Body Temperature Regulation and Fever.pptx

Body Temperature and regulations

1. Body Temperature Regulation and Fever Chapt . 74 Guyton and Hall Textbook of Medical Physiology 13 th edition, P910-920

2. Body Core and Skin Temperature : Core Temperature: Refers to the temperature of deep tissues, remaining constant within ±1°F (±0.6°C) in healthy individuals, except during febrile illness. A nude person can maintain nearly constant core temperature in dry air from 55°F to 130°F due to effective temperature regulation mechanisms. Normal core temperature: Oral measurement: 97°F to 99.5°F (36°C to 37.5°C). Rectal measurement: ~1°F higher. Core temperature increases with exercise (up to 101°F–104°F ) and may drop below 96°F in extreme cold. Skin Temperature: Varies with the environmental temperature. Plays a critical role in heat loss to the surroundings. Temperature Regulation: The body’s temperature control system functions w ell but is not perfect, leading to temporary changes during exercise or extreme environmental conditions.

3. BODY TEMPERATURE IS CONTROLLED BY BALANCING HEAT PRODUCTION AND HEAT LOSS Heat Production and Loss Mechanisms Heat Production: Heat is a by-product of metabolism, regulated by several factors, including: Basal metabolism: The baseline metabolic activity of cells. Muscle activity: Increased metabolism due to muscle contractions, including shivering. Hormonal effects: Thyroxine , growth hormone, and testosterone increase cellular metabolism. Sympathetic stimulation: Epinephrine and norepinephrine elevate metabolism. Chemical activity: Enhanced metabolism with rising cell temperature. Thermogenic effect of food: Metabolism increases during digestion, absorption, and storage of food.

4. Estimated normal range of body “core” temperature .

5. Heat Loss: Heat generated in deep organs (e.g., liver, brain, heart, and muscles) is transferred to the skin and lost to the surroundings. The rate of heat loss depends on: Heat conduction: From the body core to the skin. Heat transfer: From the skin to the environment.

6. Insulation System: The skin, subcutaneous tissues, and fat form an insulating layer that limits heat loss. Fat is particularly effective, conducting heat at only one-third the rate of other tissues. In men, insulation is comparable to wearing three-quarters of a suit of clothes. Women generally have better insulation due to subcutaneous fat. This system maintains the core temperature while allowing the skin temperature to adjust to the environment.

7. A high rate of skin flow causes heat to be conducted from the core of the body to the skin with great efficiency, whereas reduction in the rate of skin flow can decrease the heat conduction from the core to very little.

8. effect of environmental air temperature on conductance of heat from the core to the skin surface and then conductance into the air, demonstrating an approximate eightfold increase in heat conductance between the fully vasoconstricted state and the fully vasodilated state. flow of blood to the skin is a most effective mechanism for heat transfer from the body core to the skin.

9. Heat conduction to the skin is regulated by blood flow, which depends on the degree of vasoconstriction in arterioles and arteriovenous anastomoses supplying the skin's venous plexus. This vasoconstriction is predominantly controlled by the sympathetic nervous system, responding to body core and environmental temperature changes. The hypothalamus plays a central role in coordinating this temperature regulation .

10. How Heat Is Lost From the Skin Surface methods by which heat is lost from the skin to the surroundings include : radiation, conduction, and evaporation

11. The human body radiates heat rays in all directions. Heat rays are also being radiated from the walls of rooms and other objects toward the body. If the temperature of the body is greater than the temperature of the surroundings, a greater quantity of heat is radiated from the body than is radiated to the body. about 3 percent, are normally lost from the body by direct conduction from the surface of the body to solid objects, such as a chair or a bed. Loss of heat by conduction to air, however, represents a sizable proportion of the body’s heat loss (about 15 percent) even under normal conditions. unheated air is continually brought in contact with the skin, a phenomenon called air convection. heat conductivity in water is very great in comparison with that in air.Therefore , the rate of heat loss to water is usually many times greater than the rate of heat loss to air. When water evaporates from the body surface, 0.58 Calorie (kilocalorie) of heat is lost for each gram of water that evaporates.

