Thermoregulation PDF

Regulation of Body Temperature By Dr. Mona A. Hussain Assistant Prof. in Physiology FOM-PSU  Core Temperature and Skin Temperature (37.8) The temperature of the deep tissues of the body—the “core” of the body—remains very con...

Regulation of Body Temperature By Dr. Mona A. Hussain Assistant Prof. in Physiology FOM-PSU  Core Temperature and Skin Temperature (37.8) The temperature of the deep tissues of the body—the “core” of the body—remains very constant, within ±1°F (±0.6°C), day in and day out, except when a person develops a febrile illness. A nude person can be exposed t temperatures as low as 55°F or as high as 130°F in dry air and still maintain an almost constant core temperature °C = (F- 32)÷ 1.8  Skin temperature (20 - 40 C) The skin temperature, in contrast to the core temperature, rises and falls with the temperature of the surroundings. Normal Core Temperature Differences between skin temperature and core body temperature Regarding the skin temperature : It is much more variable than core temperature It is affected by skin blood flow It is affected by air temperature It is not uniform over the body surface In humans, a change of 1οC in core temperature elicits about 9 times as great a thermoregulatory response as a 1 ο C change in mean skin temperature  Body Temperature Is Controlled by Balancing Heat Production Against Heat Loss 1- Heat Production: Heat production is a principal by-product of metabolism. Different factors that determine the rate of heat production include: (1) basal rate of metabolism of all the cells of the body; (2) extra rate of metabolism caused by muscle activity, (3) extra metabolism caused by the effect of thyroxin (and, to a less extent, other hormones, such as growth hormone and testosterone) on the cells; (4) extra metabolism caused by the effect of epinephrine, norepinephrine, and sympathetic stimulation on the cells; (5) extra metabolism caused by increased chemical activity in the cells themselves, especially when the cell temperature increases; and (6) extra metabolism needed for digestion, absorption, and storage of food (thermogenic effect of food)  Heat loss The rate at which heat is lost is determined almost entirely by two factors: (1) how rapidly heat can be conducted from where it is produced in the body core to the skin and (2) how rapidly heat can then be transferred from the skin to the surroundings  Blood Flow to the Skin from the Body Core Provides Heat Transfer The rate of blood flow into the skin venous plexus can vary tremendously—from barely above zero to as great as 30 per cent of the total cardiac output. 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.  Control of Heat Conduction to the Skin by the Sympathetic Nervous System Heat conduction to the skin by the blood is controlled by the degree of vasoconstriction of the arterioles and the arteriovenous anastomoses that supply blood to the venous plexus of the skin. This vasoconstriction is controlled almost entirely by the sympathetic nervous system in response to changes in body core temperature and changes in environmental temperature. How Heat Is Lost from the Skin Surface  Sweating Sweating is an active evaporative heat-loss process under sympathetic nervous control. The rate of evaporative heat loss can be adjusted by varying the extent of sweating. At normal temperature, an average of 100 ml of sweat is produced per day; This value increases to 1.5 liters during hot weather and climbs to 4 liters during heavy exercises.  Sweating Sweat is a dilute salt solution actively extruded to the surface of the skin by sweat glands dispersed all over the body. Sweat must be evaporated from the skin for heat loss to occur. If sweat drips from the surface of skin or is wiped away, no heat loss is accomplished.  Sweating The most important factor determining the extent of evaporation of sweat is the relative humidity of the surrounding air (the percentage of H2O vapor actually present in the air compared to the greatest amount that the air can possibly hold at that temperature; for example, a relative humidity of 70% means that the air contains 70% of the H2O vapor it is capable of holding). When the relative humidity is high, the air is already almost fully saturated with H2O, so it has limited ability to take up additional moisture from the skin. Thus, little evaporative heat loss can occur on hot, humid days.  Temperature-regulating mechanisms The hypothalamus can respond to changes in blood temperature as small as 0.01°C. Two centers for temperature regulation are in the hypothalamus. The posterior region, activated by cold, triggers reflexes that mediate heat production and heat conservation. The anterior region, activated by warmth, initiates reflexes that mediate heat loss.  Receptors The body possesses two diverse groups of thermosensors. Central thermoreceptors monitor internal body temperature, whereas skin thermoreceptors provide information about the external thermal environment. 1. Central: Thermoreceptors that monitor internal temperature are located in the hypothalamus, spinal cord, and viscera, but the sensors that have the greatest influence on thermoregulatory control center output are in the preoptic area of the hypothalamus 2. Skin thermoreceptors include: a. Cold receptors (5-45 Cᵒ) b. Warmth receptors (30 – 50 Cᵒ) Effector Mechanisms in Temperature Regulation Non shivering thermogenesis in brown fat tissue Fever versus Hyperthermia Mechanism of fever Effects of changing the set-point of the hypothalamic temperature controller.  Hyperthermia when body temperature is above the set point—it is termed hyperthermia. The most common cause of hyperthermia in normal people is exercise; the rise in body temperature above set point is due to retention of some of the internal heat generated by the exercising muscles.  Heat Exhaustion and Heat Stroke Heat exhaustion is a state of collapse, often taking the form of fainting, due to hypotension brought on by: (1) depletion of plasma volume secondary to sweating, and (2) extreme dilation of skin blood vessels. Characterized by rapid, weak pulse; hypotension; profuse sweating; and disorientation. heat stroke represents a complete breakdown in heat- regulating systems so that body temperature keeps going up and up. It is an extremely dangerous situation characterized by hot, dry skin; extreme confusion or unconsciousness; and possibly death.  Heat stroke Exactly what triggers the transition to heat stroke is not clear but impaired circulation to the brain due to dehydration is one factor Heat stroke is a positive feedback situation in which the rising body temperature directly stimulates metabolism—that is, heat production— which further raises body temperature. Body temperature over 43.3C is lethal  Acclimatization The term acclimatization refers to the gradual adaptations the body makes to maintain long-term homeostasis in response to a prolonged physical change in the surrounding environment (1) The plasma volume is increased by as much as 12%. Expansion of the plasma volume provides enough blood to both supply the exercising muscles and direct blood to the skin for cooling. (2) The person begins sweating at a lower temperature so that the body does not get so hot before cooling begins. (3) The maximal sweat rate increases nearly three times, to 4 liters per hour, with a more even distribution over the body. This increase in evaporative cooling reduces the need for cooling by skin vasodilation. (4) The sweat becomes more dilute so that less salt is lost in the sweat. The retained salt exerts an osmotic effect to hold water in the body and help maintain circulating plasma volume. These adaptations take 14 days and occur only if the person exercises in the heat.