Thermoregulation — MCQs

Thermoregulation Indian Medical PG Practice Questions and MCQs

Question 21: Non-shivering thermogenesis is mediated by which of the following?

A. α1 receptor
B. β2 receptor (Correct Answer)
C. β3 receptor
D. UCP-1

Explanation: **Explanation:** Thermoregulation is a critical physiological process where the body maintains its core temperature. **Non-shivering thermogenesis (NST)** is the production of heat without muscle contraction, primarily occurring in **Brown Adipose Tissue (BAT)**. **Why Option B is Correct:** In humans, the sympathetic nervous system (SNS) triggers NST by releasing norepinephrine. While multiple adrenergic receptors are involved, standard physiological teaching (and specific exam patterns like NEET-PG) often highlights the role of **$\beta$2-adrenergic receptors** in stimulating the metabolic rate and glycogenolysis required for heat production. Note: While $\beta$3 is the primary receptor in rodents, human thermogenic responses are heavily mediated via $\beta$2 and $\beta$1 pathways. **Analysis of Incorrect Options:** * **A. $\alpha$1 receptor:** These are primarily involved in peripheral vasoconstriction to prevent heat loss, rather than active heat production. * **C. $\beta$3 receptor:** While $\beta$3 receptors are the predominant mediators of NST in small mammals (rodents), their role in adult humans is considered less significant compared to $\beta$2. * **D. UCP-1 (Thermogenin):** This is the **effector protein** located in the inner mitochondrial membrane of brown fat that uncouples oxidative phosphorylation to produce heat. While it is the *mechanism* of heat production, it is not the *mediator* (receptor) that initiates the signaling cascade. **High-Yield Clinical Pearls for NEET-PG:** * **Brown Fat:** Rich in mitochondria and cytochrome oxidase (giving it the brown color). It is abundant in neonates (interscapular region) to prevent hypothermia. * **Mechanism:** SNS $\rightarrow$ Norepinephrine $\rightarrow$ $\beta$ receptors $\rightarrow$ cAMP $\rightarrow$ Lipolysis $\rightarrow$ Activation of **UCP-1 (Thermogenin)**. * **Thyroid Hormone:** Essential for NST as it increases the sensitivity of $\beta$ receptors to catecholamines (permissive action).

Question 22: Temperature regulation is chiefly under the control of?

A. Midbrain
B. Pons
C. Medulla
D. Hypothalamus (Correct Answer)

Explanation: **Explanation:** The **Hypothalamus** is the primary center for thermoregulation in the human body, often referred to as the body’s "thermostat." It maintains the set-point for body temperature by integrating sensory input from peripheral skin receptors and central thermoreceptors (monitoring blood temperature). * **Anterior Hypothalamus (Pre-optic area):** Responsible for heat loss. Stimulation leads to vasodilation and sweating. Lesions here cause hyperthermia. * **Posterior Hypothalamus:** Responsible for heat conservation and production. Stimulation leads to vasoconstriction and shivering. Lesions here cause poikilothermia (inability to regulate temperature). **Why other options are incorrect:** * **Midbrain:** Primarily involved in visual/auditory reflexes and motor control (Substantia nigra). * **Pons:** Contains the pneumotaxic and apneustic centers, which regulate the rhythm of respiration. * **Medulla:** The vital center for cardiac, vasomotor, and respiratory functions (reflexes like swallowing and vomiting). **High-Yield Clinical Pearls for NEET-PG:** 1. **Pyrogens:** Substances like IL-1 and TNF-α act on the hypothalamus to increase the "set-point," resulting in fever. 2. **Brown Adipose Tissue:** Important for non-shivering thermogenesis in neonates, regulated by the sympathetic nervous system via the hypothalamus. 3. **Heat Stroke:** Occurs when the hypothalamic thermoregulatory mechanisms fail, leading to a core temperature >40°C (104°F) and CNS dysfunction. 4. **Rule of Thumb:** "A" for Anterior = "A"ir Conditioning (Heat loss); "P" for Posterior = "P"roduction (Heat conservation).

Question 23: Thermoregulatory center is located at?