12. Sweating and Its Regulation by the Autonomic Nervous System Control: Sweating is triggered by stimulation of the anterior hypothalamus- preoptic area due to heat or electrical signals. Signals are transmitted via sympathetic cholinergic nerves. Sweat Gland Structure: Sweat glands consist of a coiled secretory portion and a duct that modifies the sweat composition as it passes through. Sweat Secretion: Primary Secretion: Initially resembles plasma (high in sodium and chloride but lacks proteins). Modification: Sodium and chloride are reabsorbed in the duct. The extent depends on sweating rate: Low Sweating: Almost all sodium and chloride are reabsorbed, leaving concentrated other solutes (e.g., urea, lactic acid). High Sweating: Less reabsorption occurs, resulting in higher sodium and chloride content in sweat. Acclimatization: Heat acclimatization reduces sodium chloride loss despite increased sweat production, enhancing electrolyte conservation.

13. Body temperature regulation relies on hypothalamic feedback mechanisms and temperature detectors to maintain a core temperature within a safe range. Effect of high and low atmospheric temperatures of several hours’ duration, under dry conditions, on the internal body core temperature. Note that the internal body temperature remains stable despite wide changes in atmospheric temperature.

14. The anterior hypothalamic- preoptic area plays a key role in regulating body temperature by acting as a temperature control center. It contains heat-sensitive and cold-sensitive neurons that function as temperature sensors. Heat-sensitive neurons increase their activity significantly when the body temperature rises, while cold-sensitive neurons activate when the temperature drops. When the preoptic area is heated, it triggers an immediate cooling response, including profuse sweating and dilation of skin blood vessels, to promote heat loss. excess heat production is inhibited. These mechanisms work together to help maintain normal body temperature.

15. Detection of Temperature by Skin and Deep Body Receptors Role of Temperature Receptors : While hypothalamic temperature receptors strongly control body temperature, skin and deep body receptors also contribute to regulation. Skin Temperature Receptors : Detect both cold and warmth, but cold receptors outnumber warmth receptors by about 10:1 in many areas. Peripheral temperature detection primarily focuses on cool and cold temperatures. Transient receptor potential (TRP) cation channels, found in sensory neurons and epidermal cells, likely mediate thermal sensations across a range of skin temperatures.

16. Effects of Skin Chilling : When the skin experiences widespread cooling, reflexes are triggered to conserve or increase body heat: Shivering : Increases heat production. Sweating Inhibition : Reduces heat loss. Vasoconstriction : Limits heat escape through the skin. Deep Body Temperature Receptors : Found in the spinal cord, abdominal viscera, and great veins of the upper abdomen and thorax. Exposed to core body temperature rather than surface temperature. Primarily detect cold, aiding in the prevention of hypothermia. Both skin and deep receptors are integral to maintaining body temperature, with a particular focus on detecting and responding to cold to avoid hypothermia.

17. POSTERIOR HYPOTHALAMUS INTEGRATES THE CENTRAL AND PERIPHERAL TEMPERATURE SENSORY SIGNALS contains heat-sensitive neurons, as well as ,cold-sensitive neurons. heat-sensitive neurons increase their firing rate 2- to 10-fold in response to a 10°C increase in body temperature. The cold- sensitivemneurons , by contrast, increase their firing rate when the body temperature falls. When the preoptic area is heated, the skin all over the body immediately breaks out in a profuse sweat, whereas the skin blood vessels over the entire body become greatly dilated. This response is an immediate reaction to cause the body to lose heat, thereby helping to return the body temperature toward the normal level,and any excess body heat production is inhibited. it is clear that the hypothalamic- preoptic area has the capability to serve as a thermostatic body temperature control center.

18. DETECTION OF TEMPERATURE BY RECEPTORS IN THE SKIN AND DEEP BODY TISSUES. POSTERIOR HYPOTHALAMUS INTEGRATES THE CENTRAL AND PERIPHERAL TEMPERATURE SENSORY SIGNALS The body's temperature regulation involves receptors in the hypothalamus, skin, and deep tissues. Skin receptors, which include more cold receptors than warmth receptors, primarily detect cooler temperatures. These receptors trigger responses such as shivering, reduced sweating, and skin vasoconstriction to conserve heat. The transient receptor potential (TRP) channels may play a role in thermal sensation. Deep body temperature receptors, located in areas like the spinal cord and abdominal viscera, monitor core body temperature and primarily detect cold. Together, skin and deep receptors help prevent hypothermia by responding to cold stimuli. The body's temperature regulation involves receptors in the hypothalamus, skin, and deep tissues. Skin receptors, which include more cold receptors than warmth receptors, primarily detect cooler temperatures. These receptors trigger responses such as shivering, reduced sweating, and skin vasoconstriction to conserve heat. The transient receptor potential (TRP) channels may play a role in thermal sensation. Deep body temperature receptors, located in areas like the spinal cord and abdominal viscera, monitor core body temperature and primarily detect cold. Together, skin and deep receptors help prevent hypothermia by responding to cold stimuli.

