Physiology Thermoregulation

Hypothalamus regulation of temperature

Hypothalamus acts as "thermostat" that makes thermoregulatory adjustments to deviations from temperature norm in the brain (37 C ± 1 C or 98.6±1.8 F ).

Hormones in heat stress

1. Antidiuretic hormone (ADH) is released to increase water re-absorption from kidneys. 2. Aldosterone is released to increase the re-absorption of sodium. *ADH- released by brain (posterior pituitary gland) and when released it then goes to kidneys and retains just water Aldosterone- released in adrenal glands, and goes to kidneys but has an affect on sodium potassium channels (causes salt retention in the kidney) Water always follows salt and sugar as long as the membrane is permeable to it, so in our bodies if sodium or glucose travels somewhere, water will follow it Whenever have increase in blood volume will have increase in blood pressure- when body

How are the mechanisms of hypothalamus regulation of temperature activated?

1. Thermal receptors in skin provide input to central command 2. Direct stimulation of hypothalamus through changes in blood temperature perfusing area

Thermoregulation in Cold

1. Vascular adjustments: constrict peripheral blood vessels. 2. Muscular activity: exercise energy metabolism and shivering. 3. Hormonal output: epinephrine and norepinephrine increase basal heat production; prolonged cold - thyroxin

Acclimatization to Heat

Acclimatization refers to physiological changes that improve heat tolerance. 2 - 4 hours daily heat exposure produce complete acclimatization 5-10 days.

Prevention of heat illness

Allow adequate time for acclimatization. Exercise during cooler parts of day. Limit/defer exercise if heat stress index is in high risk zone. Hydrate properly prior to exercise and replace fluid loss during and after exercise. Wear clothing that is light in color and loose fitting.

Exercise in heat stress- circulatory adjustments

Cardiovascular drift - fluid loss reduces plasma volume (about 10% of fluid lost comes from plasma. About 50% comes from intracellular water). Visceral vascular constriction and skin & muscle vascular dilation. Maintaining blood pressure. Circulatory regulation and maintenance of muscle blood flow take precedence over temperature regulation often at the expense of spiraling core temperature during exercise in heat. (blood pressure has to be on a certain narrow range or else organs will shut down)

Integration of heat-dissipating mechanisms : Circulation

Circulation. Superficial venous and arterial blood vessels dilate to divert warm blood to the body shell. *If we want to maintain/retain heat in our body, the blood vessels will constrict If our body is trying to maintain water, blood vessels that are superficial will constrict but internally inside they are more dilated to return heat to our internal organs If our body is in a hot hot environment, the blood vessels that are superficial will dilate but internally many blood vessels will constrict

Effects of cold weather clothing

Cold Weather Clothing provide an air barrier to prevent convection and conduction. Layers provide more trapped air Allow water vapor to escape

Respiratory tract in cold

Cold air does not damage respiratory passages. Air warms to between 80° F to 90° F as it reaches bronchi. Humidification of inspired cold air produces water & heat loss from respiratory tract. (we have to have "enough" blood pressure to keep our organs functioning)

Exercise in the cold

Cold strain - Exposure to cold produces physiological & psychological challenges - Body fat differences effect physiological function in cold Acclimatization to the Cold - Humans adapt more successfully to chronic heat than cold exposure.

Thermal Balance

Core temperature (TCO) is in dynamic equilibrium as a result of balance between heat gain and heat loss. Mean body temperature (Tbody) represents an average of skin and internal temperatures.

Heat loss by conduction

Direct transfer of heat through a liquid, solid, or gas from one molecule to another. A small amount of body heat moves by conduction directly through deep tissues to cooler surface. Heat loss involves the warming of air molecules and cooler surfaces in contact with the skin. The rate of conductive heat loss depends on thermal gradient. *Conduction allows for heat from body to be moved to water they are laying in Depending on temp gradient, person with higher temp will lose heat faster than peson with lower high temp when put into a bath of cool water

