05_thermoregulation.pdf

THERMOREGULATION AND FEVER 2024 Learning outcomes Name the parts of the hypothalamus involved in thermoregulation List the heat loss and heat production mechanisms by giving examples Describe how the body responds to changes in core or environmental temperatures Explain what fever is and how the bod...

THERMOREGULATION AND FEVER 2024 Learning outcomes Name the parts of the hypothalamus involved in thermoregulation List the heat loss and heat production mechanisms by giving examples Describe how the body responds to changes in core or environmental temperatures Explain what fever is and how the body responds to fever Describe the process by which sweat is produced 2 What is the advantage of the ability to regulate internal body temperature ? Independence from the environment 3 Homeotherms maintain their activities over a wide range of environmental temperatures! In birds and mammals, the "warm-blooded" (homeothermic) animals, a group of reflex responses that are regulated by the hypothalamus, operate to maintain body temperature within a narrow range in spite of wide fluctuations in environmental temperature. Normal body function depends on a relatively constant body temperature because: The speed of chemical reactions varies with the temperature The enzyme systems of body have narrow temperature ranges in which their function is optimal. 4 NORMAL BODY TEMPERATURE Various parts of the body are at different temperatures, The extremities are generally cooler than the rest of the body. Core temperature: Temperature of the deep tissues of the body. Average body temperature is 37 °C (98.6 °F), with a normal range of 35.5–37.7 °C (96–99.9 °F). 5 Measuring body core temperature: Measured under the tongue (sublingually), in the ear canal, or in the rectum. The rectal temperature is most reliable since it varies least with changes in environmental temperature. The oral temperature is normally 0.5 °C lower than the rectal temperature, but it is affected by many factors, including ingestion of hot or cold fluids or mouth breathing. Axillary (armpit) temperature is around 1 °C lower than the rectal temperature. - infrared thermometers are used to measure the radiant temperature over the temporal artery 6 The human body is usually warmer than its environment and therefore loses heat. Body Temperature Is Controlled by Balancing Heat Production and Heat Loss 7 Heat Production is a by-product of metabolism. At rest, most heat is generated by: the liver, heart, brain, endocrine organs, and skeletal muscles during exercise. Important factors on metabolic rate: (1) basal rate of metabolism of all the cells of the body; (2) extra rate of metabolism caused by muscle activity (including shivering); (3) extra metabolism caused by the effects of hormones mostly thyroxine on cells (4) extra metabolism caused by sympathetic stimulation; (5) extra metabolism needed for digestion, absorption, and storage of food (thermogenic effect of food). 8 Contribution of Body Systems to Resting Metabolism During exercise – 1300 kcal/hr 50-60% of this energy evolves as heat 9 Lost heat 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. 10 Heat insulation of body: The skin, the subcutaneous tissues, and especially the fat of the subcutaneous tissues act together as a heat insulator for the body. Infrared image of human Infrared image of a sea lion sitting on a rock. – Heat loss is limited to the eye and nose (bright red parts) 11 Heat moves from the skin to the environment by - radiation - conduction - convection - evaporation Model of energy transfer between the body and the environment. 12 WAYS OF HEAT LOSS FROM SKIN Radiation is the loss of heat in the form of infrared waves (thermal energy) –(heat transfer without contact). Under normal conditions, around half of body heat loss occurs by radiation. The body also gains heat by radiation, as demonstrated by the warming of the skin during sunbathing. Conduction is heat exchange between objects or substances at different temperatures that are in contact with one another. Conduction is aided by convection, the movement of molecules away from the area of contact. *the rate of heat loss to water is usually many times greater than the rate of heat loss to air. 13 WAYS OF HEAT LOSS FROM SKIN Air convection occurs due to replacement of warmed air enveloping the body by cooler air molecules, and the cooler air absorbs heat by conduction more rapidly than the alreadywarmed air. Conduction to air is enhanced by anything that moves air more rapidly across the body surface such as wind or a fan. Evaporation: Water evaporates because its molecules absorb heat from the environment and become energetic enough (that is, vibrate fast enough) to escape as gas (water vapor) à its evaporation from body surfaces removes large amounts of body heat. 14 Room temperature : Radiation > Evaporation > Conduction to air > Conduction to objects When the temperature of the surroundings becomes greater than that of the skin, the body gains heat by both radiation and conduction. Under these conditions, the only means by which the body can rid itself of heat is by evaporation. Clothing entraps air next to the skin, thereby the rate of heat loss from the body by conduction and convection is greatly depressed. 