19. Temperature-Decreasing Mechanisms When the Body Is Too Hot The temperature control system uses three important mechanisms to reduce body heat when the body temperature becomes too great: 1. Vasodilation of skin blood vessels . In almost all areas of the body, the skin blood vessels become intensely dilated. 2. Sweating. 3. Decrease in heat production . The mechanisms that cause excess heat production, such as shivering and chemical thermogenesis , are strongly inhibited. Figure 74-7. Effect of hypothalamic temperature on evaporative heat loss from the body and on heat production caused primarily by muscle activity and shivering. This figure demonstrates the extremely critical temperature level at which increased heat loss begins and heat production reaches a minimum stable level.

20. This figure demonstrates the extremely critical temperature level at which increased heat loss begins and heat production reaches a minimum stable level.

21. Temperature-Increasing Mechanisms When the Body Is Too Cold When the body is too cold, the temperature control system institutes exactly opposite procedures. They are: 1. Skin vasoconstriction throughout the body . This vasoconstriction is caused by stimulation of the posterior hypothalamic sympathetic centers. 2 . Piloerection . Piloerection means hairs “standing on end.” Sympathetic stimulation causes the arrector pili muscles attached to the hair follicles to contract, which brings the hairs to an upright stance. in many animals, upright projection of the hairs allows them to entrap a thick layer of “insulator air”mnext to the skin, so transfer of heat to the surroundings is greatly depressed. 3. Increase in thermogenesis (heat production) . Heat production by the metabolic systems is increased by promoting shivering, sympathetic excitation of heat production, and thyroxine secretion.

22. The "set point" for temperature control is the critical body core temperature (about 37.1°C or 98.8°F) at which heat production and heat loss are balanced. If the body temperature deviates from this point, mechanisms activate to restore it. Feedback gain, which measures the control system's effectiveness, is high for temperature regulation. Human body temperature changes by about 1°C for every 25–30°C change in environmental temperature, giving a feedback gain of approximately 27. Skin and peripheral temperature signals can slightly alter the hypothalamic set point for core temperature control. These signals influence when mechanisms like sweating or shivering are triggered, complementing the role of heat receptors in the hypothalamus. Figure 74-8. Effect of changes in the internal head temperature on the rate of evaporative heat loss from the body. Note that the skin temperature determines the set point level at which sweating begins. Figure 74-9. Effect of changes in the internal head temperature on the rate of heat production by the body. Note that the skin temperature determines the set point level at which shivering begins.

23. Effect of changes in the internal head temperature on the rate of evaporative heat loss from the body. Note that the skin temperature determines the set point level at which sweating begins.

24. Effect of changes in the internal head temperature on the rate of heat production by the body. Note that the skin temperature determines the set point level at which shivering begins.

25. BEHAVIORAL CONTROL OF BODY TEMPERATURE Aside from the subconscious mechanisms for body temperature control, the body has another temperature control mechanism that is even more potent: behavioral control of temperature. person makes appropriate environmental adjustments to re-establish comfort, such as moving into a heated room or wearing well-insulated clothing in freezing weather. Behavioral control of temperature is a much more powerful system of body temperature control than most physiologists have acknowledged in the past.

26. ABNORMALITIES OF BODY TEMPERATURE REGULATION FEVER Fever, which means a body temperature above the usual range of normal, can be caused by abnormalities in the brain or by toxic substances that affect the temperature regulating centers. They include bacterial or viral infections, brain tumors, and environmental conditions that may terminate in heatstroke. Pyrogens released from toxic bacteria or those released from degenerating body tissues cause fever during disease conditions. When the set point of the hypothalamic temperature-regulating center becomes higher than normal, all the mechanisms for raising the body temperature are brought into play, including heat conservation and increased heat production. Within a few hours after the set point has been increased, the body temperature also approaches this level,

27. Figure 74-11. Effects of changing the set point of the hypothalamic temperature controller.