Thermoregulation
Across Systems
How your body maintains a core temperature near \(37\,^\circ\text{C}\) through the coordinated action of skin, blood flow, and sweating.
Anatomy & Physiology
What Is Thermoregulation?
Thermoregulation is the physiological process by which the body maintains its internal core temperature within a narrow, optimal range: approximately \(36.5\)–\(37.5\,^\circ\text{C}\). This is a prime example of homeostasis: the body's ability to maintain stable internal conditions despite external changes.
Why Does Temperature Matter?
Enzyme Function
Enzymes operate optimally near \(37\,^\circ\text{C}\). Deviations slow reactions or cause denaturation.
Cellular Metabolism
ATP production via cellular respiration depends on tightly regulated temperature conditions.
Organ Damage
Sustained temps above \(40\,^\circ\text{C}\) or below \(35\,^\circ\text{C}\) can damage the brain, heart, and kidneys.
The Three Key Systems
Skin (Integumentary)
Acts as a physical barrier and radiator. Contains thermoreceptors that detect temperature changes.
Blood Flow (Cardiovascular)
Arterioles dilate or constrict to control heat transfer between the core and the skin surface.
Sweating (Exocrine)
Eccrine sweat glands secrete fluid onto the skin. Evaporation absorbs heat, cooling the body.
The Negative Feedback Loop
Vasodilation vs. Vasoconstriction
Sweating and Evaporative Cooling
Cold Response: Shivering and Heat Generation
When core temperature drops below the set point, the hypothalamus triggers shivering thermogenesis: rapid, involuntary muscle contractions that convert chemical energy (ATP) into heat.
1
Thermoreceptors in the skin detect a drop in surface temperature and send signals via afferent neurons.
2
The hypothalamus compares input to the set point and activates the shivering center in the posterior hypothalamus.
3
Skeletal muscles contract rapidly and rhythmically, generating heat as a byproduct of ATP hydrolysis.
4
Vasoconstriction simultaneously reduces heat loss from the skin surface, conserving core warmth.
Heat Response: Cooling Pathway
1
Peripheral thermoreceptors detect rising skin temperature above \(37\,^\circ\text{C}\).
2
The hypothalamus activates sympathetic signals to the skin's arterioles and sweat glands.
3
Vasodilation increases blood flow to the skin, promoting radiation and convection heat loss.
4
Eccrine sweat glands secrete sweat, and its evaporation absorbs heat energy from the skin.
5
Core temperature returns toward the set point, and the hypothalamus reduces the cooling response: negative feedback.
Four Mechanisms of Heat Exchange
Radiation
Emission of infrared heat waves from the body to cooler surroundings. No contact needed.
Conduction
Direct heat transfer between the body and a solid surface in contact (e.g., sitting on cold metal).
Convection
Heat carried away by moving air or water currents flowing over the skin surface.
Evaporation
Sweat absorbs heat energy as it transitions from liquid to gas on the skin. Most effective cooling method.
Example 1: Running on a Hot Day
A runner exercises on a \(35\,^\circ\text{C}\) day. Their core temperature begins to rise above \(37\,^\circ\text{C}\). Trace the thermoregulatory response.
1
Stimulus: Muscle activity generates metabolic heat, raising core temperature to approximately \(38.5\,^\circ\text{C}\).
2
Receptor: Central thermoreceptors in the hypothalamus and peripheral receptors in the skin detect the temperature increase.
3
Control Center: The hypothalamus sends signals via sympathetic pathways to sweat glands and cutaneous arterioles.
4
Effectors: Vasodilation increases skin blood flow (skin turns flushed). Eccrine glands produce sweat. Evaporation cools the skin.
5
Result: Core temp returns toward \(37\,^\circ\text{C}\). Negative feedback reduces sweating and vasodilation.
Example 2: Swimming in Cold Water
A swimmer enters \(15\,^\circ\text{C}\) lake water. Their skin temperature drops rapidly. Trace the thermoregulatory response.
1
Stimulus: Cold water conducts heat away from skin rapidly. Skin temp drops to approximately \(28\,^\circ\text{C}\).
