temperature_regulation_Physiology_22-02-2019.pdf

BODY TEMPERATURE AND ITS REGULATION Learning Objectives List the factors affecting the body temperature Understand the mechanism of heat generation and transfer in the body Understand the mechanism of heat preservation and heat loss in the body Describe the regulation of body temperature Underst...

BODY TEMPERATURE AND ITS REGULATION Learning Objectives List the factors affecting the body temperature Understand the mechanism of heat generation and transfer in the body Understand the mechanism of heat preservation and heat loss in the body Describe the regulation of body temperature Understand the physiological basis of hypothermia and hyperthermia REGULATION OF BODY TEMPERATURE Body temperature reflects the balance between heat production and heat loss At rest, the liver, heart, brain, kidneys, and endocrine organs generate most heat During exercise, heat production from skeletal muscles increases dramatically REGULATION OF BODY TEMPERATURE Normal body temperature = 37C  5C (98.6F) Optimal enzyme activity occurs at this temperature Increased temperature denatures proteins and depresses neurons Heat production Heat loss Basal metabolism Radiation Muscular activity Conduction/ (shivering) convection Thyroxine and Evaporation epinephrine (stimulating effects on metabolic rate) Temperature effect on cells Figure 24.25 CORE AND SHELL TEMPERATURE Organs in the core have the highest temperature Blood is the major agent of heat exchange between the core and the shell Core temperature is regulated Core temperature remains relatively constant, while shell temperature fluctuates substantially (20C–40C) MECHANISMS OF HEAT EXCHANGE Four mechanisms 1. Radiation is the loss of heat in the form of infrared rays 2. Conduction is the transfer of heat by direct contact 3. Convection is the transfer of heat to the surrounding air 4. Evaporation is the heat loss due to the evaporation of water from body surfaces MECHANISMS OF HEAT EXCHANGE Insensible heat loss accompanies insensible water loss from lungs, oral mucosa, and skin Evaporative heat loss becomes sensible (active) when body temperature rises and sweating increases water vaporization ROLE OF THE HYPOTHALAMUS Hypothalamus contains the two thermoregulatory centers Anterior Hypothalamus : activates the mechanism that promote Heat-loss. Posterior Hypothalamus : activates the mechanism that increase Heat-production and promote heat gain. ROLE OF THE HYPOTHALAMUS (THERMOSTAT) The hypothalamus receives afferent input from Peripheral thermoreceptors in the skin Central thermoreceptors (some in the hypothalamus) Initiates appropriate heat-loss and heat- promoting activities HEAT-PROMOTING MECHANISMS Constriction of cutaneous blood vessels Shivering Increased metabolic rate via epinephrine and norepinephrine Enhanced thyroxine release HEAT-PROMOTING MECHANISMS Voluntary measures include Putting on more clothing Drinking hot fluids Changing posture or increasing physical activity HEAT-LOSS MECHANISMS Dilation of cutaneous blood vessels Enhanced sweating Voluntary measures include -Reducing activity and seeking a cooler environment -Wearing light-colored and loose-fitting clothing  Skin blood vessels dilate: capillaries become flushed with warm blood; heat radiates from skin surface Activates heat- loss center in hypothalamus Sweat glands activated: secrete perspiration, Body temperature Stimulus which is vaporized by decreases: blood Increased body body heat, helping to temperature declines temperature; cool the body and hypothalamus blood warmer heat-loss center than hypothalamic “shuts off” set point Figure 24.27, step 1  Stimulus Decreased body tempera- ture; blood cooler than Skin blood vessels constrict: hypothalamic blood is diverted from skin set point capillaries and withdrawn to Body temperature deeper tissues; minimizes increases: blood overall heat loss from skin temperature rises surface and hypothalamus Activates heat- heat-promoting promoting center center “shuts off” in hypothalamus Skeletal muscles activated when more heat must be generated; shivering begins Figure 24.27, step 2 HEAT ILLNESS HOMEOSTATIC IMBALANCE Hyperthermia  Elevated body temperature depresses the hypothalamus  Positive-feedback mechanism (heat stroke) begins at core temperature of 41C  Can be fatal if not corrected HOMEOSTATIC IMBALANCE Heat exhaustion  Heat-associated collapse after vigorous exercise  Due to dehydration and low blood pressure  Heat-loss mechanisms are still functional  May progress to heat stroke HOMEOSTATIC IMBALANCE Hypothermia  Low body temperature where vital signs decrease  Shivering stops at core temperature of 30 - 32C  Can progress to coma a death by cardiac arrest at ~ 21C FEVER Controlled hyperthermia Due to infection (also cancer, allergies, or CNS injuries) Macrophages release interleukins (“pyrogens”) that cause the release of prostaglandins from the hypothalamus FEVER Prostaglandins reset the hypothalamic thermostat higher Natural body defenses or antibiotics reverse the disease process; cryogens (e.g., vasopressin) reset the thermostat to a lower (normal) level