Thermoregulation Practice Questions

Q: Which of the following is endogenous pyrogen?

PG E2. ***PG E2*** - **Prostaglandin E2 (PGE2)** is considered the **endogenous mediator of fever** in the context of this question (NEET-PG 2018). - **Mechanism:** PGE2 acts on the **preoptic area of the hypothalamus** to raise the body's temperature set point, leading to fever. - **Production pathway:** Endogenous pyrogens like **IL-1, IL-6, and TNF-α** stimulate **cyclooxygenase-2 (COX-2)** in hypothalamic endothelial cells → **PGE2 synthesis** → binds to EP3 receptors → raises set point. - **Clinical relevance:** This is why **antipyretics** (aspirin, paracetamol, NSAIDs) work by inhibiting COX enzymes, thereby reducing PGE2 production. - **Terminology note:** Strictly speaking, cytokines (IL-1, IL-6, TNF-α) are the true "endogenous pyrogens," while PGE2 is the "final common mediator" - but for exam purposes, PGE2 is accepted as the endogenous pyrogenic mediator. *PG D2* - **Prostaglandin D2 (PGD2)** is primarily involved in **allergic reactions**, **sleep regulation**, and modulating immune responses. - Main roles include **bronchoconstriction** in asthma, **vasodilation**, and mast cell activation. - It does **not** act as a fever mediator in the hypothalamus. *PGF2 alpha* - **Prostaglandin F2 alpha (PGF2α)** functions primarily in **reproductive physiology**. - Causes **uterine contractions** (labor induction), **luteolysis** (corpus luteum regression), and **bronchoconstriction**. - It has **no direct role** in fever generation or thermoregulation. *PG I2* - **Prostacyclin (PGI2)** is a potent **vasodilator** and **inhibitor of platelet aggregation**. - Produced by vascular endothelium, it has **anti-thrombotic** and **anti-inflammatory** properties. - It does **not** function as a pyrogenic mediator and may actually have **antipyretic effects** in some contexts.

Q: Fever increases water loss by how much for each degree Celsius increase in body temperature?

200 ml/day. ***200 ml/day*** - For every 1-degree Celsius (or 1.8-degree Fahrenheit) increase in body temperature, there is an approximate **200 ml increase in insensible water loss** per day due to increased metabolism and sweating. - This value highlights the importance of **adequate fluid replacement** in febrile patients to prevent dehydration. *100 ml/day* - This value is **insufficient** to account for the increased insensible fluid losses associated with fever. - Using this estimate could lead to **underestimation of fluid requirements** and potential dehydration in febrile patients. *400 ml/day* - This value is **higher than the typical estimated increase** in water loss per degree Celsius of fever. - While extreme fever might cause higher losses, 200 ml/day is the standard clinical approximation for a 1-degree rise. *800 ml/day* - This value represents a **significant overestimation** of the fluid loss per degree Celsius increase in fever. - Such a high estimate would generally be seen only in very severe conditions or with much larger temperature increases.

Q: What is the primary mechanism underlying hyperthermia?

Failure of thermoregulation. ***Failure of thermoregulation*** - **Hyperthermia** is fundamentally caused by the body's inability to dissipate heat effectively, leading to an uncontrolled rise in core body temperature. - This differentiates it from fever, where the **hypothalamic set point** is elevated, and the body actively tries to reach that higher temperature. *Temperature > 41.5°C* - While a temperature exceeding **41.5°C** is often seen in severe hyperthermia, it is a *consequence* of the failure of thermoregulation, not its primary cause. - This extreme temperature indicates a critical state, but the underlying problem is the body's inability to control internal heat. *Temperature > 40°C with autonomic dysfunction* - A temperature above **40°C** combined with **autonomic dysfunction** (e.g., altered mental status, seizures) describes a severe *manifestation* of hyperthermia, often seen in heat stroke. - This is a symptom complex resulting from, rather than the primary cause of, the body's thermoregulatory failure. *No change in hypothalamic set point* - This statement is a *characteristic* of hyperthermia, distinguishing it from fever, where the **hypothalamic set point** is elevated. - However, the *absence* of this change is not the primary cause; rather, the underlying issue is the body's inability to manage its heat load despite a normal set point.

Q: Shivering is controlled by?

Posterior hypothalamus. ***Posterior hypothalamus*** - The **posterior hypothalamus** plays a crucial role in **thermoregulation**, particularly in generating heat. - It receives input from cold receptors and initiates mechanisms like **shivering** and vasoconstriction to increase body temperature. *Dorsomedial nucleus* - The **dorsomedial nucleus** of the hypothalamus is primarily involved in **feeding behavior**, obesity, and some aspects of stress response. - It does not have a direct, primary role in the control of shivering. *Perifornical nucleus* - The **perifornical nucleus** is linked to **arousal**, stress responses, and **feeding behavior**. - Its functions are distinct from the direct control of thermogenesis and shivering. *Lateral hypothalamic area* - The **lateral hypothalamic area** is often referred to as the "**feeding center**" and is involved in hunger and motivation. - While it has broad roles in various autonomic functions, it is not the primary site for shivering control.

