Thermoregulation Lecture Notes PDF

Thermoregulation by Dr.Mai Mohammed Hasan Professor of Medical physiology  Regulation of body temperature Vertebrate animals are divided into: ▪ Cold blooded or poikilothermic ▪ Warm blooded or homeothermic Human body temperature: 1. Oral temperature 37°C + 0....

Thermoregulation by Dr.Mai Mohammed Hasan Professor of Medical physiology  Regulation of body temperature Vertebrate animals are divided into: ▪ Cold blooded or poikilothermic ▪ Warm blooded or homeothermic Human body temperature: 1. Oral temperature 37°C + 0.3 2. Rectal temperature is higher by 0.5°C 3. Axillary temperature is lower by 0.5°C 4. Skin temperature (shell temperature) is lower than the central (core) temperature.  Factors affecting body temperature 1.Age: body temperature is higher in younger 2.Sex: the basal body temperature in females increases by 0.5°C in the second half of menstrual cycle and during pregnancy. 3.Diurnal variation: it is lowest at 3 am and higher at 6 pm (+0.5°C) 4.Environmental temperature: if increased, it leads to increase in the body temperature. 5.Emotions :  body temperature (tension of muscles& hormones) 6.Muscular exercise: body temperature 7. Food intake:  body temperature due to specific dynamic action 8.Diseases: ▪Hyperthyroidism  body temperature. ▪Febrile diseases → fever ▪Peripheral circulatory disorder causes coldness of the skin  Heat production I. Basal rate of metabolism: metabolic activity of all cells II. Extra metabolic rate a- Muscular activity: - Muscle tone → 25% of heat - Voluntary muscle Contraction → 75% of heat. -Shivering b-Thyroxin secretion: increases heat production slowly but for long time. c- Catecholamines secretion: Catecholamines and sympathetic stimulation  heat rapidly but for short period.  Heat loss I- Non-evaporative heat loss a) Radiation: The most important → 60% of heat loss. b) Conduction: 3 % of heat loss as it's self limited. c) Convection: 15 % of heat loss. II- Evaporative heat loss → 22% of heat loss a) Insensible perspiration At 24°C, during rest 25 ml (gm) H2O are evaporated/hour from the skin and lungs. As 0.58 k calorie of heat is lost by evaporation of 1 gm H2O, the heat loss = 12-16 k. calorie/hour. b) Sweat secretion:  body temperature by 1°C → enough sweating to remove ten times the basal rate of body heat production.. Sweat secretion starts when environmental temp. is 32-34°C. Sweat evaporation is helped by moving dry atmosphere. Heat loss via urine and stool: (1%) To Maintain a Steady-State Core Temperature, the Body Must Balance Heat Gain With Heat Loss Body heat transfer  Control of body temperature The thermoregulatory system is composed of 1. Sensory receptors (thermoreceptors) 2. Central integrator (centre) 3. Effector organs  I-Thermoreceptors A) Peripheral thermoreceptors Site: Skin contain both cold and warmth receptors with more cold receptors. - Deep body temperature receptors are present in abdominal viscera and spinal cord Pathway: the peripheral thermoreceptors via the lateral spinothalamic tract to the thalamus and somatosensory cortex pass to activate the heat regulatory center in hypothalamus. B) Central thermoreceptors - The anterior hypothalamus and the preoptic area contain large number of heat sensitive neurons and cold sensitive neurons. - These receptors are sensitive to core temperature  II-Thermoregulatory center (thermostat) - It is present in the hypothalamus. - It receives impulses from the thermoreceptors and compares it with its specific standard reference temperature (set - point) which almost 37.1°C body core temperature. - If body temp. > set point → stimulation of anterior hypothalamus → heat loss. If body temp. < set point → stimulation of posterior hypothalamus → heat production &  heat loss. III- The effectors organs ▪ skin (blood vessels and sweat gland) ▪ Skeletal muscle ▪ Endocrine glands  Disorders of temperature regulation Fever (pyrexia) -It is hyperthermia caused by resetting of the setpoint of the hypothalamus to a higher level. Mechanism of fever: -Toxins of bacteria + degenerated tissue → exogenous pyrogens act on the monocytes and macrophages which release interleukin and TNF are called endogenous pyrogens(IL-1) reach the hypothalamic thermosensitive neurons cause fever within ten minutes by formation of prostaglandin E2 (PGE2) which activates resetting of the central thermostat ( set-point) by synthesis of cAMP. -Because body temp. is still less than the set-point, the person has false feeling of cold and mechanisms of elevation of body temperature occur. Mechanism of fever  The crisis (Flush) If the factor that causes fever is removed (treated), the set point returns to the normal level and the body temp. is still more than the set-point → sensation of hotness → mechanism of heat loss Control of fever - PGE2 has a negative feed back on interleukin I. -  interleukin I → down regulation of its receptors. - Glucocorticoids as cortisol → interleukin I. - Aspirin as antipyretic drugs synthesis of PGE2 from arachidonic acid set point level  heat loss by sweating Typical fever curves have the four stages in common: 1.The ascent phase 2.The peak 3.The beginning of the decrease 4.Normalization: end of decrease