Heat loss at high ambient temperatures

Effectiveness of heat loss via conduction, convection, and radiation decreases. When ambient temperature exceeds body temperature, heat is gained. The only effective mechanism is evaporation of sweat and respiratory tract vaporization of water. *This is when you have a temperature like 100 and if it is like 110 degrees outside then it will be very hard for us to lose heat so the only ways to do so are through evaporation and sweat

Integration of heat-dissipating mechanisms - Evaporation

Evaporation. Sweating begins within 1.5 s after start of vigorous exercise. Hormonal adjustments. Certain hormonal adjustments are initiated in heat stress as body attempts to conserve fluids and sodium. *When body is under stress, hormones can be released to maintain heat #1 way to get rid of excess heat is evaporation

Factors that improve heat tolerance

Fitness Level Age (see FYI) Aging delays the onset of sweating and blunts the magnitude of sweating response Gender Body fatness (the older or the more younger makes it difficult)

Heat loss by convection

Heat Loss by Convection (+ conduction 35%) Effectiveness depends on how rapidly the air (or water) adjacent to the body is exchanged. Air currents at 4 mph are about twice as effective for cooling air currents at 1 mph.

Heat loss by evaporation

Heat Loss by Evaporation (~ 55%) Heat transferred as water is vaporized from respiratory passages and skin surfaces. For each liter of water vaporized, 580 kcal transferred to the environment. When sweat comes in contact with the skin, a cooling effect occurs as sweat evaporates. The cooled skin serves to cool the blood (major way of all the ways that body will get rid of excess heat)

Heat loss by radiation

Heat Loss by Radiation (~ 10%) - Objects emit electromagnetic heat waves without molecular contact with warmer objects. - When temperature of things in environment exceeds the skin temperature, radiant heat energy is absorbed from the surroundings (ex. heat leaving a woodstove)

Water loss in heat

Magnitude of Fluid Loss in Exercise. Consequences of Dehydration. - plasma volume peripheral blood flow & sweat rate Water Replacement - Primary aim of fluid replacement during exercise is to maintain plasma volume - The most effective defense against heat stress is adequate hydration Electrolyte Replacement. *Magnitude of fluid loss depends on how much exercise we do as well as thigns like how hot it is outside and the humidity Fluid replacement orally and IV wise IV is most efficient way, but it may not always be needed - iv fluid is more efficient going in 1L of pedialite is not the same as 1L of IV, the L of IV is much more efficient

Exercise in heat stress- core temperature

More than likely a modest increase in core temperature reflects favorable internal adjustments. (Its okay because our body will get warmer but we have many mechanisms to bring it back down)

Evaluating heat stress

Prevention remains most effective way to manage heat-stress injuries Wet bulb-globe temperature relies on ambient temperature, relative humidity, and radiant heat. Heat stress index (HR will be up and BP will be low because losing fluid)

Thermoregulation

the maintenance of a particular temperature in a living body

Heat loss in high humidity

Total sweat vaporized from skin depends on: - Surface area exposed to environment - Temperature and humidity of ambient air - Convective air currents about the body Most important factor is relative humidity. When relative humidity is high, the ambient water vapor pressure approaches that of the moist skin and evaporation is impeded. *The greater the surface area, the greater the chances that stuff will move across the membrane The higher the humidity around the body, the more difficult it is to lose heat in an evaporative way Water is less readily going to leave our bodies in areas of high humidity because the outside environment is more wetter so harder to use evaporation to give off heat

Effects of warm weather clothing

Warm Weather Clothing loose fitting to permit free convection. The less surface covered the more evaporative cooling. Clothing should be loosely woven to allow skin to breathe.

Factors that improve heat tolerance: Acclimatization

improved cutaneous blood flow effective distribution of cardiac output lowered threshold for start of sweating more effective distribution of sweat over skin surface increased rate of sweating decreased salt concentration of sweat transports metabolic heat from deep tissues to body's shell appropriate circulation to skin and muscles to meet demands evaporative cooling begins early in exercise optimum use of surface for effective evaporative cooling maximize evaporative cooling dilute sweat preserves electrolyte in fluids (the larger the surface area of anything, the more things can go across, there are receptors in our skin that put salt into the water and then push it out)

Evaluating environmental cold stress

wind chill index