2
Receptor: Cold thermoreceptors in the skin fire at increased rates, signaling the hypothalamus.
3
Control Center: The hypothalamus activates the heat-conserving and heat-producing pathways.
4
Effectors: Vasoconstriction minimizes heat loss at the skin. Shivering generates heat via involuntary muscle contractions. Piloerection (goosebumps) reduces convective heat loss.
5
Result: Core temperature is maintained near \(37\,^\circ\text{C}\) despite the cold environment.
Example 3: Fever and the Reset Set Point
During an infection, pyrogens (from pathogens or immune cells) cause the hypothalamus to raise the set point above \(37\,^\circ\text{C}\), for example to \(39\,^\circ\text{C}\).
Rising Phase
The body perceives its current \(37\,^\circ\text{C}\) as "too cold" relative to the new set point. Vasoconstriction and shivering occur: you feel chills.
Plateau Phase
Core temperature reaches \(39\,^\circ\text{C}\). The body now maintains this elevated temperature. You feel hot and flushed.
Defervescence
As pyrogens are cleared, the set point returns to \(37\,^\circ\text{C}\). Vasodilation and profuse sweating cool the body back down.
Common Mistakes
✕ Confusing the Feedback Type
Thermoregulation uses negative feedback (opposing the change), not positive feedback. Positive feedback would amplify the temperature deviation.
✕ Mixing Up Dilation and Constriction
Vasodilation = vessels widen = heat loss (cooling). Vasoconstriction = vessels narrow = heat retention (warming). Students often reverse these.
✕ Thinking Sweat Itself Cools
Sweat on the skin does not cool you: the evaporation of sweat does. In 100% humidity, sweat drips off without evaporating, and cooling fails.
Comparing Hot vs. Cold Responses
| Too Hot (\(\gt 37\,^\circ\text{C}\)) | Too Cold (\(\lt 37\,^\circ\text{C}\)) | |
|---|---|---|
| Blood Vessels | Vasodilation | Vasoconstriction |
| Sweat Glands | Active: produce sweat | Inactive: minimal output |
| Skeletal Muscles | Relaxed (no shivering) | Shivering: generate heat |
| Skin Appearance | Flushed (red, warm) | Pale (cool, blanched) |
| Hair (Arrector Pili) | Relaxed (hair lies flat) | Contracted (goosebumps) |
| Primary Heat Transfer | Evaporation + Radiation | Conduction minimized |
Question 1 of 5
Which structure acts as the control center for thermoregulation in the human body?
The hypothalamus receives input from thermoreceptors and coordinates both the heat-loss and heat-conservation responses. The cerebral cortex handles conscious thought, the medulla controls autonomic functions like breathing, and the thalamus relays sensory information.
Question 2 of 5
During exercise on a hot day, which combination of responses would the body initiate to lower core temperature?
Vasodilation brings warm blood to the skin surface for heat dissipation, while sweating promotes evaporative cooling. Vasoconstriction and shivering are heat-conservation responses, not cooling responses.
Question 3 of 5
Why does a person experience chills at the onset of a fever, even though their core temperature is rising?
Pyrogens raise the hypothalamic set point (e.g., to \(39\,^\circ\text{C}\)). The body's current \(37\,^\circ\text{C}\) is now below the new set point, so the hypothalamus triggers heat-generating responses (vasoconstriction, shivering), which the person perceives as chills.
Question 4 of 5
A person is sweating profusely but not cooling down on a very humid day. What is the most likely explanation?
Cooling via sweat depends on evaporation, not the liquid itself. When ambient humidity is high, the air is already saturated with water vapor, so sweat cannot evaporate efficiently: it simply drips off without absorbing heat. This is why humid heat feels worse than dry heat.
Question 5 of 5
Thermoregulation is an example of which type of feedback mechanism?
Negative feedback opposes the direction of change: if temperature rises, cooling mechanisms activate; if temperature falls, warming mechanisms activate. This returns the variable toward the set point. Positive feedback amplifies change (e.g., labor contractions), which is the opposite of thermoregulation's goal.