200 متن (TEXTS 200)،(متون تخصصی انگلیسی)

TEXT 22 .Redistribution of blood flow during exercise At rest 15-20% of blood goes to skeletal muscle during exercise this amount increases to 80- 85%. The percentage of blood to the brain decreases but the absolute amount increases. The same percentage of blood goes to cardiac muscle but the absolute amount increases. Blood flow to visceral tissues and inactive skeletal muscle reduces. In addition the cutaneous blood flow initially decreases but it later increases during the course of exercise The redistribution of the blood is brought about by several mechanisms. During exercise generalized vasodilatation occurs due to the accumulation of vasodilatory metabolites. This leads to a decrease in the peripheral resistance which in turn elicits a strong increase in the sympathetic activity through the activation of baroreceptors. The increase in sympathetic activity leads to vasoconstriction in the visceral organs while the vasodilatation predominates in the blood vessels of the muscles and the coronary circulation due to the local vasodilatory metabolites. The cutaneous blood vessels initially respond to the sympathetic activity by vasoconstriction. As the exercise coutinues temperature reflexes are activated and cause cutaneous vasodilatation to dissipate the heat produced by the muscle activity resulting in an increase in the cutaneous blood flow