15 16 17 Stimulate release of free fatty acids from adipose tissue There are threshold core temperatures for each of the main temperatureregulating responses and when the threshold is reached the response begins. The threshold is 37 °C for sweating and vasodilation, 36.8 °C for vasoconstriction, 36 °C for nonshivering thermogenesis 18 35.5 °C for shivering. Regulation of Body Temperature-Role of the Hypothalamus The temperature of the body is regulated (almost entirely) by nervous feedback mechanisms mostly operating through temperature-regulating centers located in the hypothalamus. The preoptic area and anterior hypothalamic nuclei of hypothalamus Thermoreceptors located in preoptic area: The heat-sensitive neurons increase their firing rate in response to increase in body temperature. The cold-sensitive neurons increase their firing rate when the body temperature falls. Posterior nucleus integrates the central and peripheral sensory signals Signals from the preoptic area and the signals from the body are combined and integrated to control the heat-producing and heat-conserving reactions of the body. If the preoptic area is heated : 1- an immediate profuse sweating , 2-skin blood vessels become greatly dilated 3-body heat production is inhibited. Outputs of hypothalamus in thermoregulation: The anterior nucleus mediates temperature decreasing mechanisms The posterior region mediates temperature increasing mechanisms. Blood Flow to the Skin from the Body Core Provides Heat Transfer Blood is the main medium for transporting heat from core to skin regions. Blood vessels are distributed profusely beneath the skin. For heat regulation, the venous plexus that is supplied by inflow of blood from the skin capillaries is very important. Hands, feet, and ears (most exposed areas) blood is also supplied to the plexus directly from the small arteries through arteriovenous anastomoses. Heat conduction through 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. 24 This vasoconstriction is controlled almost entirely by the sympathetic nervous system in response to changes in body core temperature and changes in environmental temperature. Effect of environmental air temperature on conductance of heat from the core to the skin surface: 8 fold increase in heat conductance between the fully vasoconstricted state and the fully vasodilated state. 25 Restricting blood flow to the skin by vasoconstriction is not a problem for a brief period, but if it is extended (as during prolonged exposure to very cold weather), skin cells deprived of oxygen and nutrients begin to die à frostbite (can be a serious condition). 26 27 Sweating and Its Regulation by the Autonomic Nervous System Nerve impulses from preoptic area of hypothalamus that stimulate sweating are transmitted through sympathetic pathways to the skin everywhere in the body. Sweat glands are innervated by cholinergic nerve fibers 28 The three temperature-decreasing mechanisms when the body is too hot 1. Vasodilation of skin blood vessels. 2. Sweating. When the body core temperature rises above the critical level of 37°C. 3. Decrease in heat production. The mechanisms that cause excess heat production, such as shivering and chemical thermogenesis, are strongly inhibited. The temperature-increasing mechanisms when the body is too cold 1. Skin vasoconstriction. Stimulation of sympathetic centers in the posterior hypothalamus. 2. Piloerection. Sympathetic stimulation causes contraction of arrector pili muscles which brings the hairs to an upright stance. ( Not important in human beings, but in some animals, upright projection of the hairs provides a thick layer of "insulator air" next to the skin). 