Answer
When we engage in unusually hard physical activity( work, exercise , sports) the muscles generate a great deal of heat. This heat which is produced not in the body core but in blood vessels of the the shell or adjacent regions must not be allowed to travel back to the core ( brani, thoracic, abdominopelvic organs) in such amounts that it disturbs the homoiothermic balance of core temperature. Although core temperature is normally warmer than shell temperature it must not be allowed to rise or fall out of its set range. Since the blood is the principal heat exchanger of the body this warm blood form the skeletal muscles must be exposed to mechanisms that lower its heat content (enthalpy). The body therefore acts to get rid of some of the heat before the blood gets to core structures like brain and heart--especially the brain. This desirable heat loss, is effected through the physical processes of radiation, convection, evaporation and sometimes conduction.
Work Step by Step
Normal core body temperature is usually in the range ( measured orally) of 97.6F to 99.6deg.F. The shell temperature may vary much more widely --68-104 deg F.. A core temperature over 100 F indicates fever, and a core temperature below 95 F is a threat to health.
Ordinarily, most of the body's heat is generated by metabolism in core organs-- brain, heart, liver etc.-- the skeletal muscles accounting for just about 20% of the heat production. However, under conditions of vigorous exercise, the skeletal muscles may generate 30 to 40 times the amount of heat being generated by the rest of the body. When the body starts exercising, blood is shunted to skeletal muscles and away from deeper organs. But when the heat of the body rises, blood is shunted into shell arteries and capillaries to increase heat loss by radiation, convection ,if necessary, by evaporation of sweat --and maybe by conduction..
Exercise increases blood flow blood and blood pressure. Muscle activity and increased respiratory rate and depth increase venous pressure and return. The increase in the blood volume being returned and the increased venous pressure account for the bulging of the veins.
