Thermoregulation: MDCAT Biology notes
Thermoregulation for MDCAT: endotherms vs ectotherms, the hypothalamic thermostat, responses to heat and cold, shivering, vasodilation, sweating and fever.
What is thermoregulation?
Thermoregulation is the maintenance of body temperature within a narrow range despite changes in the environment. It is one part of homeostasis, alongside osmoregulation and excretion. Enzymes work best near a particular temperature, so keeping core temperature steady (about 37 °C in humans) keeps metabolism efficient.
Endotherms and ectotherms
| Feature | Endotherms | Ectotherms |
|---|---|---|
| Main heat source | Own metabolism | Environment |
| Body temperature | Kept fairly constant (homeotherms) | Varies with surroundings |
| Examples | Birds and mammals | Fishes, amphibians, reptiles, most invertebrates |
Ectotherms regulate mainly by behaviour, such as basking in the sun or moving into shade.
The hypothalamic thermostat
In humans the temperature control centre is in the hypothalamus of the brain (not the thalamus, pons, cerebellum or spinal cord). It works as a thermostat with a set point. It receives information from temperature receptors in the skin and from the temperature of blood flowing through it, and it triggers corrective responses by negative feedback.
Responses to heat and cold
| Too hot (e.g. heat stroke, hot weather) | Too cold |
|---|---|
| Vasodilation: skin arterioles widen, more blood flows near the surface and heat is lost by radiation | Vasoconstriction: skin arterioles narrow, reducing heat loss |
| Sweating: evaporation of sweat removes heat | Shivering: rapid involuntary contractions of skeletal muscle generate heat |
| Hairs lie flat | Hairs raised (erector muscles), trapping air |
| Metabolic rate not raised | Metabolic rate raised (non-shivering thermogenesis, helped by thyroxine and adrenaline; brown fat in infants) |
If a person in hot weather cannot hold body temperature down, it is vasodilation and sweating that have failed. Heat stroke results when these cooling responses are overwhelmed.
Fever
During infection, substances called pyrogens (released by pathogens and by white blood cells) raise the set point of the hypothalamus. The body then feels cold at normal temperature and shivers and constricts skin vessels until it reaches the new, higher set point. Moderate fever may help fight infection; very high fever is dangerous because enzymes and proteins denature.
Common MDCAT traps
- The thermostat is in the hypothalamus, not the thalamus.
- Response to heat is vasodilation, not vasoconstriction.
- Shivering is involuntary, even though skeletal muscle is used.
- Birds are endotherms; fishes and amphibians are ectotherms.
- Fever is caused by pyrogens resetting the hypothalamus.
Quick revision
- Thermoregulation is part of homeostasis.
- Hot: vasodilation and sweating.
- Cold: vasoconstriction, shivering, raised hairs, higher metabolic rate.
- Endotherms: birds and mammals.
- Pyrogens raise the hypothalamic set point.