3. Increase in thermogenesis (heat production). Heat production by the metabolic systems is increased by promoting shivering, nonshivering heat production by sympathetic excitation , and by thyroxine secretion. Hypothalamic Stimulation of Shivering Center for shivering : The dorsomedial portion of the posterior hypothalamus. This area is inhibited by signals from the heat center (preoptic area) but is excited by cold signals from the skin and spinal cord. Chemical thermogenesis by sympathetic stimulation Sympathetic stimulation à immediate increase in the rate of cellular metabolism. The degree of chemical thermogenesis directly proportional to the amount of brown fat. In infants, who do have a small amount of brown fat, chemical thermogenesis can increase the rate of heat production 100 percent Brown fat is not found in adults à limited contribution to heat production. Chemical thermogenesis by thyroxine secretion secretion of thyroxine from thyroid gland increases the rate of cellular metabolism throughout the body. This mechanism requires several weeks Behavioral Control of Body Temperature Whenever the internal body temperature is disrupted, a psychic sensation of being overheated or cold discomfort occurs. Therefore, the person makes appropriate behavioural adjustments to cease this feeling of discomfort. To increase body temperature: -Changing posture to reduce exposed body surface area (hunching over or clasping the arms across the chest) -Putting on more or warmer clothing to restrict heat loss (a hat, gloves, and “insulated” outer garments) -Increasing physical activity to generate more heat (jumping up and down, clapping the hands) -Drinking hot fluids To decrease body temperature: Seeking a cooler environment (a shady spot) or using a device to increase convection (a fan) or cooling (an air conditioner) Reducing activity (“laying low”) Wearing light-colored, loose clothing that reflects radiant energy. 34 FEVER Hypothalamic “thermostat” has been reset to a new point above 37°C The termoreceptors signal that the body temperature is below the new set point à the temperature-raising mechanisms are activated. Monocytes, macrophages, and Kupffer cells digest toxins released from bacteria such as endotoxin or other pyrogens from degenerating body tissues (due to inflammation) act on to produce cytokines that act as endogenous pyrogens (e.g. IL-1 , IL-6, -IFN, and TNF). Cytokines are mediators coordinate the local and systemic inflammatory response to microbial products. But they also act on the brain in inducing the central symptoms specific to sickness, fever is one of these. 35 The fever produced by cytokines is due to local synthesis and release of prostaglandins within the hypothalamus. 36 OVLT: organum vasculosum laminae terminalis - leaky portion of the BBB *Antipyretic effect of Acetylsalicylic acid (aspirin) is exerted directly on the hypothalamus by inhibiting prostoglandin synthesis. Benefits of fever for infection? The value of fever in fighting infection is still debated. A popular hypothesis is that the elevated temperature enhances the host's response to infection. (The rate of T-lymphocyte proliferation in response to interleukins is many-fold higher at 39°C than it is at 37°C) Very high temperatures are harmful. As body core temperature rises, excessive cutaneous vasodilation can lead to a fall in arterial pressure and, therefore, to a decrease in brain perfusion. As T core approaches 41°C, confusion and, ultimately, loss of consciousness occur. 37 When heat gain exceeds heat loss, body core temperature rises Excessive hyperthermia (>41°C) leads to the clinical condition known as heat stroke. High temperature can cause fibrinolysis and consumption of clotting factors and thus disseminated intravascular coagulation, which results in uncontrolled vascular thrombosis and hemorrhage. 38 Hyperthermia and heat stroke: Under conditions of overexposure to a hot and humid environment, normal heat-loss processes become ineffective. The hyperthermia (elevated body temperature) that may depress the hypothalamus. At core body temperature above 40°C heat-control mechanisms begin to get less effective. With accompanying dehydration a fatal positive feedback cycle can be triggered : heat stroke Increased temperature à promotion of most chemical reactions à more heat production Multiple organ damage including brain damage becomes a distinct possibility,. Immersing the body in cool water and administering fluids is necesarry. 39 Daily (circadian) cycle of temperature: The normal human core temperature undergoes a regular circadian fluctuation of 0.5–0.7 °C in individuals. Lowest at about 6:00 a.m. and highest in the evenings at about 6:00 p.m. Typical temperature chart of a hospitalized patient who does not have a febrile disease. Note the slight rise in temperature, due to excitement and apprehension, at the time of admission to the hospital, and the regular circadian temperature cycle. 40