Vasodilation, convective heating

Figure 13.3 Changes in cutaneous blood flow experienced as a result of vasoconstriction to reduce heat dissipation and convective heat transfer from the skin and vasodilation with the opposite effects.

If these processes produce too much heat, the body attempts to lose heat by vasodilation within the skin (via convection) and sweating (via evaporation of the water in the sweat). Both are well-known characteristics of fever. The patient s experience of alternate shivering and sweating (so well described by Hippocrates) probably represents an impairment of the thermorequlatory centre in the hypothalamus that regulates the balance between heat loss and heat production, resulting in fluctuations in body temperature. 

Thermal challenges are met in several ways. Blood sent to the limbs and blood returning from the limbs are normally conveyed by arteries and veins in close proximity deep inside the limb. This tends to conserve heat by countercurrent heat exchange between the arteries and veins. Thermal stress causes blood to return via surface veins rather than deep veins. Skin surface temperature increases and heat loss by convection and radiation also increases. In addition, vasodilation of cutaneous blood vessels augments surface heat loss but puts an additional burden on the heart to deliver added blood to the skin as well as the muscles. Heart rate increases as body temperature rises. 

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