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ACUTE PHYSIOLOGICAL RESPONSES TO HEAT STRESS. Basic Mechanisms of Thermoregulation. Core temperature maintained between 35 to 41 o C despite environmental extremes which fluctuate between -88 to 58 o C via:
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Basic Mechanisms of Thermoregulation • Core temperature maintained between 35 to 41o C despite environmental extremes which fluctuate between -88 to 58o C via: 1. Behavioral temperature regulation such as choice of clothing, shelter, ventilation, air conditioning, heating, humidifiers, and dehumidifiers. 2. Physiological temperature regulation controlled by rate of metabolic heat production, heat flow from core to skin, and sweating.
Basic Mechanisms of Thermoregulation • Physiological control systems operate on a graded or proportional response in which changes in controlled variables (e.g., sweating and skin blood flow) are proportional to displacements of the regulated variable (e.g., core temperature) from a threshold value. • Note: each physiological response has a core temperature at which the responses start to increase and the actual response is dependent on mean skin temperature; the lower the skin temperature, the higher the increase in core temperature before the response is initiated.
Basic Mechanisms of Thermoregulation • Thus, thermoregulatory responses are related to both core and mean skin temperature and hence, (1) at any given skin temperature each response is proportional to core temperature and (2) an increase in skin temperature will decrease the core temperature threshold and increase the response at any given core temperature.
CORE TEMPERATURE • Temperatures within body regions are dependent on: 1. The metabolic rate of surrounding tissues. 2. The source and magnitude of blood flow. 3. The temperature gradients between contiguous body regions.
SKIN TEMPERATURE • Determination of skin temperature is useful for: 1. Estimating the input of skin temperature receptors into the hypothalamus for thermoregulatory control. 2. Predicting core temperature. 3. Calculating mean body temperature for heat storage determination. 4. Calculating sensible (radiation and convection) heat exchange.
Exercise Intensity and Core Temperature • At rest, 70% of metabolic heat comes from internal organs and viscera within the body core. During dynamic exercise, metabolic rate increases rapidly by 5-15 fold with 70-90% of metabolic rate released as heat (humans are least efficient at slow and fast speeds of movement). Thermoregulatory effectors for heat dissipation respond more slowly. • Core temperature increases rapidly, then gradually leveling off at a steady-state value when heat production equals heat loss.
Exercise Intensity and Core Temperature • Magnitude of core temperature at steady-state is largely independent of environmental conditions within a fairly wide prescription zone. Increases in core temperature are proportional to increases in metabolic rate. 1 watt = 6 kgm/min = .01433 kcal/min. • The prescription zone is smaller at higher exercise intensities.
Exercise Intensity and Core Temperature • If exercise intensity is expressed in absolute terms (L/min), large individual differences exist in steady-state core temperature; however, if exercise intensity is expressed in relative terms (%VO2max),the inter-individual differences disappear.
Exercise Intensity and Core Temperature • There is a curvilinear relation between steady-state core temperature and relative workload. The prescription zone is smaller at higher relative exercise intensities and it is more difficult to reach steady-state core temperature as core temperature and relative exercise intensity increases. • Note: An increase in VO2max from training would decrease relative workload and hence, reduce core temperature, particularly in a heat acclimated person.
Exercise Intensity and Core Temperature • An increase in ambient air temperature increases the metabolic rate (energy requirements) and hence relative workload of an absolute exercise task as well as the relative contribution of anaerobic energy metabolism to the total energy requirements.
Effects of Mode of Exercise on Relative Workload • Although VO2max of upper body exercise (e.g., arm cranking) is lower than lower body exercise (e.g., leg cycling) and consequently relative work intensity is much higher when performing the same absolute exercise task during arm cranking, core temperature responses appear to be quite similar when working at the same absolute metabolic workload.
Acute Metabolic and Muscular Effects of Heat 1. Decreased VO2max due to diversion of blood to the skin (cutaneous) vasculature. - Decreased blood flow (% Q) to muscle. - Increased peripheral pooling of blood, which decreases central blood volume and hence end- diastolic volume (EDV). SV = EDV - ESV, Q = SV X HR, and VO2 = Q X [A - V O2 Diff]
Acute Metabolic and Muscular Effects of 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 • Increased reliance on fast-twitch motor units. 9. Decreased efficiency of skeletal muscle contraction as fast-twitch 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 (i.e.,breathing frequency) with minimal changes in respiratory (i.e.,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 exercise-induced 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; secretion is a watery substance that contains lipids, trace of color, and odor; most commonly found on the palms of the hands, soles of the feet, arm pits, groin area, face, and upper lip.
Types of Sweat Glands 2. Eccrine glands - respond to thermal stress by secreting a watery substance which contains electrolytes and is generally colorless and odorless; there are 1.6 to 4 million eccrine glands and are most numerous on the sole of the feet and least numerous on the back, although the back eccrine glands are the first to respond to increases in core temperature.
Sweat Rate • People who sweat more (1) have larger sweat glands, (2) have a greater amount of sweating per gland, (3) have a higher sweating rate per unit of tubular length or unit volume of secretory coil, (4) have a reduced hidromeiotic effect, and (5) the sweat glands have a greater cholinergic (AcH) sensitivity. • Hidromeiotic Effect - increased skin wettedness decreases sweating.
Sweat Rate • Sweat rate is dependent on both core temperature and skin temperature as: - Increased sweat rate is proportional to core temperature at any given skin temperature. - Increased skin temperature will decrease the core temperature threshold.
Generally, the heat dissipating responses are sufficient to meet the rise in core temperature during exercise; if not however, hyperthermia may occur leading to heat strain and/or other heat-related illnesses.
Heat Strain • Heat strain results in a decrease in end-diastolic volume as blood pools in the peripherally dilated veins and/or plasma volume decreases, which leads to a decrease in stroke volume and thus heart rate must increase to maintain cardiac output. Plasma volume decreases due to an increased movement of fluid from the plasma to tissue (affected by temperature, exercise intensity and mode, hydration level, and status of heat acclimation) and an increased fluid loss through sweating.
Compensation of Heat Strain • Decreased splanchnic (visceral) and renal blood flow, which is proportional to relative exercise intensity and skin temperature.
Compensation of Heat Strain • Decreased skin blood flow at high intensities as cardiovascular strain is increased (i.e., skin blood flow is proportionally lower than expected for a given skin or core temperature); this response occurs more quickly in the upright position as compared to the supine position and is probably controlled by cardiopulmonary baroreceptors.
