Thermoregulation — MCQs

Thermoregulation Indian Medical PG Practice Questions and MCQs

Question 31: In extreme cold, which is NOT a mechanism of thermogenesis?

A. Shivering
B. Increased secretion of epinephrine
C. Increased thyroxine
D. Piloerection (Correct Answer)

Explanation: ### Explanation The core of this question lies in distinguishing between **thermogenesis** (heat production) and **thermoproduction/heat conservation**. **1. Why Piloerection is the Correct Answer:** Piloerection (contraction of the *arrector pili* muscles, causing "goosebumps") is a **heat conservation mechanism**, not a thermogenic one. In animals with thick fur, it traps a layer of stagnant air near the skin, acting as an insulator. In humans, while the reflex remains, it is vestigial and provides negligible thermal benefit. Crucially, it does not generate significant metabolic heat; therefore, it is not classified as thermogenesis. **2. Analysis of Incorrect Options (Thermogenic Mechanisms):** * **Shivering (Option A):** This is the most potent form of **shivering thermogenesis**. It involves involuntary rhythmic muscle contractions where nearly all energy consumed is converted into heat rather than mechanical work. * **Increased Epinephrine (Option B):** This triggers **non-shivering thermogenesis**. Epinephrine and norepinephrine increase the metabolic rate and promote lipolysis (especially in brown adipose tissue), leading to rapid heat production. * **Increased Thyroxine (Option C):** This is a long-term adaptation. Thyroxine increases the Basal Metabolic Rate (BMR) by stimulating $Na^+$-$K^+$ ATPase activity across most tissues, leading to sustained heat production. **3. NEET-PG High-Yield Pearls:** * **Primary Center:** The **Posterior Hypothalamus** is the primary center for integration of cold responses (heat gain), while the **Anterior Hypothalamus** handles heat loss. * **Brown Fat:** Rich in **UCP-1 (Thermogenin)**, which uncouples oxidative phosphorylation to produce heat instead of ATP. This is vital in neonates. * **Efficiency:** Shivering can increase body heat production by **3 to 5 times** the basal level.

Question 32: When an individual is resting in a room with an air temperature of 21°C and 80% humidity, what is the greatest method of heat loss from the body?

A. Elevation of body metabolism
B. Vaporization of sweat
C. Respiration
D. Radiation and conduction (Correct Answer)

Explanation: **Explanation:** The primary mechanism of heat loss from the human body depends heavily on the ambient temperature and environmental conditions. **1. Why Radiation and Conduction are correct:** At a comfortable room temperature (21°C), the body temperature (37°C) is significantly higher than the surroundings. Under these conditions, **radiation** is the most significant contributor, accounting for approximately **60%** of total heat loss. **Conduction** to the air (and subsequent convection) accounts for another **15-18%**. Together, these passive physical processes are the dominant methods of thermolysis when the environmental temperature is lower than the body temperature. **2. Why the other options are incorrect:** * **Elevation of body metabolism:** This is a mechanism for **heat production** (thermogenesis), not heat loss. * **Vaporization of sweat:** While evaporation is the most effective cooling method in hot environments or during exercise, it is less dominant at 21°C. Furthermore, the **80% humidity** mentioned in the question reduces the efficiency of sweat evaporation, as the air is already saturated with moisture. * **Respiration:** Heat loss through respiration (insensible water loss and warming of air) occurs but only accounts for a small fraction (about 10%) of total heat loss. **High-Yield NEET-PG Pearls:** * **Radiation** is the primary mode of heat loss at room temperature (60%). * **Evaporation** becomes the **only** effective method of heat loss when the ambient temperature exceeds the body temperature (>37°C). * High humidity impairs evaporation, leading to a higher risk of heat exhaustion. * The **Hypothalamus** is the central thermostat: the Anterior hypothalamus (Pre-optic area) regulates heat loss, while the Posterior hypothalamus regulates heat production.

Question 33: What is true regarding animals that are chronically exposed to cold?