Q: Nonshivering thermogenesis in adults is due to:

Brown fat between the shoulders. ***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.

Q: During acclimatization to hot environments, increased sweating efficiency is primarily due to enhanced sensitivity of which receptors?

Cholinergic receptors. ***Cholinergic receptors*** - **Sweat glands** are innervated by **sympathetic postganglionic fibers** that release **acetylcholine**, acting on **muscarinic cholinergic receptors** to stimulate sweating. - During **acclimatization**, the sweat glands become more sensitive to acetylcholine, resulting in **increased sweating efficiency** and a lower threshold temperature for sweating. *Adrenergic receptors* - Adrenergic receptors are primarily involved in the sympathetic nervous system's response to **norepinephrine** and **epinephrine**, mediating effects like **vasoconstriction** and **bronchodilation**. - They are not the primary receptors responsible for stimulating **eccrine sweat gland** secretion in response to heat. *Noradrenergic receptors* - **Noradrenergic receptors** are a type of adrenergic receptor that respond to **norepinephrine** (noradrenaline). - While sympathetic activity increases in hot environments, the primary stimulation of **sweat glands** is via **acetylcholine** acting on cholinergic receptors, not noradrenergic receptors. *Dopaminergic receptors* - **Dopaminergic receptors** respond to **dopamine** and are involved in various functions including **motor control**, **reward**, and **neuroendocrine regulation**. - These receptors are not directly involved in the physiological regulation of **sweating efficiency** during heat acclimatization.

Q: Lesion of preoptic nucleus of hypothalamus is associated with which of the following conditions?

Hyperthermia. ***Hyperthermia*** - The **preoptic nucleus** of the anterior hypothalamus is the primary **heat-loss center** containing warm-sensitive neurons. - Lesion of this area impairs **heat dissipation mechanisms** (sweating, cutaneous vasodilation), preventing the body from lowering its temperature. - Results in **hyperthermia** - a pathological elevation of core body temperature due to failure of heat dissipation, not a change in set point. - This is the **most specific and clinically accurate** term for this condition. *Impaired thermoregulation* - While technically true, this is too **broad and non-specific**. - Impaired thermoregulation could refer to inability to either increase or decrease temperature. - In medical terminology, we use more specific terms like "hyperthermia" to describe the actual clinical condition. *Increased body temperature* - This is a **general descriptive term** rather than a specific clinical diagnosis. - While the body temperature is indeed increased, **hyperthermia** is the precise medical term that indicates the mechanism (impaired heat dissipation). - Less specific than "hyperthermia" for exam purposes. *Normal thermoregulation* - Clearly incorrect - a lesion in the primary thermoregulatory center would **abolish normal temperature control**. - The preoptic nucleus is essential for detecting and responding to temperature changes.

Q: The temperature centre is?

Preoptic nucleus of hypothalamus. **Preoptic nucleus of hypothalamus** - The **preoptic nucleus** within the **hypothalamus** serves as the primary **thermoregulatory center** in the brain. - It contains both **heat-sensitive** and **cold-sensitive neurons** that monitor core body temperature and initiate appropriate responses to maintain homeostasis. *Supraoptic nucleus of hypothalamus* - The **supraoptic nucleus** is primarily involved in the production of **antidiuretic hormone (ADH)**, which regulates water balance. - It plays a crucial role in **fluid and electrolyte balance**, not temperature regulation. *Paraventricular nucleus of hypothalamus* - The **paraventricular nucleus** is multifunctional, producing **oxytocin** and **vasopressin** (ADH), and is involved in stress response and feeding. - While it has broad regulatory roles, it is not the primary center for **temperature control**. *Suprachiasmatic nucleus of hypothalamus* - The **suprachiasmatic nucleus (SCN)** is the body's main **circadian clock**, regulating daily rhythms like the sleep-wake cycle. - Its primary function is to synchronize biological activities with the **24-hour light-dark cycle**, not directly control body temperature.

Q: What is the PRIMARY pathophysiological feature that defines severe heat stress?

