11 Thermoregulation Notes

Thermoregulation

Thermoregulation is the process by which the human body maintains its temperature in a constantly changing environment.
When the outside temperature is cold, the body employs several mechanisms:
- Increased metabolic rate to generate more heat.
- Blood-flow redistribution (vasoconstriction) to reduce heat loss from the skin.
- Countercurrent blood flow to minimize heat loss in limbs.
- Shivering to produce heat through muscle activity.
- Insulation through fat, feathers, or fur (though humans primarily rely on fat).
- Behavioral adaptations like huddling to conserve heat.

Counter-current exchange minimizes conductive and radiative heat loss in cold environments.
Heat lost by outgoing arterial blood re-heats returning venous blood, which minimizes heat loss in limbs.
The design of the heat exchange vascular network varies from simple side-by-side arrangements to highly complex “Rete” structures.
Counter current exchange: Exchange of heat between two vessels that run in opposite directions /same direction and are in close contact

Warm arterial blood moving away from the heart loses heat to the cooler venous blood returning to the heart.
At each stage, the warm blood losing heat is always warmer than the cool blood, ensuring heat transfer to the cool blood.
This constant transfer of heat increases the temperature of the cool blood as it moves back into the body.

A rete mirabile (Latin for “wonderful net”) is a complex intertwined network of veins and arteries.
Blood flow from the small arteries into the capillaries is controlled by vascular constrictors.

A great example of counter current exchange in human anatomy is in mechanism for keeping the testes at optimum temp. for producing healthy sperm which is 3-4 ^{\circ}C lower than normal body temp.
As body temp increases, the cremaster and dartos muscles relax, allowing the scrotum (& testes) to be lowered further away from the body & increasing the effective length of the veinous pampiniform plexus.
This cools the blood which, in turn, keeps the testis at optimum temp.
Increased cooling is also facilitated by the larger scrotal surface area over which body heat is lost.
Despite the enhanced testicular cooling, the body minimizes total heat loss from the blood via counter-current exchange by passing it through the pampiniform plexus.

Piloerection (goosebumps) is a vestigial response that tries to trap a thicker layer of insulating air.

Veinous blood flow near the skin reduces as outside temperature decreases.
Recall flow rate reduces with r^4 (Poisseuille Eq.).
Blood flow through subcutaneous capillary networks is controlled by sphincter muscles and shunts.
In persistent cold conditions, blood is redirected away from extremities, supplying only vital organs in the core of the body.
This can lead to frostbite (= ischaemic damage to the extremities).
Example: Beck Weathers' frostbite on Mt. Everest, leading to amputations.

Vasodilation increases blood flow to sub-cutaneous capillaries by relaxing and opening sphincters, allowing heat to be transferred to surroundings.
Increase blood flow to the skin and therefore increase heat loss to the environment. This is achieved by using:
- Sphincter muscles that increase the cross- sectional area of the vessel
- Shunt blood vessels that redirect blood flow.

Sweating (perspiration) steals Latent Heat of Vapourization from the skin to form vapour and the loss of heat energy cools the body.
Humans perspire more than any other mammal.
This is seen as an evolutionary advantage that allowed early ancestors to hunt prey by running them down over long distances without overheating (persistence hunting).
Perspiration is the only cooling mechanism that works when outside T > 37 ^{\circ}C.

Before the introduction of paracetamol, fever patients were rubbed with damp cloths or cloths soaked with alcohol.
Alcohol has a lower boiling point than water, so it evaporates more vigorously at 37 ^{\circ}C and accelerates cooling (but with serious side-effects).

Thermal homeostasis is controlled by the hypothalamus, which activates all the thermoregulatory processes
The hypothalamus also activates heat-loss centers and heat-promoting centers, depending on whether the blood is warmer or cooler than the hypothalamic set point

Perspiration is not efficient if you are covered in fur or feathers.
Cats & dogs use evaporative cooling of saliva over their tongues (panting).
Some marsupial species cover their fore-arms with saliva to increase evaporative cooling.
Shoebills & Pelicans increase the surface area for evaporative cooling.
Storks squirt liquidy faeces over their bare legs to evaporatively cool themselves.