A. Increased sympathetic stimulation (Correct Answer)
B. Increased vagal action
C. Increased insulin levels in blood
D. Decreased blood supply to adipose tissue

Explanation: **Explanation:** Chronic exposure to cold triggers a physiological adaptation known as **cold acclimatization**, which is primarily mediated by the **Sympathetic Nervous System (SNS)**. **1. Why Option A is Correct:** When the body is chronically exposed to cold, the hypothalamus stimulates the sympathetic nerves. This leads to the release of **Norepinephrine**, which acts on **$\beta_3$-adrenergic receptors** in **Brown Adipose Tissue (BAT)**. This process, known as **Non-Shivering Thermogenesis (NST)**, involves the activation of **Uncoupling Protein-1 (UCP-1 or Thermogenin)**. UCP-1 uncouples oxidative phosphorylation from ATP production, dissipating energy as heat to maintain core body temperature. **2. Why the Other Options are Incorrect:** * **Option B:** Vagal (parasympathetic) action is associated with "rest and digest" functions. It decreases metabolic rate and heart rate, which would be counterproductive in a cold environment where heat production is required. * **Option C:** Insulin is an anabolic hormone. In cold stress, the body prioritizes catabolism (lipolysis and glycogenolysis) to provide fuel for thermogenesis. Therefore, counter-regulatory hormones like glucagon and catecholamines rise, while insulin effectiveness may decrease. * **Option D:** Blood supply to adipose tissue (especially Brown Fat) **increases** significantly during cold exposure to deliver oxygen and nutrients required for thermogenesis and to distribute the generated heat to the rest of the body. **NEET-PG High-Yield Pearls:** * **Brown Fat vs. White Fat:** Brown fat has high mitochondrial density and contains UCP-1 (Thermogenin). * **Primary site of NST:** In neonates, brown fat is the primary source of heat as they cannot shiver effectively. * **Thyroxine's Role:** Chronic cold exposure also increases TSH and Thyroxine ($T_4$) levels, which enhances the metabolic rate (long-term adaptation).

Question 34: A newborn infant prevents hypothermia by which mechanism?

A. Coupling of oxidation with phosphorylation
B. Uncoupling of oxidation with phosphorylation
C. Non-shivering thermogenesis (Correct Answer)
D. Shivering thermogenesis

Explanation: **Explanation:** The correct answer is **Non-shivering thermogenesis (NST)**. Newborns have a high surface-area-to-volume ratio, making them prone to rapid heat loss. Unlike adults, neonates cannot effectively generate heat through shivering because their skeletal muscle mass is immature. Instead, they rely on NST, which occurs primarily in **Brown Adipose Tissue (BAT)**. **Why the options are right/wrong:** * **Non-shivering thermogenesis (Correct):** This is the primary heat-production mechanism in neonates. When exposed to cold, norepinephrine stimulates the breakdown of triglycerides in brown fat. * **Uncoupling of oxidation with phosphorylation (Underlying Mechanism):** While this is the *biochemical process* that drives NST, the question asks for the *mechanism/method*. In brown fat, a protein called **Thermogenin (UCP-1)** uncouples the mitochondrial electron transport chain from ATP synthesis. Instead of producing ATP, the energy from oxidation is dissipated as **heat**. * **Coupling of oxidation with phosphorylation (Incorrect):** This is the standard process of aerobic respiration (ATP production) and does not generate the excess heat required for thermoregulation. * **Shivering thermogenesis (Incorrect):** This is the primary mechanism in adults. Newborns lack the muscle mass and neurological maturity to shiver effectively. **High-Yield Clinical Pearls for NEET-PG:** 1. **Brown Fat Distribution:** Located in the interscapular region, axillae, mediastinum, and around the kidneys/adrenals. 2. **Thermogenin (UCP-1):** The specific protein located in the inner mitochondrial membrane of brown fat responsible for heat production. 3. **Brown vs. White Fat:** Brown fat contains numerous mitochondria and iron-containing cytochromes (giving it the brown color) and is highly vascularized. 4. **Clinical Significance:** Hypothermia in a neonate leads to increased oxygen consumption and metabolic acidosis, emphasizing the importance of "Kangaroo Mother Care" (KMC).

