexus *Incorrect: Causes relaxation* - LES relaxation during swallowing is mediated by **nitric oxide (NO)** and **vasoactive intestinal peptide (VIP)**, NOT acetylcholine - These inhibitory neurotransmitters are released from separate **inhibitory motor neurons** - The relaxation response during swallowing is due to activation of the inhibitory pathway, which suppresses cholinergic tone *Incorrect: No effect on LES* - Acetylcholine has a significant effect on the LES - It is one of the key neurotransmitters maintaining basal LES tone - Loss of cholinergic input can lead to decreased LES pressure *Incorrect: Contraction followed by relaxation* - Acetylcholine itself causes only contraction - The swallowing reflex involves coordinated activation of inhibitory (NO/VIP) and suppression of excitatory (acetylcholine) pathways - The sequence of events is neural, not a biphasic response to acetylcholine alone
Question 76: Which of the following is most important in sodium and water retention ?
- A. Renin angiotensin system (Correct Answer)
- B. ANP
- C. BNP
- D. Vasopressin
Explanation: ***Renin angiotensin system*** - The **renin-angiotensin-aldosterone system (RAAS)** is the most important mechanism for **both sodium AND water retention**, which is what the question specifically asks about. - **Aldosterone** directly promotes **sodium reabsorption** in the principal cells of the collecting duct by increasing apical ENaC channels and basolateral Na-K-ATPase pumps. - **Angiotensin II** stimulates sodium reabsorption in the proximal tubule and also stimulates ADH release, contributing to water retention. - When sodium is retained, **water follows passively** due to the osmotic gradient, resulting in effective volume expansion. - RAAS is the primary system activated in states of volume depletion and is most important for combined sodium and water retention. *Vasopressin* - **Vasopressin (ADH)** primarily controls **water retention only** by increasing aquaporin-2 channels in the collecting duct. - While crucial for water balance, it has minimal direct effect on sodium reabsorption. - It causes retention of **free water**, which can actually dilute plasma sodium concentration. - ADH is the answer if the question asked about water retention alone, but not for combined sodium and water retention. *ANP* - **Atrial natriuretic peptide (ANP)** promotes **sodium and water excretion** (natriuresis and diuresis). - Released in response to atrial stretch from volume expansion. - Acts to *oppose* retention mechanisms, making it incorrect for this question. *BNP* - **Brain natriuretic peptide (BNP)** similarly promotes **natriuresis and diuresis**. - Released from ventricular myocytes in response to volume overload. - Like ANP, it acts to *excrete* sodium and water, not retain them.
Question 77: ANP acts at which site?
- A. Glomerulus
- B. Loop of Henle
- C. PCT
- D. Collecting duct (Correct Answer)
Explanation: ***Collecting duct*** - Atrial Natriuretic Peptide (**ANP**) exerts its primary effect on the **collecting duct** by inhibiting sodium reabsorption, leading to increased sodium and water excretion (natriuresis and diuresis). - This action helps to reduce blood volume and blood pressure in conditions like **hypervolemia**. *Glomerulus* - While ANP does cause **afferent arteriolar dilation** and **efferent arteriolar constriction**, increasing **glomerular filtration rate** (GFR), its direct tubular action is most prominent in the collecting duct. - The primary function of the glomerulus is **filtration**, influenced by many factors including pressure, but it is not the main site of ANP's direct tubular reabsorptive effects. *Loop of Henle* - The loop of Henle is responsible for establishing the **medullary osmotic gradient** and reabsorbing a significant amount of sodium and water. - ANP has minor effects on the loop of Henle, but its most impactful reabsorptive modulation occurs downstream in the collecting duct. *PCT* - The **proximal convoluted tubule (PCT)** is where the bulk of reabsorption of filtered substances (e.g., glucose, amino acids, most sodium and water) occurs. - ANP has very little direct influence on the reabsorptive processes of the PCT.
Question 78: What is the normal range of interstitial pressure?
- A. -3 to 0 mmHg
- B. -5 to 0 mmHg (Correct Answer)
- C. 0 to 5 mmHg
- D. 5 to 10 mmHg
Explanation: ***-5 to 0 mmHg*** - The interstitial fluid is normally under a **slight negative pressure**, typically ranging from **-5 to 0 mmHg** - This negative pressure helps pull fluid from the capillaries into the interstitial space and facilitates **lymphatic drainage** - Maintained by continuous drainage of fluid and proteins by the **lymphatic system** - This range is the commonly accepted value in standard physiology references for Indian medical exams *-3 to 0 mmHg* - While this range acknowledges the typically **negative nature** of interstitial pressure, it represents a slightly narrower range - Some sources cite this as the average range, but **-5 to 0 mmHg** is the more commonly accepted standard range - Not the most precise or widely cited range for exam purposes *0 to 5 mmHg* - This range suggests a **positive interstitial pressure**, which is generally **abnormal** - Indicates **edema formation** due to excess fluid accumulation in the interstitial space - Positive pressure impairs fluid reabsorption and lymphatic drainage - Represents pathological fluid dynamics *5 to 10 mmHg* - Represents significant **positive interstitial pressure** leading to severe **interstitial edema** - Markedly impairs tissue function and fluid exchange - Indicates pathological conditions where capillary filtration far exceeds lymphatic drainage capacity - Associated with severe edematous states
Question 79: According to some older studies, which sperm chromosome was hypothesized to be associated with faster initial motility?
- A. None of the options
- B. X chromosome
- C. Y chromosome (Correct Answer)
- D. Both same
Explanation: ***Y chromosome*** - **Older hypothesis** suggested that Y chromosome-bearing sperm might be faster due to being slightly smaller and lighter - However, **modern rigorous studies have largely debunked this theory** - The chromosomal size difference (X vs Y) represents less than 0.02% of total sperm mass, making any speed difference negligible - **Current scientific consensus**: No consistent, reproducible motility difference has been demonstrated *X chromosome* - X-bearing sperm are marginally larger due to more DNA content - Early theories suggested they were slower but more robust - **Modern evidence does not support consistent motility differences** between X and Y bearing sperm *Both same* - This actually reflects the **current scientific consensus** based on modern flow cytometry and separation studies - Most rigorous contemporary research shows no reliable motility differences between X and Y chromosome-bearing sperm - The Ericsson albumin method (based on speed separation) has been largely discredited *None of the options* - This option is incorrect as the question asks about historical hypotheses - Early studies did propose the Y chromosome theory, even though it's now considered largely inaccurate
Question 80: Which structure of the eye has the maximum refractive power?
- A. Anterior surface of lens
- B. Posterior surface of lens
- C. Anterior surface of cornea (Correct Answer)
- D. Posterior surface of cornea
Explanation: ***Anterior surface of cornea*** - The **cornea** accounts for approximately two-thirds of the eye's total refractive power due to the large difference in refractive index between air and the corneal tissue. - The **anterior surface of the cornea** is the primary site of light refraction as light enters the eye from the air. *Anterior surface of lens* - The **lens** contributes significantly to accommodation, but its overall refractive power is less than that of the cornea in the unaccommodated state. - The change in refractive index between the aqueous humor and the lens is less pronounced compared to the air-cornea interface. *Posterior surface of lens* - The **posterior surface of the lens** also contributes to the focusing power of the lens, but its curvature and refractive index difference are typically less than the anterior surface of the cornea. - Its contribution to total refractive power is secondary to the anterior corneal surface and the anterior lens surface. *Posterior surface of cornea* - The **posterior surface of the cornea** has a much smaller refractive power compared to the anterior surface due to the smaller difference in refractive index between the cornea and the aqueous humor. - This interface does contribute to refraction but is not the primary focusing component.