W10 Temperature Regulation (Connolly) PDF

Temperature Regulation Jennifer Connolly, PhD Email: [email protected] Learning Objectives 1. Differentiate between core body temperature (Tb), skin temperature (Ts), ambient 2. 3. 4. 5. 6. 7. 8. temperature (Ta), and hypothalamic set-point temperature (Tset). Describe the circadian rh...

Temperature Regulation Jennifer Connolly, PhD Email: [email protected] Learning Objectives 1. Differentiate between core body temperature (Tb), skin temperature (Ts), ambient 2. 3. 4. 5. 6. 7. 8. temperature (Ta), and hypothalamic set-point temperature (Tset). Describe the circadian rhythm and temperature variability during the female menstrual cycle. Describe the mechanisms of heat production, heat storage and heat dissipation. Describe the mechanisms of heat transport from core of the body to skin. Describe normothermia, hypothermia and hyperthermia and regulatory mechanisms for maintaining normothermia. Describe the functions of the thermoregulatory center. Describe how the body regulates the body temperature back to normal if body temperature is higher or lower than hypothalamic thermal set point. Define fever and describe the mechanisms of heat production and heat dissipation during fever and recovery. Overview • Body heat is produced by muscular exercise, assimilation of • • • • • food & all of the vital processes that contribute to Basal Metabolic Rate (BMR). Body heat is lost by radiation, conduction, convection & by evaporation of water. The balance between heat production & heat loss determines body temperature. Chemical reactions & enzyme systems have narrow temperature ranges; thus, normal body function depends on a relatively constant body temperature. Body temperature is tightly regulated by reflex adjustments in heat production & heat loss. Thermoregulation is primarily under hypothalamic control. Regional Body Temperatures • “Normal” Core Body Temperature (Tb) o 37ᵒC (98.6 ᵒF) o Range: 36 - 37.5 ᵒC (97 - 99.5 ᵒF) o Tb is homeostatically regulated • Skin temperature (Ts) o Varies according to: • Ambient Temperature (Ta) • Cutaneous blood flow Variations in Regional Temperatures • Rectal temp ↑ ~0.5 °C compared to oral o Rectal > ear > oral > axillary • Tb varies with activity & Ta o ↑ during exercise : ~40 °C o ↓ extreme cold weather: ~35 °C • Skin temperature (Ts) varies widely with changes in Ta • Naked individual at rest in a room exposed to Ta ~23 °C to 35 °C in dry air can maintain an almost constant core Tb despite wide changes in Ta & Ts Rhythmic Changes in Body Temperature (Tb) Circadian (diurnal) rhythm Female monthly rhythm 37±0.6 ᵒC (98.6±1 ᵒF) +0.5 ᵒC during luteal phase of menstrual cycle Post-ovulation (due to progesterone) Pre-ovulation Importance of Maintaining a Stable Core Body Temperature Skin vasoconstriction Skin vasodilation Basic Mechanism of Heat Balance • • • • • Metabolism Muscular activity Food intake Brown adipose tissue Environment • • • • Heat Gain Evaporation Conduction Convection Radiation Heat Loss Body temperature is constant when heat gain = heat loss Gain > Loss = ↑ Heat Storage (↑ Tb) Gain < Loss = ↓ Heat Storage (↓ Tb) Heat Storage Heat capacity • Energy required to change the temperature of 1 kg of a substance by 1 ᵒC • Rate at which the body temperature rises or falls as its heat content ↑ or ↓ • Water has a high heat capacity o 1 kcal of heat energy is required to ↑ temperature of 1 kg of water by 1 ᵒC • Body is ~70% water: ➢ ~1 kcal of heat energy is required to ↑ temperature of 1 kg of the body by 1 ᵒC ➢ ~70 kcal of heat energy is required to ↑ body temperature of 70 kg individual by 1 ᵒC • Varies little between individuals Mechanisms of Heat Production • Catabolism liberates energy from organic molecules & the energy liberated is used to do work (40%) or is released as heat (60%) Metabolism • Basal Metabolic Rate (BMR): minimum level of energy required to exist accounts for 50-70% of daily energy expenditure • BMR: i.e., heat production rate, averages ~70 kcal/hr = ~1ᵒC/hr • BMR is influenced by a number of factors: • Age: MR declines with age • Sex: 5-10% ↑ metabolic rate in men • Hormones: Thyroid hormone & catecholamines ↑ cellular MR (chemical thermogenesis) • Digestive state: 10-20% post-prandial ↑ metabolic rate (thermal effect of food) Mechanisms of Heat Production Muscular activity Voluntary or involuntary muscle activity = ↑ energy consumption rate = ↑ metabolic heat production Physical activity: • • 75% of energy is released as heat Moderate exercise (e.g., jogging) ↑ metabolic rate ~ 500 kcal/hr = ~6ᵒC/hr Shivering: • Asynchronous involuntary skeletal muscle contraction, ↑ muscle tone & tremors • ↑ rate of heat production X 5 BMR Heat generated in active muscle… is convected to the blood…. & conducted to the skin… Thus, active muscle = ↑ Tb Mechanisms of Heat Production Non-shivering thermogenesis • Occurs in Brown Adipose Tissue (BAT) • Stored in depots • Quantity ↓ with age,↑ with chronic exposure to cold • NE stimulates lipase & release of FFAs • Thyroid hormone stimulates upregulation of mitochondrial UCP1 (thermogenin) • Respiration uncoupled to ATP production→ heat release • ↑ rate of heat production by 10-15% Mechanisms of Heat Loss • • • • RADIATION : heat transfer in the form of electromagnetic waves between solid objects Amount & direction of heat transfer is determined by the temperature of the radiator Heat net-radiates from the body to solid objects which are cooler than Ts (e.g., walls) Heat net-radiates to the body from objects that are warmer than Ts (e.g., sun) • CONDUCTION: transfer of heat between objects which are in direct contact (e.g., chair) • Conduction of heat requires a temperature gradient • CONVECTION: transfer of heat to the environment by a moving fluid (e.g., air or water) • Convective heat loss is proportional to the temperature gradient & the velocity of the liquid • • • EVAPORATION of 1 L of water from the skin surface removes 580 kcal of heat Primary mechanism of heat loss at high Ta & during strenuous physical activity Rate of evaporation depends on ambient humidity:↑ humidity = ↓ evaporation Heat Transfer Between the Core & the Environment Heat produced in the body enters the blood & is conveyed to the body surface from where heat is dissipated/lost Heat transfer occurs from area of high temperature to area of lower temperature. For heat loss, surface temperature must be lower than at the core. Heat Transfer From the Core to the Skin • Heat is transferred to the skin by convection (mostly) & conduction (minor) • Rate of heat transfer by conduction across the subcutaneous fat is relatively constant • Rate of heat transfer by convection depends on cutaneous blood flow • Blood flow to the skin varies from 0-30% of CO • Vasoconstriction → Heat storage • Vasodilation → Heat loss Heat dissipation at low Ta Heat dissipation at high Ta Regulation of Skin Temperature (Ts) Controlled by the ANS ↑ Sympathetic activity (NE via a1 adrenoceptors) → vasoconstriction → heat storage ↑ SYMPATHETIC NERVOUS ACTIVITY ↓ SYMPATHETIC NERVOUS ACTIVITY ↓Sympathetic tone → vasodilation → heat loss Heat Transfer Between the Core & the Environment Naked individual in a room with Ta of 20 ᵒC, loses heat predominantly by radiation to surrounding solid objects. At Ta of 30 ᵒC, evaporative heat loss increases. Radiative heat loss decreases owing to smaller gradient between Ts & Ta At Ta >36 ᵒC or so, heat loss occurs by evaporation only, owing to loss of thermal gradient for conduction & radiation Regulation of Sweat Production • Loss of heat by evaporation of water from the skin surface is regulated by controlling the rate of sweat production • Evaporation of 1 L of water from the skin surface removes 580 kcal of heat • ~600 mL/day of water evaporates from skin & respiratory surfaces - “Insensible heat loss” Sweat glands: • are innervated by the sympathetic cholinergic nerves (ACh) • can deliver up to 6L fluid/hr to skin surface • Evaporation rate depends on ambient temperature & humidity ↑ Ambient humidity = ↓evaporation = ↓ heat loss = ↑ heat storage Heat Transfer Between the Core & the Environment At high Ta with high ambient humidity, evaporation rate declines, the body cannot effectively lose heat and, thus ↑Tb >37ᵒC Humidity >75% Evaporation rate approaches 0 & Tb rises Heat transfers from areas of high temperature to lower temperature. Heat is gained if surface temperature exceeds core temperature. This is particularly relevant in conditions of high Ta & high humidity Heat Balance Equation • If Tb is to remain unchanged, ↑ or ↓ heat production must be balanced with ↑ or ↓ in heat loss resulting in negligible heat storage within the body • In equilibrium: • Capabilities of thermoregulatory machinery are not limitless • Shifts in heat storage can shift the balance from normothermia to hypothermia or hyperthermia • The thermoregulatory center is located in the anterior hypothalamus. It determines the temperature set point (Tset) • It receives information about ambient temperature from cold & warmth thermoreceptors in the skin & about core Tb from thermoreceptors in the spinal cord & anterior hypothalamus itself • It compares actual core Tb with Tset & initiates measures to counteract any deviations Thermoregulatory Center Hypothalamic temperature Hypothalamic Control of Thermoregulation cholinergic Heat loss by evaporation a1- adrenergic Cutaneous vasoconstriction b 3- adrenergic Non-shivering thermogenesis cholinergic Heat production by shivering Thermoregulatory Responses to Changes in Core Tb Activates heat-loss center in hypothalamus Blood warmer than hypothalamic set-point Skin blood vessels dilate: capillaries becomes flushed with warm blood; heat is lost from skin surface Sweat glands activated & sweat production ↑ evaporation of