Question 35: What is the most important hormone that increases gallbladder contraction after a fatty meal?

A. Gastrin
B. Secretin
C. Cholecystokinin (CCK) (Correct Answer)
D. Gastric inhibitory peptide (GIP)

Explanation: **Explanation:** The correct answer is **Cholecystokinin (CCK)**. **Why CCK is correct:** CCK is a peptide hormone secreted by the **I-cells** of the duodenum and jejunum in response to the presence of fatty acids and amino acids in the intestinal lumen. It is the primary mediator of gallbladder contraction. CCK acts by: 1. Causing forceful **contraction of the gallbladder wall**. 2. Simultaneously causing **relaxation of the Sphincter of Oddi**, allowing bile to flow into the duodenum to emulsify fats. **Analysis of Incorrect Options:** * **Gastrin (Option A):** Secreted by G-cells of the stomach; its primary role is stimulating gastric acid secretion and mucosal growth. While it shares structural similarity with CCK, it has minimal effect on the gallbladder. * **Secretin (Option B):** Secreted by S-cells in response to low pH (acid). Its main function is to stimulate the secretion of bicarbonate-rich pancreatic juice and "alkaline bile" from the bile ducts, rather than gallbladder contraction. * **Gastric Inhibitory Peptide (GIP) (Option D):** Secreted by K-cells; its primary role is stimulating insulin release (incretin effect) and inhibiting gastric acid secretion. **High-Yield Clinical Pearls for NEET-PG:** * **Stimulus for CCK:** Most potent stimulus is the presence of **fatty acids** and peptides in the duodenum. * **Diagnostic Use:** CCK analogues (e.g., Sincalide) are used in HIDA scans to assess gallbladder ejection fraction. * **Other CCK functions:** It inhibits gastric emptying (to allow more time for fat digestion) and stimulates the secretion of enzyme-rich pancreatic juice. * **Vagal Influence:** While CCK is the most important hormonal stimulus, the **Vagus nerve** provides the primary neural stimulus for gallbladder contraction during the cephalic phase of digestion.

Question 36: What is the physiological effect in an unacclimatized person suddenly exposed to cold?

A. Tachycardia
B. Shift of blood from shell to core (Correct Answer)
C. Non-shivering thermogenesis
D. Hypertension

Explanation: **Explanation:** The primary physiological response to sudden cold exposure is the preservation of core body temperature through **peripheral vasoconstriction**. **1. Why Option B is Correct:** When an unacclimatized person is exposed to cold, the sympathetic nervous system triggers intense vasoconstriction of the cutaneous blood vessels (the "shell"). This reduces blood flow to the skin, minimizing heat loss via radiation and convection. Consequently, blood is shunted from the peripheral circulation to the deep visceral organs (the "core"). This **shift of blood from shell to core** acts as an internal insulator, protecting vital organs and maintaining the core temperature. **2. Why Other Options are Incorrect:** * **Tachycardia:** While cold can cause a transient sympathetic surge, the shift of blood to the core increases venous return (preload), which often triggers the **Bainbridge reflex** or a baroreceptor response, leading to a compensatory **bradycardia** rather than sustained tachycardia. * **Non-shivering thermogenesis:** This is a metabolic heat production mechanism primarily mediated by **brown adipose tissue**. It is the dominant mechanism in **neonates** but is negligible in unacclimatized adults, who rely primarily on shivering. * **Hypertension:** While peripheral resistance increases, "cold diuresis" (due to inhibited ADH and increased ANP from the core blood shift) eventually helps regulate volume; hypertension is not the primary physiological goal of thermoregulation. **High-Yield Clinical Pearls for NEET-PG:** * **Shivering** is the most potent mechanism for heat production in adults (increasing metabolic rate by 3–5 times). * **The Posterior Hypothalamus** is the integration center for responses to **cold**, while the **Anterior Hypothalamus** handles **heat**. * **Cold Diuresis:** The shift of blood to the core is perceived by the heart as fluid overload, leading to decreased ADH secretion and increased urine output.

