Thermoregulation — MCQs

Thermoregulation Indian Medical PG Practice Questions and MCQs

Question 41: Nonshivering thermogenesis is due to what?

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

Explanation: **Explanation:** **Non-shivering thermogenesis (NST)** is a critical mechanism for heat production, especially in neonates who lack the ability to shiver effectively. The primary site for NST is **Brown Adipose Tissue (BAT)**. 1. **Why Norepinephrine is correct:** When the body is exposed to cold, the sympathetic nervous system is activated. Sympathetic nerve endings release **Norepinephrine**, which acts on **$\beta_3$-adrenergic receptors** in brown fat. This triggers a signaling cascade that activates **Uncoupling Protein-1 (UCP-1)**, also known as **Thermogenin**, located in the inner mitochondrial membrane. Thermogenin uncouples oxidative phosphorylation from ATP production, causing the energy generated by the electron transport chain to be dissipated as **heat** instead of being stored as ATP. 2. **Why the other options are incorrect:** * **Acetylcholine:** This is the primary neurotransmitter of the parasympathetic nervous system and the neuromuscular junction. While it triggers sweating (heat loss) via eccrine glands, it does not mediate heat production. * **Dopamine:** Primarily acts as a neurotransmitter in the CNS (reward/motor control) and a vasodilator in the periphery; it has no direct role in brown fat thermogenesis. * **Serotonin:** Involved in mood regulation and GI motility. While central serotonin can influence the hypothalamic set-point, it is not the effector molecule for NST. **High-Yield Clinical Pearls for NEET-PG:** * **Brown Fat Distribution:** In neonates, it is found in the interscapular region, axilla, and around deep organs (kidneys/adrenals). * **Thermogenin (UCP-1):** The hallmark protein of brown fat that allows protons to leak across the mitochondrial membrane. * **Thyroid Hormone:** Acts synergistically with Norepinephrine to increase the metabolic rate and enhance NST. * **Adults:** Once thought to disappear, functional brown fat is now known to persist in adults (cervical and supraclavicular regions) and can be visualized on PET scans.

Question 42: Which of the following is seen in cold temperature?

A. Increased Thyroxine release (Correct Answer)
B. Decreased Thyroxine release
C. Unaltered Thyroxine release
D. None of the above

Explanation: **Explanation:** **1. Why Option A is Correct:** Thermoregulation is primarily controlled by the **Hypothalamus**. When the body is exposed to cold temperatures, the anterior hypothalamus triggers the release of **Thyrotropin-Releasing Hormone (TRH)**. TRH stimulates the anterior pituitary to release **Thyroid-Stimulating Hormone (TSH)**, which in turn increases the secretion of **Thyroxine (T4)** from the thyroid gland. Thyroxine is a potent stimulator of **Basal Metabolic Rate (BMR)**. It increases cellular metabolism and oxygen consumption, leading to **obligatory thermogenesis** (heat production). This chemical thermogenesis is a crucial long-term adaptation to maintain core body temperature in cold environments. **2. Why Other Options are Incorrect:** * **Option B & C:** Decreased or unaltered thyroxine levels would fail to increase the metabolic rate. Without an increase in T4, the body would be unable to sustain the heat production required to counteract environmental cold, leading to hypothermia. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Primary Center:** The **Posterior Hypothalamus** is responsible for heat conservation and production (activated by cold), while the **Anterior Hypothalamus** (Pre-optic area) is responsible for heat loss (activated by warmth). * **Mechanisms of Heat Production:** 1. **Shivering Thermogenesis:** Involuntary muscle contractions (most rapid response). 2. **Non-Shivering Thermogenesis:** Occurs in **Brown Adipose Tissue** (rich in mitochondria and **UCP-1/Thermogenin**), primarily in neonates. 3. **Chemical Thermogenesis:** Mediated by Thyroxine and Epinephrine/Norepinephrine. * **Piloerection:** A sympathetic response to cold ("goosebumps") intended to trap a layer of insulating air, though it is vestigial in humans.

Question 43: Which of the following is true about arteriovenous shunt vessels?

A. Evenly distributed throughout the skin
B. Role in nutrition
C. Play a role in thermoregulation (Correct Answer)
D. No autonomic nervous regulation

Explanation: **Explanation:** Arteriovenous (AV) shunts (or anastomoses) are direct connections between arterioles and venules that bypass the capillary bed. Their primary function is **thermoregulation** rather than nutrient exchange. **Why Option C is Correct:** AV shunts are located primarily in the "apical" skin (fingertips, toes, ears, nose, and lips). When the body needs to dissipate heat, these shunts dilate, allowing a high volume of warm blood to flow rapidly into the superficial venous plexuses. This facilitates heat loss through radiation and conduction. Conversely, in cold environments, these shunts constrict to conserve core body heat. **Why the Other Options are Incorrect:** * **Option A:** AV shunts are **not evenly distributed**. They are highly concentrated in acral (apical) regions. Non-apical skin (trunk, limbs) contains few shunts and relies more on capillary flow and sweat gland activity for cooling. * **Option B:** These vessels have **no role in nutrition**. Because they bypass the capillary beds, no significant exchange of gases, nutrients, or waste occurs across their thick muscular walls. * **Option C:** They are under **intense autonomic regulation**. They are primarily innervated by **sympathetic adrenergic fibers**. A decrease in sympathetic tone causes vasodilation (heat loss), while an increase causes vasoconstriction (heat conservation). **High-Yield Clinical Pearls for NEET-PG:** * **Glomus Body:** A specialized AV shunt found in the fingertips, often the site of a painful "Glomus tumor." * **Lewis Triple Response:** While involving capillaries and arterioles, remember that skin blood flow can increase up to 30 times its basal rate during maximal vasodilation. * **Neurogenic Control:** The hypothalamus acts as the "thermostat" of the body, controlling these shunts via the sympathetic nervous system.