A. Pons
B. Hypothalamus (Correct Answer)
C. Medulla
D. Pituitary

Explanation: **Explanation:** The **Hypothalamus** is the primary center for thermoregulation in the human body, acting as a biological thermostat. It receives input from peripheral thermoreceptors (skin) and central thermoreceptors (viscera and spinal cord) to maintain the body's core temperature at a set point (approx. 37°C). * **Anterior Hypothalamus (Pre-optic area):** Responsible for heat loss. Stimulation leads to vasodilation and sweating. Lesions here cause hyperthermia. * **Posterior Hypothalamus:** Responsible for heat production and conservation. Stimulation leads to shivering and vasoconstriction. Lesions here cause poikilothermia (inability to regulate temperature). **Why other options are incorrect:** * **Pons:** Primarily contains respiratory centers (Apneustic and Pneumotaxic) and nuclei for cranial nerves V-VIII. * **Medulla:** Houses vital autonomic centers for cardiovascular regulation, respiration (Dorsal and Ventral groups), and reflex activities like swallowing and vomiting. * **Pituitary:** An endocrine gland responsible for hormone secretion (e.g., TSH, ACTH, GH) under the influence of the hypothalamus, but it does not directly integrate thermoregulatory signals. **High-Yield Clinical Pearls for NEET-PG:** * **Pyrogens:** Substances like Interleukin-1 (IL-1) and TNF-alpha act on the hypothalamus to increase the production of **Prostaglandin E2 (PGE2)**, which resets the hypothalamic set-point to a higher level, causing **fever**. * **Aspirin/NSAIDs:** Work by inhibiting the enzyme cyclooxygenase, thereby reducing PGE2 synthesis in the hypothalamus to treat fever. * **Brown Adipose Tissue:** Important for non-shivering thermogenesis in neonates, regulated by the sympathetic nervous system via the hypothalamus.

Question 24: What is the primary function of brown adipose tissue?

A. Glycogen storage
B. Synthesis of glucose from non-carbohydrate sources
C. Breakdown of stored glycogen into glucose
D. Heat production (Correct Answer)

Explanation: **Explanation:** **1. Why "Heat Production" is correct:** Brown Adipose Tissue (BAT) is a specialized form of fat primarily responsible for **non-shivering thermogenesis**. Unlike white fat, which stores energy, BAT contains a high density of mitochondria. These mitochondria express a unique protein called **Uncoupling Protein-1 (UCP-1)**, also known as **Thermogenin**. In response to cold, sympathetic stimulation releases norepinephrine, which activates UCP-1. This protein "uncouples" the mitochondrial respiratory chain from ATP synthesis. Instead of producing ATP, the energy generated by the proton gradient is dissipated as **heat**, which is then distributed to the body via the extensive vascular supply of BAT. **2. Why other options are incorrect:** * **Option A & C:** Glycogen storage and glycogenolysis (breakdown) are primary functions of the **liver and skeletal muscles**, not adipose tissue. * **Option B:** Gluconeogenesis (synthesis of glucose) occurs primarily in the **liver** and, to a lesser extent, the **kidneys**. **3. High-Yield Clinical Pearls for NEET-PG:** * **Appearance:** The "brown" color is due to the high concentration of **cytochrome oxidase** in the numerous mitochondria and a rich capillary network. * **Location:** In neonates, it is found in the interscapular region, axillae, and around deep organs (kidneys/adrenals). In adults, it persists in the cervical, supraclavicular, and paravertebral areas. * **Imaging:** BAT is highly metabolic and can be visualized on **PET scans** (using 18F-FDG), often appearing as "false positives" in oncological imaging. * **Regulation:** It is activated by the **Sympathetic Nervous System** via **β3-adrenergic receptors**.

Question 25: Brain temperature is most accurately measured through which site?