fluid from the skin enhances heat loss greatly Stimulus: ↓ Tb (cold Ta, exposure to cold water) Stimulus:↑Tb (during exercise, high Ta) Skin blood vessels constrict, blood is diverted from skin capillaries; minimizes overall heat loss from the skin Skeletal muscles contraction activated, shivering begins Blood cooler that hypothalamic set-point Activates heat-promoting center in hypothalamus When heat transfer to or from the environment overwhelms the body’s regulatory capacity: Hypothermia & Hyperthermia Hyperthermia • Elevation of Tb > normal Tset range (36-37.5 ᵒC) due to overwhelming of thermoregulatory mechanisms • Environmental conditions: Prolonged exposure to heat (↑Ta), high ambient humidity, physical exertion Thermoregulatory mechanisms become overwhelmed & fail; sweating ceases Inability to maintain adequate CO • Heat Collapse: inability to maintain CO leads to transient collapse • Heat Stroke: thermoregulatory failure (sweating ceases), CNS dysfunction (disorientation, headache, irritability, confusion, coma, seizures), death. Malignant Hyperthermia: Hypermetabolic crisis characterized by excessive accumulation of calcium in skeletal muscle & resultant sustained contraction following exposure to certain volatile anesthetic agents (e.g., isoflurane) in susceptible (MHS) individuals. Hypothermia I. Mild Hypothermia (32-35 ᵒC) II. Moderate Hypothermia (28-32 ᵒC) III. Severe Hypothermia (<28 ᵒC) Most common environmental cause is prolonged immersion in cold water. The convective heat transfer coefficient (h) in water is ~100X that of air Frostbite: Perfusion of extremities (ears, nose, cheeks, chin, fingers & toes) reduced markedly even with mild hypothermia. Prolonged cold exposure leads to freezing of tissue & cold injury (loss of sensation → blister formation → extensive tissue n ecrosis) Induced hypothermia: use of sedatives to depress the hypothalamic temperature controller & ice packs to reduce Tb < 34ᵒC. Permits ↓HR & ↓MR without tissue damage for periods 30-60 mins during surgery. Fever • Fever is an elevation of Tb due to resetting of the hypothalamic thermoregulatory set-point to a higher level (↑37 ᵒC) RISING PHASE FALLING PHASE PLATEAU 1. In response to a variety of infectious & inflammatory stimuli, macrophages & lymphocytes release cytokines into the circulation 2. Cytokines cross the BBB & stimulate PGE2 release from endothelial cells 3. PGE2 acts on the POAH to elevate thermoregulatory set-point resulting in fever • Cytokines are endogenous pyrogens & mediate their fever-inducing effects via stimulation of PG production. Inhibitors of PG synthesis (e.g., NSAIDs, aspirin) thus help to reduce temperature in fever • Fever is beneficial? • ↑ Tb = ↓micro-organism growth, ↑ AB production, slows the growth of some tumors • Prolonged Tb > 41 °C = hyperthermia...death By The End Of This Lecture, You Should Be Able To Answer The Following Questions: • • • • • • • • • • • • • • • • • • What are the primary mechanisms by which heat is lost from the body? During what phase of fever is Body temperature equal to Set-point temperature? Vasoconstriction of cutaneous blood vessels in response to changes in ore body temperature is controlled by what hormone/ neurotransmitter? At increased ambient temperatures, increased sweat production is achieved by stimulation of what receptor? What effect does intensive exercise have on temperature set-point? What are the most common causes of hypothermia and heatstroke? If core body temperature decreases, what 2 mechanisms that are activated to help bring temperature back to normal. By what brain center is the temperature set-point set and monitored? At room temperature, what is the primary mechanism by which heat is lost from the body? To diagnose heat stroke, core body temperature must exceed what temperature (in degree C)? If someone has a fever and is shivering, what phase of fever are they most likely in? At what core body temperature can hypothermia be diagnosed? Vasoconstriction of cutaneous blood vessels in response to changes in ore body temperature is mediated by binding of a hormone/ neurotransmitter to what receptor? During the falling phase of fever, which is highest – body temperature or set point temperature? If core body temperature increases, what 2 mechanisms that are activated to help bring temperature back to normal. What hormone is responsible for increasing female body temperature by 0.5 degree C during the luteal phase of the menstrual cycle? At ambient temperature approaches body temperature, what is the primary heat loss mechanism used by the body to maintain normal core body temperature? Increased sympathetic stimulation of blood vessels in the skin increases heat loss or heat storage? Which of the following temperatures (core body temperature, skin temperature, temperature set-point) is monitored and homeostatically regulated by thermoregulatory responses?