Which of the following statements about volume receptors is NOT true?
What is the normal range for the CSF/plasma glucose ratio?
Which of the following contains the PRIMARY central chemoreceptors responsible for detecting CO2 and pH changes in cerebrospinal fluid?
Tetanic contraction is due to accumulation of?
What percentage of gastric secretion is attributed to the cephalic phase?
Which hormone primarily inhibits gastric acid secretion in response to acidic chyme?
Prostaglandins (PGs) in semen are secreted by?
Wolff–Chaikoff effect is due to?
Growth hormone level is highest during
Insensible water loss per day is ?
NEET-PG 2012 - Physiology NEET-PG Practice Questions and MCQs
Question 41: 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 42: What is the normal range for the CSF/plasma glucose ratio?
- A. 1.2 - 1.6
- B. 0.6 - 0.8 (Correct Answer)
- C. 0.2 - 0.4
- D. 1.0 - 1.2
Explanation: ***Correct: 0.6 - 0.8*** - This ratio indicates that cerebrospinal fluid (CSF) glucose concentration is typically 60-80% of plasma glucose concentration - This range is crucial for identifying metabolic or infectious pathologies affecting the central nervous system - Normal CSF glucose is approximately 50-80 mg/dL when plasma glucose is 70-120 mg/dL *Incorrect: 0.2 - 0.4* - A ratio in this range indicates significantly low CSF glucose, suggesting conditions like bacterial meningitis or hypoglycorrhachia - This is well below the normal physiological proportion of glucose in the CSF relative to plasma - Seen in bacterial/tuberculous meningitis, fungal infections, or malignancy *Incorrect: 1.0 - 1.2* - A CSF/plasma glucose ratio close to or above 1.0 would imply that CSF glucose levels are equal to or higher than plasma levels, which is physiologically impossible under normal conditions - Glucose transport into the CSF is regulated by GLUT-1 transporters and typically results in lower concentrations than in plasma - The blood-brain barrier maintains this gradient *Incorrect: 1.2 - 1.6* - This range is even more exaggerated and physiologically impossible, as CSF glucose cannot exceed plasma glucose in a healthy individual - Such a high ratio would contradict the mechanisms of glucose transport across the blood-brain barrier - Would suggest laboratory error if observed
Question 43: Which of the following contains the PRIMARY central chemoreceptors responsible for detecting CO2 and pH changes in cerebrospinal fluid?
- A. Medulla (Correct Answer)
- B. Baroreceptors in carotid sinus
- C. Peripheral chemoreceptors in carotid bodies
- D. All of the above
Explanation: ***Medulla*** - The **medulla oblongata** in the brainstem houses the primary central chemoreceptors. - These chemoreceptors are located on the **ventral surface of the medulla** and are highly sensitive to changes in the **pH of the cerebrospinal fluid (CSF)**, which is indirectly affected by the partial pressure of carbon dioxide (PCO2) in arterial blood. - CO2 diffuses across the blood-brain barrier, combines with water to form H+ ions, which directly stimulate these central chemoreceptors. *Baroreceptors in carotid sinus* - **Baroreceptors** primarily detect changes in **arterial blood pressure**, not CO2 or pH levels. - They are located in the carotid sinus and aortic arch and are involved in cardiovascular reflexes, not direct chemoreception for respiratory drive. *Peripheral chemoreceptors in carotid bodies* - **Peripheral chemoreceptors** in the carotid bodies (and aortic bodies) detect changes in **arterial blood O2, CO2, and pH**. - However, they are **peripheral**, not central chemoreceptors, and are the primary detectors of **hypoxemia**. - They contribute to respiratory drive but are secondary to central chemoreceptors for CO2 detection. *All of the above* - This option is incorrect because only the **medulla** contains the primary central chemoreceptors for CO2 and pH detection in CSF. - Baroreceptors detect blood pressure, and peripheral chemoreceptors are not central chemoreceptors.
Question 44: Tetanic contraction is due to accumulation of?