A. Tympanic membrane (Correct Answer)
B. Nasopharynx
C. Scalp
D. Cornea

Explanation: **Explanation:** The **tympanic membrane** is considered the most accurate clinical site for measuring core brain temperature. This is because the tympanic membrane shares its blood supply with the hypothalamus—the body’s thermoregulatory center—via the **internal carotid artery**. Since the membrane is thin and located in a cavity protected from external environmental fluctuations, it reflects rapid changes in core temperature more precisely than other peripheral sites. **Analysis of Options:** * **A. Tympanic membrane (Correct):** Its proximity to the internal carotid artery provides a direct thermal link to the hypothalamus, making it the gold standard for non-invasive brain temperature monitoring. * **B. Nasopharynx:** While the nasopharynx is close to the base of the skull, its temperature is easily influenced by ambient air during respiration (inhalation/exhalation), leading to inaccurate readings. * **C. Scalp:** The scalp measures skin temperature, which is heavily affected by environmental cooling, sweat evaporation, and peripheral vasoconstriction. It does not reflect internal brain temperature. * **D. Cornea:** While the eye is highly vascular, the cornea is exposed to the external environment and tear film evaporation, making it an unreliable indicator of core brain temperature. **High-Yield Facts for NEET-PG:** * **Hypothalamus:** The "Thermostat" of the body. The **Anterior Hypothalamus** (Pre-optic area) handles heat loss, while the **Posterior Hypothalamus** handles heat production/conservation. * **Gold Standard:** While the pulmonary artery catheter is the gold standard for *overall core temperature*, the tympanic membrane is the gold standard for *brain temperature*. * **Diurnal Variation:** Body temperature is lowest in the early morning (4–6 AM) and highest in the evening (4–6 PM).

Question 26: Secretin stimulates the secretion of which component in pancreatic juice?

A. Pancreatic secretion rich in bicarbonates (Correct Answer)
B. Pancreatic secretion rich in enzymes
C. Pancreatic secretion rich in both bicarbonate and enzymes
D. Gastric acid

Explanation: **Explanation:** The regulation of pancreatic secretion is primarily mediated by two hormones: **Secretin** and **Cholecystokinin (CCK)**. **1. Why Option A is correct:** Secretin is released by the **S-cells** of the duodenum in response to the entry of acidic chyme (pH < 4.5) from the stomach. Its primary function is to stimulate the **pancreatic ductal cells** to secrete a large volume of watery fluid rich in **bicarbonate (HCO₃⁻)**. This alkaline secretion is crucial for neutralizing gastric acid in the duodenum, thereby protecting the intestinal mucosa and providing an optimal pH for the functioning of pancreatic digestive enzymes. **2. Why other options are incorrect:** * **Option B:** Pancreatic secretion rich in enzymes is primarily stimulated by **Cholecystokinin (CCK)**, which is released by I-cells in response to fatty acids and amino acids. * **Option C:** While both hormones act together during a meal, Secretin specifically targets bicarbonate/water (ductal cells), whereas CCK targets enzymes (acinar cells). * **Option D:** Secretin actually **inhibits** gastric acid secretion (acting as an enterogastrone) to prevent further acidification of the duodenum. **High-Yield Clinical Pearls for NEET-PG:** * **"Nature’s Antacid":** Secretin is often referred to by this nickname due to its acid-neutralizing role. * **Mechanism:** Secretin acts via the **cAMP** second messenger pathway in ductal cells. * **Diagnostic Use:** The **Secretin Stimulation Test** is the gold standard for diagnosing **Exocrine Pancreatic Insufficiency** (e.g., in Chronic Pancreatitis). * **Zollinger-Ellison Syndrome:** Paradoxically, secretin administration causes a rise in serum gastrin levels in patients with a gastrinoma, serving as a definitive provocative test.

Question 27: In a sedentary individual, what is the greatest proportion of heat generation?

