acutephysiologicalresponsestoheatstress-全文預(yù)覽
【正文】 f Heat 2. The decreased muscle blood flow increases tissue hypoxia and the relative contribution of anaerobic metabolism to total energy requirements. 3. Increased carbohydrate utilization, primarily from blood glucose. Acute Metabolic and Muscular Effects of Heat 4. Increased blood lactate levels, due to increased lactate production as well as reduced lactate removal due to splanchnic vasoconstriction and hence, decreased hepatic clearance of lactate. 5. Decreased free fatty acid utilization as an energy source. Acute Metabolic and Muscular Effects of Heat 6. Increased blood glucose levels, as glucose is released from the liver. 7. Decreased blood triglyceride levels, due to decreased mobilization of fat. Acute Metabolic and Muscular Effects of Heat 8. Increased reliance on fasttwitch motor units. 9. Decreased efficiency of skeletal muscle contraction as fasttwitch motor units expend greater energy than ST motor units to develop the same tension. Acute Metabolic and Muscular Effects of Heat 10. Increased expired ventilation rate, primarily due to an increase respiratory rate (.,breathing frequency) with minimal changes in respiratory (.,tidal) volume. Effector Mechanisms of Acute Heat Exposure ? Vasodilation of Cutaneous Vasculature (Increased Skin Blood Flow) ? Increased Sweat Rate Vasodilation of Cutaneous Vasculature (Increased Skin Blood Flow) ? There is threshold above which skin blood flow will increase. Skin blood flow carries heat by convection from the deep body tissues to the skin, which may lead to sensible and/or insensible heat loss. Vasodilation of Cutaneous Vasculature (Increased Skin Blood Flow) ? Skin blood flow is dependent on both core temperature and skin temperature as: Increased skin blood flow is proportional to core temperature at any given skin temperature. Increased skin temperature will decrease the core temperature threshold. Heat Transfer by Skin Blood Flow ? Dependent on the rate of blood flow and on the differences in temperature between arterial blood leaving the core on its way to the skin and venous blood returning to the core from the skin. Increases in the rate of blood flow, conduction of tissues, and the difference between core and skin temperatures will increase the heat transfer. ? As ambient temperature increases, there is an increased dependence on insensible (evaporative) heat loss mechanisms and a decreased dependence on sensible (convection and radiation) heat loss mechanisms to minimize exerciseinduced increases in core temperature. ? Evaporative heat loss is dependent on skin blood flow (provides latent heat for the evaporation of sweat) and on secretion of perspiration from the sweat glands. Types of Sweat Glands 1. Apocrine glands nervous or emotional sweat due to neurochemical stimuli。 the lower the skin temperature, the higher the increase in core temperature before the response is initiated. Basic Mechanism
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