Question 37: Which of the following is not a feature of heat stroke?

A. Very high body temperature
B. Dry, hot skin
C. Sweating (Correct Answer)
D. Constricted pupils

Explanation: **Explanation:** Heat stroke is a life-threatening medical emergency characterized by a failure of the body's thermoregulatory mechanisms. It is clinically defined by a core body temperature exceeding **40°C (104°F)** and central nervous system dysfunction. **Why "Sweating" is the correct answer:** The hallmark of classic heat stroke is the **cessation of sweating (anhidrosis)**. This occurs because the hypothalamic thermoregulatory center fails, and the sweat glands become exhausted or dysfunctional due to extreme heat stress. Therefore, the skin becomes **dry and hot**, rather than moist. **Analysis of other options:** * **A. Very high body temperature:** This is the primary feature of heat stroke. The body's heat gain (from the environment or metabolic activity) far exceeds its heat loss capacity. * **B. Dry, hot skin:** As sweating stops, the skin loses its primary cooling mechanism (evaporation), leading to a characteristically dry, flushed, and hot appearance. * **D. Constricted pupils:** In the advanced stages of heat stroke and associated neurological collapse, miosis (constricted pupils) is frequently observed, though pupillary responses can vary as the condition progresses to coma. **High-Yield Clinical Pearls for NEET-PG:** * **Two Types:** *Classic Heat Stroke* (seen in elderly/infants during heatwaves; anhidrosis is common) vs. *Exertional Heat Stroke* (seen in athletes/military; sweating may still be present initially). * **Neurological Signs:** Confusion, seizures, and coma are key diagnostic criteria. * **Management:** The gold standard treatment is **rapid cooling**, specifically **evaporative cooling** or **ice-water immersion**, to bring the core temperature below 39°C immediately. * **Complication:** Disseminated Intravascular Coagulation (DIC) and Multi-Organ Dysfunction Syndrome (MODS) are common causes of death.

Question 38: Which of the following is the primary neurotransmitter responsible for lowering the thermoregulatory set point in triggering hot flushes?

A. Acetylcholine
B. Norepinephrine (Correct Answer)
C. Dopamine
D. Glycine

Explanation: ### Explanation **Correct Answer: B. Norepinephrine** **Mechanism and Concept:** The thermoregulatory center is located in the **preoptic area of the hypothalamus**. In conditions like menopause (estrogen deficiency), there is a significant alteration in the neurochemical environment of the hypothalamus. Estrogen normally modulates the activity of neurotransmitters that maintain the "thermoneutral zone." When estrogen levels drop, there is an **elevation in brain norepinephrine (NE)** levels. This excess norepinephrine acts on **$\alpha_2$-adrenergic receptors**, narrowing the thermoneutral zone and lowering the thermoregulatory set point. Consequently, even a minor increase in core body temperature triggers an exaggerated heat-dissipation response, manifesting as a "hot flush" (vasodilation and sweating). **Analysis of Incorrect Options:** * **A. Acetylcholine:** While acetylcholine is the primary neurotransmitter for the *efferent* limb of thermoregulation (stimulating eccrine sweat glands via sympathetic cholinergic fibers), it is not the trigger for resetting the hypothalamic set point. * **C. Dopamine:** Dopamine is involved in various hypothalamic functions (like prolactin inhibition), but it does not play a primary role in the narrowing of the thermoneutral zone seen in hot flushes. * **D. Glycine:** Glycine is a major inhibitory neurotransmitter in the spinal cord and brainstem; it is not involved in the hypothalamic regulation of the thermal set point. **High-Yield Clinical Pearls for NEET-PG:** * **The "Estrogen-Withdrawal" Theory:** Hot flushes are not caused by low estrogen alone, but by the *withdrawal* of estrogen, which leads to increased NE and Serotonin (5-HT) activity. * **Pharmacotherapy:** Because NE and Serotonin are involved, **SSRIs and SNRIs** (like Venlafaxine) are effective non-hormonal treatments for hot flushes. * **Clonidine:** An $\alpha_2$-agonist, clonidine can sometimes be used to reduce the frequency of hot flushes by modulating noradrenergic activity.