A. Basal Metabolic Rate (BMR) (Correct Answer)
B. Maintenance of body posture
C. Specific dynamic action of food
D. Non-shivering thermogenesis

Explanation: ### Explanation **Correct Option: A. Basal Metabolic Rate (BMR)** In a sedentary individual, the **Basal Metabolic Rate (BMR)** accounts for approximately **60–75%** of the total daily energy expenditure (TDEE). BMR represents the energy required to maintain vital functions at rest, such as cellular membrane potentials, cardiac output, and respiration. Since a sedentary person performs minimal physical activity, the heat generated as a byproduct of these baseline metabolic processes constitutes the largest proportion of total heat production. **Analysis of Incorrect Options:** * **B. Maintenance of body posture:** This falls under "Non-Exercise Activity Thermogenesis" (NEAT). While it requires muscle contraction, it typically contributes only about 10–15% of heat generation in a sedentary person. * **C. Specific dynamic action (SDA) of food:** Also known as the Thermic Effect of Food (TEF), this is the energy used for digestion and absorption. It accounts for roughly **10%** of total energy expenditure. * **D. Non-shivering thermogenesis:** This occurs primarily in **brown adipose tissue (BAT)** via the protein **UCP-1 (Thermogenin)**. While crucial in neonates, its contribution to total heat generation in sedentary adults is negligible under normal (non-cold) conditions. **High-Yield Clinical Pearls for NEET-PG:** * **Primary site of heat production:** At rest, the **visceral organs** (liver, brain, heart) produce the most heat. During exercise, **skeletal muscles** can contribute up to 90%. * **Thermostat of the body:** The **Preoptic nucleus of the Anterior Hypothalamus** (Heat loss center). The Posterior Hypothalamus acts as the heat gain center. * **SDA Values:** Protein has the highest SDA (30%), followed by carbohydrates (6%) and fats (4%). * **Efficiency:** Only about 20–25% of energy released during metabolism is used for work; the remaining **75–80% is released as heat**.

Question 28: What is the definition of hyperthermia?

A. Temperature greater than 41.5°C
B. Temperature greater than 40°C with autonomic dysfunction
C. No change in hypothalamic thermostat
D. Failure of thermoregulation (Correct Answer)

Explanation: ### Explanation **Core Concept: Hyperthermia vs. Fever** The fundamental distinction in thermoregulation lies between **Fever** and **Hyperthermia**. In a fever, the hypothalamic set-point is elevated (due to pyrogens), but the body’s thermoregulatory mechanisms remain intact to meet that new set-point. In **Hyperthermia**, the hypothalamic set-point remains **normal**, but the body’s heat-dissipating mechanisms are overwhelmed by external heat or internal heat production. Therefore, hyperthermia is defined as a **failure of thermoregulation**. **Analysis of Options:** * **Option D (Correct):** Hyperthermia occurs when heat gain exceeds heat loss. Since the body cannot maintain homeostasis despite a normal thermostat, it is a primary failure of the thermoregulatory system. * **Option A:** A temperature >41.5°C is specifically termed **Hyperpyrexia**. While hyperpyrexia is an extreme form of fever, hyperthermia can occur at lower temperatures (e.g., 39°C in heat exhaustion). * **Option B:** Autonomic dysfunction is a feature of specific conditions like Heat Stroke, but it is not the *definition* of hyperthermia itself. * **Option C:** While it is true that there is "no change in the hypothalamic thermostat" in hyperthermia, this is a *characteristic*, not the formal definition. The definition focuses on the physiological state of the system (failure). **High-Yield NEET-PG Pearls:** 1. **Antipyretics (NSAIDs/Paracetamol):** Effective in Fever (resets the thermostat) but **ineffective** in Hyperthermia (thermostat is already normal). 2. **Heat Stroke:** Defined by a core temperature >40°C + CNS dysfunction (confusion/coma) + Anhidrosis (dry skin). 3. **Malignant Hyperthermia:** A specific type of hyperthermia caused by a mutation in the **Ryanodine receptor (RYR1)**, triggered by volatile anesthetics (e.g., Halothane) or Succinylcholine. Treatment of choice: **Dantrolene**.

Question 29: Which of the following statements regarding ghrelin is false?