- A. Na+
- B. K+
- C. Ca<sup>2+</sup> (Correct Answer)
- D. Cl<sup>-</sup>
Explanation: ***Ca<sup>2+</sup>*** - **Tetanic contraction** results from a rapid succession of muscle stimulations, leading to the sustained elevation of **intracellular calcium (Ca<sup>2+</sup>)** levels. - This persistent high Ca<sup>2+</sup> concentration in the sarcoplasm allows for continuous binding to **troponin**, maintaining the activation of cross-bridge cycling. *Na<sup>+</sup>* - **Sodium (Na<sup>+</sup>)** influx is primarily responsible for the **depolarization** of the muscle cell membrane, leading to an **action potential**. - While essential for initiating the contraction, Na<sup>+</sup> accumulation itself does not directly cause the sustained high Ca<sup>2+</sup> levels characteristic of tetany. *K<sup>+</sup>* - **Potassium (K<sup>+</sup>)** efflux is crucial for the **repolarization** of the muscle cell membrane after an action potential. - Accumulation of K<sup>+</sup> in the extracellular space can contribute to muscle fatigue and reduce excitability, but it does not directly lead to tetanic contraction. *Cl<sup>-</sup>* - **Chloride (Cl<sup>-</sup>)** ions play a significant role in stabilizing the resting membrane potential and contributing to muscle **repolarization**, particularly in skeletal muscle. - While important for muscle function, changes in Cl<sup>-</sup> concentration do not directly cause the sustained Ca<sup>2+</sup> release required for tetanic contraction.
Question 45: What percentage of gastric secretion is attributed to the cephalic phase?
- A. 20% (Correct Answer)
- B. 70%
- C. 10%
- D. 100%
Explanation: ***20%*** - The **cephalic phase** of gastric secretion is initiated by the sight, smell, taste, or even thought of food and accounts for approximately **20-30%** of total gastric acid secretion. - This phase is mediated by the **vagus nerve**, stimulating parietal cells (via acetylcholine) and G cells (via gastrin-releasing peptide) to release acid and gastrin, respectively. *70 %* - **70%** represents the approximate contribution of the **gastric phase** to total gastric secretion, which is the largest phase. - This phase is activated by the presence of food in the stomach, distension, and the presence of amino acids and peptides. *10%* - **10%** is a value that is too low for the cephalic phase; it typically accounts for a more significant portion of initial acid secretion. - This percentage is sometimes associated with the intestinal phase, which produces a smaller amount of acid secretion after chyme enters the duodenum. *100%* - **100%** is incorrect because gastric secretion is a complex process involving multiple phases (cephalic, gastric, intestinal), each contributing a portion of the total secretion. - Each phase has distinct stimuli and regulatory mechanisms, ensuring a coordinated digestive response.
Question 46: Which hormone primarily inhibits gastric acid secretion in response to acidic chyme?
- A. Secretin
- B. Somatostatin (Correct Answer)
- C. Insulin
- D. Gastrin
Explanation: ***Somatostatin*** - **Somatostatin** is the **primary hormone** that inhibits gastric acid secretion in response to acidic chyme. - Released by D cells in the stomach and duodenum when pH drops below 3.0. - **Direct inhibitory effects:** Inhibits parietal cells directly, suppresses gastrin release from G cells, and blocks histamine release from ECL cells. - Acts as the main **negative feedback mechanism** to prevent excessive gastric acidification. *Secretin* - **Secretin** is released by S cells in the duodenum in response to acidic chyme (pH < 4.5). - Its **primary function** is to stimulate pancreatic bicarbonate secretion to neutralize duodenal acid. - While it does have a **secondary effect** of inhibiting gastric acid secretion, this is not its primary role. *Gastrin* - **Gastrin** is a hormone that **stimulates** gastric acid secretion, not inhibits it. - Released by G cells in the gastric antrum in response to peptides, amino acids, and gastric distension. - Promotes acid secretion by stimulating parietal cells and ECL cells (which release histamine). *Insulin* - **Insulin** is a pancreatic hormone primarily involved in **glucose metabolism** and cellular glucose uptake. - It has **no significant role** in the regulation of gastric acid secretion.
Question 47: Prostaglandins (PGs) in semen are secreted by?
- A. Prostate
- B. Seminal vesicle (Correct Answer)
- C. Sperms
- D. Testes
Explanation: ***Seminal vesicle*** - The **seminal vesicles** are the primary source of **prostaglandins (PGs)** in semen, contributing significantly to the seminal fluid volume. - These PGs play a crucial role in promoting **sperm motility** and facilitating fertilization. *Prostate* - The **prostate gland** primarily secretes **citrate**, **acid phosphatase**, and **prostate-specific antigen (PSA)**, which contribute to sperm activation and semen liquefaction. - It does not significantly contribute to the prostaglandin content of semen. *Sperms* - **Spermatozoa** themselves primarily contribute genetic material and are not a significant source of prostaglandin synthesis in semen. - Their main function is fertilization, not the production of accessory gland secretions. *Testes* - The **testes** are responsible for **spermatogenesis** (sperm production) and the synthesis of **androgens** like testosterone. - They do not secrete prostaglandins into the seminal fluid.
