PET3380C Exam 4 Book Notes Chapter 25 9 degrees F temperature Thermal Balance Humans can tolerate a decline in deep body temperature of 18 degrees F but an increase of only Hypothalamic Temperature Regulation hypothalamus represents the mean kinetic energy of a substance s atoms as they move central tissues organs etc the periphery exterior of the body skin toes fingers etc the dynamic equilibrium between factors that add and subtract body heat core shell chemical reactions of energy metabolism produce body heat gains that can reach considerable levels during muscular activity from shivering alone whole body metabolism increases 3 to 5 fold the body absorbs heat from solar radiation and objects warmer than the body heat leaves the body via radiation conduction convection and water vaporization most imp circulatory adjustments provide the fine tuning for temperature regulation heat conservation occurs when blood pushes rapidly to the deep cranial thoracic and abdominal cavities and portions of the muscle mass this optimizes insulation from subcutaneous fat and other components of the body s shell conversely increases in internal heat dilate peripheral vessels as warm blood flows to the cooler periphery group of specialized neurons at the floor of the brain that contains the central coordinating center for temperature regulation acts as thermostat that continually makes thermoregulatory adjustments to deviations from a temperature norm it can t turn off the heat it can only initiate responses to protect the body for buildup or loss two ways to activate the body s heat regulating mechanisms thermal receptors in the skin that provide input to the central control center changes in the temperature of blood that perfuses the hypothalamus directly stimulate this area free nerve endings embedded in the peripheral respond to rapid changes in heat and cold the more numerous cutaneous cold receptors generally exist near the skin surface and play an important role in initiated regulatory responses to a cold environment the cutaneous thermal receptors act as an early warning system that relays sensory information to the hypothalamus and cortex the central hypothalamic regulatory center plays the primary role in maintaining thermal balance cells in the hypothalamus detect slight changes in blood temperature these cells heightened activity stimulates the other parts of the hypothalamus anterior portion controls heat loss posterior portion controls heat conservation unlike receptors that detect cold the temperature of the blood that perfuses the hypothalamus provides the primary monitoring system to assess body warmth Thermoregulation in Cold Stress Heat Conservation and Heat Production the normal heat transfer gradient flow from the body to the environment when it comes to the core temperature in cases of extreme cold the body produces heat to Exam 4 Book Notes Vascular Adjustments minimize the heat lost to the environment conservation PET3380C stimulation of cutaneous cold receptors constricts peripheral blood vessels this immediately reduces the flow of warm blood to the body s cooler surface and redirects it the warmer core it s more important that the major organs are protected skin temperature declines toward ambient temperature to maximize the insulatory benefits of skin muscle and subcutaneous fat a person with a lot of body fat who is exposed to cold stress greatly benefits from this heating conserving mechanism why overweight people tend to be warmer on average Muscular Activity shivering generates heat but physical activity provides the greatest contribution in defending against the cold when exercise metabolism decreases shivering alone may not prevent a decline in core temperature internal temperature mediates the thermoregulatory response to cold shivering still occurs during vigorous exercise if the core temperature remains low in result cold stress often induces higher exercise oxygen consumptions from shivering compared to performing the same exercise in a warmer environment Hormonal Output there are two calorigenic adrenal medulla hormones epinephrine and norepinephrine they increase heat production during cold exposure prolonged cold exposure stimulates thyroxine release to increase resting metabolism Thermoregulation in Heat Stress Heat Loss the body s thermoregulatory mechanisms primarily protect against overheating dissipating heat is crucial during exercise in hot weather there is inherent competition between mechanisms that maintain large muscle blood flow and thermoregulatory mechanisms body heat loss occurs by four physical processes radiation conduction convection evaporation Heat Loss by Radiation all objects emit electromagnetic heat waves radiant energy usually our bodies are warmer than the environment so the net exchange goes from us to cooler objects in the environment this does not require contact between objects we absorb heat when objects are warmer than our skin temperature a person can remain warm by absorbing radiant heat energy from direct sunlight or by reflection from snow sand or water Heat Loss by Conduction cooler surface this involves direct heat transfer from one molecule to another through a liquid solid or gas most body heat is transferred to the surface by blood but some comes by conduction to the Exam 4 Book Notes PET3380C heat loss by conduction then involves warming air molecules and cooler surfaces that contact the skin the rate of conductive heat loss depends on two factors temperature gradient between the skin and surrounding surfaces thermal qualities of the surfaces Heat Loss by Convection the effectiveness of heat loss by convection depends on how rapidly the air or water adjacent to the body exchanges once it warms if it is slow the air water next to the skin warms and acts as a zone of insulation if cooler air water continually replaces warmer air water around the body then there is considerable heat loss the wind chill index indicates the equivalent still air temperature for a particular ambient temperature at different wind velocities Heat Loss by Evaporation each vaporized liter of water exacts 580 kCal from the body and transfers it to the environment the body s surface contains approximately 2 to 4 million sweat glands during stress these eccrine glands which are controlled by cholinergic sympathetic nerve fibers secrete large quantities of hypotonic saline solution Evaporative Heat Loss at High Ambient Temperatures evaporation
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