Question 44: Pyrogens act on which site in the brain?

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

Explanation: ### Explanation **Correct Answer: D. Hypothalamus** **Why it is correct:** The **hypothalamus** is the primary center for thermoregulation in the human body, often referred to as the body's "thermostat." Pyrogens (substances that cause fever) act by inducing the production of **Prostaglandin E2 (PGE2)**. Specifically, exogenous pyrogens (like bacterial toxins) trigger immune cells to release endogenous pyrogens (IL-1, IL-6, TNF-α). These cytokines act on the **Organum Vasculosum of the Lamina Terminalis (OVLT)** in the hypothalamus. This triggers the release of PGE2, which acts on the **Preoptic Area (POA)** of the hypothalamus to "reset" the thermal set-point to a higher level, resulting in fever. **Why the other options are incorrect:** * **A. Basal Ganglia:** This area is primarily involved in the control of posture and voluntary motor movements (e.g., Parkinson’s disease pathology). It has no role in temperature regulation. * **B. Limbic System:** This system (including the amygdala and hippocampus) is responsible for emotions, behavior, and long-term memory. * **C. Thalamus:** Known as the "relay station" of the brain, it processes sensory and motor signals to the cerebral cortex but does not contain the thermoregulatory set-point. **High-Yield Clinical Pearls for NEET-PG:** * **Anterior Hypothalamus (Preoptic area):** Controls heat dissipation (response to heat). Lesions here cause hyperthermia. * **Posterior Hypothalamus:** Controls heat conservation (response to cold). Lesions here cause poikilothermia (inability to regulate temperature). * **Mechanism of Antipyretics:** Drugs like Aspirin and Paracetamol reduce fever by inhibiting the enzyme **Cyclooxygenase (COX)**, thereby blocking the synthesis of PGE2 in the hypothalamus. * **Heat Stroke vs. Fever:** In fever, the hypothalamic set-point is raised; in heat stroke, the set-point is normal, but the body's heat-dissipating mechanisms are overwhelmed.

Question 45: All of the following changes occur in chronic starvation except?

A. Hyperthermia (Correct Answer)
B. Exhaustion
C. Heart atrophy
D. Hypotension

Explanation: ***Hyperthermia*** - Chronic starvation triggers a compensatory decrease in the **Basal Metabolic Rate (BMR)**, which is intended to conserve energy and heat production. - The resulting physiological state is typically **hypothermia** (low body temperature) due to both decreased metabolism and loss of insulating subcutaneous fat, meaning hyperthermia does *not* occur. *Exhaustion* - **Profound exhaustion** occurs due to the catabolism of muscle protein and fat reserves, leading to overall muscle wasting and weakness. - Lack of adequate caloric intake also results in states like **anemia** and **hypoglycemia**, which severely limit physical stamina and mental capacity. *Heart atrophy* - The heart, like skeletal muscle, undergoes **autocannibalism** (breakdown of protein) to supply amino acids for essential organ function. - This reduction in myocardial mass (atrophy) severely compromises **cardiac output** and contributes significantly to clinical instability and potential sudden death. *Hypotension* - **Low blood pressure** results from decreased effective circulating volume and reduced **cardiac output** stemming from bradycardia and myocardial atrophy. - Severe fluid and electrolyte shifts, often coupled with volume depletion, further exacerbate the tendency toward **orthostatic hypotension**.

Question 46: Patient presents to OPD with fever. Which area is most likely involved?

A. Periventricular hypothalamus
B. Pre-optic nucleus (Correct Answer)
C. Dorsomedial hypothalamus
D. Insular cortex

Explanation: ***Pre-optic nucleus*** - The **pre-optic nucleus** in the anterior hypothalamus contains heat-sensitive and cold-sensitive neurons and functions as the body's primary **thermoregulatory center** or **thermostat**. - Fever results when **pyrogens** (like **IL-1, IL-6**, and **TNF-α**) raise the set-point of this nucleus, leading to heat production and conservation. *Insular cortex* - The insular cortex is involved in processing emotions, interoception (awareness of the body's internal state), and pain, not primarily in regulating core body temperature. - It has a role in complex functions like taste, visceral sensation, and autonomic control but is not the central area for initiating fever. *Periventricular hypothalamus* - The periventricular zone of the hypothalamus is involved in various neuroendocrine functions, including releasing hormones like **somatostatin** and **vasopressin**. - It is not the principal area responsible for setting the body's core temperature or initiating the febrile response. *Dorsomedial hypothalamus* - The **dorsomedial nucleus** of the hypothalamus mainly regulates gastrointestinal activity and some affective behaviors, like fear and aggression. - While the hypothalamus is a thermal regulation hub, this specific nucleus is not the primary target for pyrogens causing fever.