Question 48: Wolff–Chaikoff effect is due to?
- A. Decreased iodination of MIT
- B. Excess iodine intake (Correct Answer)
- C. Suppression of TSH secretion
- D. Decreased conversion of T4 to T3
Explanation: ***Excess iodine intake*** - The **Wolff-Chaikoff effect** is a phenomenon where a high intake of iodine acutely **inhibits thyroid hormone synthesis** and release. - This effect protects the body from excessive thyroid hormone production during periods of very high iodine availability. *Decreased iodination of MIT* - While the Wolff-Chaikoff effect does inhibit **iodination**, the direct cause is the excessive iodine itself, which triggers an autoregulatory shutdown. - Decreased iodination is a *consequence* of the high iodine leading to inhibition of thyroid peroxidase activity, but not the primary cause of the effect. *Suppression of TSH secretion* - **TSH (Thyroid Stimulating Hormone)** secretion is primarily regulated by negative feedback from thyroid hormones (T3 and T4) and TRH from the hypothalamus. - The Wolff-Chaikoff effect directly involves the thyroid gland's response to iodine and is not primarily mediated by TSH suppression. *Decreased conversion of T4 to T3* - The **conversion of T4 to T3** primarily occurs in peripheral tissues, mediated by deiodinase enzymes. - The Wolff-Chaikoff effect focuses on the inhibition of **iodine organification** and hormone release within the thyroid gland itself, not peripheral conversion.
Question 49: Growth hormone level is highest during
- A. Sleep (Correct Answer)
- B. Hypoglycemia
- C. Fasting
- D. Exercise
Explanation: ***Sleep*** - Growth hormone (GH) secretion is **pulsatile**, with the largest and most consistent pulses occurring during **slow-wave sleep** (deep sleep). - This nocturnal surge contributes significantly to the overall daily GH output and is crucial for growth and metabolic regulation. *Hypoglycemia* - While **hypoglycemia** is a potent stimulus for GH release, it is an acute stress response rather than a state where GH levels are consistently highest. - The body's primary response to hypoglycemia is to raise blood glucose, and while GH helps, it is not the peak physiological secretion time. *Fasting* - **Prolonged fasting** can increase GH secretion as a mechanism to mobilize fat stores and conserve glucose. - However, the peak levels due to fasting are generally less pronounced than the dramatic surge observed during deep sleep. *Exercise* - **Vigorous exercise** can acutely stimulate GH release, particularly with sustained effort. - This increase is typically transient and not as high or consistently cyclical as the secretion during nocturnal sleep.
Question 50: Insensible water loss per day is ?
- A. 100 ml
- B. 1000 ml (Correct Answer)
- C. 700 ml
- D. 300 ml
Explanation: ***1000 ml*** - **Insensible water loss** occurs through the skin (evaporation) and respiratory tract (exhalation) without conscious perception. - The typical daily insensible water loss in an adult is approximately **800-1000 ml/day**. - **Breakdown**: Skin evaporation (~400-500 ml) + Respiratory tract (~300-400 ml) = **~900-1000 ml total**. - **1000 ml** is the standard value cited in major physiology textbooks (Guyton & Hall, Ganong) and is the most commonly accepted answer for NEET PG examinations. *100 ml* - This value is significantly **lower** than the actual insensible water loss, which occurs continuously throughout the day. - Such a low volume would imply negligible evaporation and respiratory loss, which is not physiologically accurate. *300 ml* - While greater than 100 ml, 300 ml is still **far below** the typical range for daily insensible water loss. - This amount represents only about one-third of the actual insensible losses from the skin and respiratory system combined. *700 ml* - Although this value is sometimes mentioned in literature, it is at the **lower end** of the physiological range. - The more widely accepted standard value for insensible water loss in a healthy adult under normal conditions is **900-1000 ml/day**. - 700 ml would underestimate the normal daily insensible losses.