Question 39: Which of the following statements about the hormone leptin is FALSE?

A. Leptin levels are typically increased in obese individuals.
B. Leptin exerts its primary action on the hypothalamus.
C. Leptin is synthesized and secreted by adipose tissue.
D. Leptin stimulates appetite. (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Leptin stimulates appetite.** **Why Option D is the correct (False) statement:** Leptin is an **anorexigenic** hormone, meaning it **suppresses appetite**. It is often referred to as the "satiety hormone." Leptin acts on the arcuate nucleus of the hypothalamus to inhibit Neuropeptide Y (NPY) and Agouti-related peptide (AgRP)—which are hunger stimulants—while simultaneously stimulating Pro-opiomelanocortin (POMC) neurons to promote satiety and increase energy expenditure. **Analysis of Incorrect Options:** * **Option A:** In most cases of human obesity, leptin levels are high. However, obese individuals often suffer from **leptin resistance**, where the brain fails to respond to the satiety signal despite high circulating levels. * **Option B:** The primary site of action for leptin is the **Hypothalamus** (specifically the arcuate nucleus), where it regulates long-term energy balance and body weight. * **Option C:** Leptin is a peptide hormone primarily synthesized and secreted by **white adipose tissue** (adipocytes). The amount of leptin secreted is directly proportional to the total body fat mass. **High-Yield NEET-PG Pearls:** * **Gene Association:** Leptin is encoded by the *ob* (obese) gene; its receptor is encoded by the *db* (diabetes) gene. * **Thermoregulation:** Leptin increases metabolic rate and heat production by stimulating sympathetic activity and upregulating **uncoupling protein-1 (UCP1/Thermogenin)** in brown adipose tissue. * **Opposing Hormone:** **Ghrelin**, produced by the stomach (P/D1 cells), is the "hunger hormone" and acts as the physiological antagonist to leptin. * **Reproduction:** Leptin is necessary for the onset of puberty; very low levels (as seen in anorexia or extreme exercise) can lead to hypothalamic amenorrhea.

Question 40: Shivering is controlled by which part of the brain?

A. Medulla
B. Hypothalamus (Correct Answer)
C. Thalamus
D. Basal ganglia

Explanation: **Explanation:** The **Hypothalamus** is the primary center for thermoregulation in the body, often referred to as the "body's thermostat." Specifically, shivering is mediated by the **Posterior Hypothalamus**. When the body is exposed to cold, the posterior hypothalamus triggers the **Primary Motor Center for Shivering** (located in the dorsomedial portion). This center sends signals down the lateral columns of the spinal cord to increase muscle tone, eventually leading to the involuntary, rhythmic muscle contractions known as shivering, which generates heat through ATP hydrolysis. **Analysis of Incorrect Options:** * **Medulla:** Primarily contains autonomic centers for cardiovascular (vasomotor) and respiratory control, but does not regulate body temperature. * **Thalamus:** Acts as the major sensory relay station for the brain. While it relays temperature sensations to the cortex, it does not initiate the motor response of shivering. * **Basal Ganglia:** Involved in the regulation of voluntary motor movements, procedural learning, and posture, but plays no direct role in thermoregulatory reflexes. **High-Yield Clinical Pearls for NEET-PG:** * **Anterior Hypothalamus/Preoptic Area:** Responsible for heat loss (e.g., sweating, vasodilation). *Mnemonic: **A**nterior = **A**ir Conditioning.* * **Posterior Hypothalamus:** Responsible for heat conservation and production (e.g., shivering, vasoconstriction). *Mnemonic: **P**osterior = **P**roduction.* * **Brown Adipose Tissue:** In neonates, non-shivering thermogenesis occurs here via the protein **Thermogenin (UCP-1)**. * **Pyrogens:** Substances like IL-1 reset the hypothalamic set-point upward, leading to fever.

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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