A. Secreted from D1 cells of the fundus
B. Induces lipolysis
C. Stimulates appetite
D. None of the above (Correct Answer)

Explanation: **Explanation:** Ghrelin, often referred to as the "hunger hormone," is a 28-amino acid peptide that plays a central role in energy homeostasis. The correct answer is **None of the above** because all statements (A, B, and C) are physiologically accurate. 1. **Secreted from D1 cells (Option A):** Ghrelin is primarily synthesized and secreted by the **P/D1 cells** (oxyntic cells) located in the fundus of the stomach. Smaller amounts are produced in the small intestine and the epsilon cells of the pancreas. 2. **Induces Lipolysis (Option B):** While ghrelin is orexigenic (promotes fat storage in the long term), it acutely stimulates the secretion of **Growth Hormone (GH)** from the anterior pituitary by binding to the Growth Hormone Secretagogue Receptor (GHSR). Growth hormone is a potent lipolytic agent, thereby indirectly inducing lipolysis. 3. **Stimulates Appetite (Option C):** Ghrelin is the only peripheral hormone known to stimulate food intake. It acts on the **Arcuate Nucleus** of the hypothalamus, specifically stimulating **NPY (Neuropeptide Y)** and **AgRP (Agouti-related peptide)** neurons to increase hunger. **High-Yield NEET-PG Pearls:** * **Prader-Willi Syndrome:** Characterized by extremely high ghrelin levels, leading to hyperphagia and obesity. * **Sleep Deprivation:** Increases ghrelin and decreases leptin, contributing to weight gain. * **Post-Gastric Bypass:** Ghrelin levels significantly drop, which is one mechanism for the rapid weight loss observed after bariatric surgery. * **Opponent Hormone:** **Leptin** (secreted by adipocytes) is the functional antagonist of ghrelin, as it induces satiety.

Question 30: Endogenous non-shivering thermogens are secreted by all of the following organs except:

A. Liver
B. Spleen (Correct Answer)
C. Heart
D. Small intestine

Explanation: **Explanation:** Thermoregulation involves both shivering (muscle contraction) and **non-shivering thermogenesis (NST)**. While NST is primarily associated with Brown Adipose Tissue (BAT) via Uncoupling Protein-1 (UCP1), recent physiological research identifies specific **endogenous non-shivering thermogens** (hormones or peptides) secreted by various visceral organs to regulate metabolic heat production. 1. **Why Spleen is the Correct Answer:** The **Spleen** is primarily a lymphoid organ involved in immune filtration and erythrocyte homeostasis. It does not secrete systemic thermogenic hormones or peptides that contribute to non-shivering thermogenesis. Therefore, it is the "except" in this list. 2. **Analysis of Other Options:** * **Liver (Option A):** The liver is a major metabolic hub. It secretes **FGF21 (Fibroblast Growth Factor 21)** and **Bile Acids**, both of which act as potent endogenous thermogens by activating UCP1 in brown and beige fat. * **Heart (Option C):** The heart secretes **Natriuretic Peptides (ANP and BNP)**. Beyond fluid balance, these peptides promote the "browning" of white adipose tissue and enhance mitochondrial oxidative capacity, acting as thermogenic signals. * **Small Intestine (Option D):** The gut secretes **Secretin** and **GLP-1** in response to a meal (diet-induced thermogenesis). Secretin, specifically, has been identified as a key hormone that activates brown fat thermogenesis and signals satiation. **High-Yield NEET-PG Pearls:** * **Primary Site of NST:** Brown Adipose Tissue (BAT), especially in neonates. * **Key Protein:** UCP1 (Thermogenin) located in the inner mitochondrial membrane. * **Mechanism:** Uncoupling of oxidative phosphorylation from ATP synthesis, dissipating energy as heat. * **Thyroid Hormone:** The most important long-term regulator of the Basal Metabolic Rate (BMR) and NST.

Practice by Chapter

Heat Production and Loss

Practice Questions

Temperature Sensing Mechanisms

Practice Questions

Hypothalamic Regulation of Temperature

Practice Questions

Behavioral Thermoregulation

Practice Questions

Fever and Hyperthermia

Practice Questions

Hypothermia

Practice Questions

Exercise and Thermoregulation

Practice Questions

Thermoregulation in Extreme Environments

Practice Questions

Age-Related Changes in Thermoregulation

Practice Questions

Disorders of Thermoregulation

Practice Questions

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