Hyperpyrexia. ***Hyperpyrexia*** - **Hyperpyrexia** (a core body temperature typically exceeding 40°C or 104°F) is the hallmark of severe heat stress, indicating a failure of the body's thermoregulatory mechanisms. - This extreme elevation in body temperature can lead to **cellular damage**, organ dysfunction, and is a defining characteristic of conditions like heat stroke. *Syncope* - **Syncope** (fainting) can occur in heat exhaustion due to **vasodilation** and **orthostatic hypotension**, but it is not the primary defining pathophysiological feature of severe heat stress itself. - While a symptom, it does not represent the overarching systemic thermoregulatory failure that defines severe heat stress. *Cramps* - **Heat cramps** are painful muscle spasms resulting from **electrolyte imbalances** and **dehydration**, usually occurring during strenuous activity in a hot environment. - Though a heat-related illness, cramps represent a milder form of heat stress and do not signify the systemic danger of **thermoregulatory collapse** seen in severe cases. *Hypothermia* - **Hypothermia** is a condition where the body loses heat faster than it can produce it, leading to a dangerously low body temperature, usually below 35°C (95°F). - This is the **opposite** of what occurs in heat stress; severe heat stress is characterized by an excessive **increase** in body temperature.

Q: The diagram shows the effects of changing the set point of the hypothalamic temperature controller. Which set of changes occurs at point W compared with point V?

Shivering and vasoconstriction. ***Shivering and vasoconstriction*** - At point V, the physiological response is aiming to maintain a normal body temperature, while at point W, the set point is elevated, indicating the body is trying to **increase its core temperature** to match the new set point. - To increase body temperature, the body initiates heat-generating mechanisms like **shivering** (increased muscle activity) and heat-conserving mechanisms like **vasoconstriction** (reducing heat loss through the skin). *Shivering and vasodilation* - While **shivering** helps generate heat, **vasodilation** would increase blood flow to the skin, leading to greater heat loss, which is counterproductive when trying to raise body temperature. - Therefore, these two responses would typically not occur simultaneously in this scenario. *Sweating and vasoconstriction* - **Sweating** is a mechanism for heat loss, which would occur if the body needed to cool down, not warm up to reach a higher set point. - **Vasoconstriction** helps conserve heat, but it would not be coupled with sweating in this context. *Sweating and vasodilation* - Both **sweating** and **vasodilation** are mechanisms the body uses to **lose heat** and cool down. - These responses would be observed if the body temperature was above the set point and needed to decrease, not when aiming to increase the core temperature to match an elevated set point.

Question 1

Which of the following is endogenous pyrogen?

Accepted Answer

PG E2

Answer

PG D2

Answer

PGF2 alpha

Answer

PG I2

Question 2

Fever increases water loss by how much for each degree Celsius increase in body temperature?

Accepted Answer

200 ml/day

Answer

100 ml/day

Answer

400 ml/day

Answer

800 ml/day

Question 3

What is the primary mechanism underlying hyperthermia?

Accepted Answer

Failure of thermoregulation

Answer

Temperature > 40°C with autonomic dysfunction

Answer

Temperature > 41.5°C

Answer

No change in hypothalamic set point

Question 4

Shivering is controlled by?

Accepted Answer

Posterior hypothalamus

Answer

Dorsomedial nucleus

Answer

Perifornical nucleus

Answer

Lateral hypothalamic area

Question 5

Nonshivering thermogenesis in adults is due to:

Accepted Answer

Brown fat between the shoulders

Answer

Muscle metabolism

Answer

Thyroid hormone

Answer

Noradrenaline

Question 6

During acclimatization to hot environments, increased sweating efficiency is primarily due to enhanced sensitivity of which receptors?

Accepted Answer

Cholinergic receptors

Answer

Adrenergic receptors

Answer

Noradrenergic receptors

Answer

Dopaminergic receptors

Question 7

Lesion of preoptic nucleus of hypothalamus is associated with which of the following conditions?

Accepted Answer

Hyperthermia

Answer

Impaired thermoregulation

Answer

Increased body temperature

Answer

Normal thermoregulation

Question 8

The temperature centre is?

Accepted Answer

Preoptic nucleus of hypothalamus

Answer

Supraoptic nucleus of hypothalamus

Answer

Paraventricular nucleus of hypothalamus

Answer

Suprachiasmatic nucleus of hypothalamus

Question 9

What is the PRIMARY pathophysiological feature that defines severe heat stress?

Accepted Answer

Hyperpyrexia

Answer

Hypothermia

Answer

Syncope

Answer

Cramps

Question 10

The diagram shows the effects of changing the set point of the hypothalamic temperature controller. Which set of changes occurs at point W compared with point V?

Accepted Answer

Shivering and vasoconstriction

Answer

Sweating and vasoconstriction

Answer

Shivering and vasodilation

Answer

Sweating and vasodilation

Thermoregulation — MCQs

Thermoregulation — MCQs

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