Question 47: An individual is in an environment of a temperature of 47°C. What is the mechanism of heat loss?

A. Conduction
B. Radiation
C. Sweating (Correct Answer)
D. Convection

Explanation: ***Sweating*** - In an environment where the ambient temperature (47°C) is **higher than body temperature**, heat can only be lost through the evaporation of sweat. - **Evaporation** is the primary mechanism for cooling the body in hot environments when other forms of heat loss become ineffective or even cause heat gain. *Conduction* - **Conduction** involves the transfer of heat through direct contact between surfaces. - In an environment hotter than the body, conduction would cause **heat gain** by the body, not heat loss. *Radiation* - **Radiation** involves the transfer of heat in the form of electromagnetic waves. - When the ambient temperature is higher than body temperature, the body will **absorb radiant heat** from the environment, leading to heat gain, not loss. *Convection* - **Convection** is the transfer of heat through the movement of fluids (air or water). - In an environment with a temperature of 47°C, convection would cause **heat gain** as the surrounding hot air transfers heat to the body.

Question 48: Body temperature regulation centre is located at:

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

Explanation: ***Hypothalamus*** - The **hypothalamus** acts as the body's **thermostat**, integrating signals from temperature receptors and initiating appropriate responses to maintain **homeostasis**. - It contains both **heat-sensitive and cold-sensitive neurons** that monitor blood temperature and receive input from peripheral thermoreceptors. *Thalamus* - The **thalamus** functions primarily as a **relay station** for sensory information, directing it to the appropriate cortical areas. - While involved in perception, it does not directly regulate body temperature. *Basal ganglia* - The **basal ganglia** are crucial for **motor control**, learning, and executive functions. - They are not directly involved in the central regulation of body temperature. *Pituitary* - The **pituitary gland** is the "master gland" of the endocrine system, secreting hormones that regulate growth, metabolism, and reproduction. - Although it is anatomically and functionally connected to the hypothalamus, the pituitary itself does not directly regulate body temperature.

Question 49: A 72-year-old man undergoes resection of an abdominal aneurysm. He arrives in the ICU with a core temperature of 33°C (91.4°F) and shivering. Which of the following is a physiologic consequence of the shivering?

A. Rising mixed venous O2 saturation
B. Rising base excess
C. Increased production of CO2 (Correct Answer)
D. Decreased consumption of O2

Explanation: ***Increased production of CO2*** - Shivering is a physiological response to **hypothermia** that involves rapid, involuntary muscle contractions. - This muscle activity significantly increases **metabolic rate**, leading to higher oxygen consumption and consequently, increased **carbon dioxide production**. *Rising mixed venous O2 saturation* - Shivering increases tissue oxygen demand, therefore, the peripheral tissues extract more oxygen from the blood. - This increased extraction would lead to a *decrease* in mixed venous O2 saturation, as less oxygen returns to the heart. *Rising base excess* - Increased metabolic activity from shivering can lead to the production of **lactic acid** if oxygen demand outstrips supply (anaerobic metabolism). - This would result in **metabolic acidosis**, which is characterized by a *negative* base excess (or decreasing base excess), not a rising one. *Decreased consumption of O2* - Shivering is an active process that requires energy, and this energy is primarily generated through **aerobic metabolism** in the muscles. - Therefore, shivering leads to a significant *increase* in oxygen consumption, not a decrease.

Question 50: Which of the following physiological changes occur during fever?

A. Resetting of skin temperature
B. Thermoregulatory centre to shift to new level (Correct Answer)
C. Both of the above
D. None of the above

Explanation: ***Thermoregulatory centre to shift to new level*** - During a fever, **pyrogens** (e.g., interleukins, prostaglandins) act on the **hypothalamus**, which is the thermoregulatory center. - This action causes the hypothalamus to **reset its set point** to a higher temperature. The body then works to raise its core temperature to this new, elevated set point. *Resetting of skin temperature* - The **skin temperature** is a result of the body trying to achieve the new set point, but it's not the primary physiological change that *initiates* the fever. - Changes in skin temperature, like **vasoconstriction** leading to cold skin or **vasodilation** leading to warm skin, are mechanisms for heat retention or dissipation, driven by the change in the hypothalamic set point. *Both of the above* - While skin temperature does change, the fundamental physiological event driving fever is the **resetting of the hypothalamic thermoregulatory set point**. - Skin temperature changes are a **consequence** of the body's efforts to reach this new set point, not a primary cause or an independent "resetting" event itself. *None of the above* - This option is incorrect because the **thermoregulatory center** indeed shifts to a new, higher set point during fever. - This shift is the hallmark physiological response to pyrogens.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free