Basal Metabolic Rate & Body Temperature Regulation (PDF)

Basal Metabolic Rate & Regulation of Body Temperature Basal Metabolic Rate (BMR) Def: It is the rate of heat loss in the body/unit time under three basal conditions. It represents the activity of vital organs (heart, resp muscle, liver, brain, nervous system, kidney) under basal conditions. Bas...

Basal Metabolic Rate & Regulation of Body Temperature Basal Metabolic Rate (BMR) Def: It is the rate of heat loss in the body/unit time under three basal conditions. It represents the activity of vital organs (heart, resp muscle, liver, brain, nervous system, kidney) under basal conditions. Basal conditions: 1. Complete mental, physical & emotional rest. 2. Post-absorptive state (12-14 hours after the last meal) 3. Comfortable external temp (neither shivering nor sweating). Normal values of basal metabolic rate:  Newborn infant 25 C/m2/hour  2-5 years 60 C/m2/hour  Adult male (20years) 40 C/m2/hour  Adult female (20years) 36 C/m2/hour  Every 10 years BMR decrease by 1 C/m2/hour  Factors affecting BMR Factors increasing BMR Factors decreasing BMR Physiological factors Male Female Athletes Sedentary people People living in cold zones People living in hot zones High protein diet Sleep Pregnancy & lactation Dark colored people Stress Pathological factors Hyperthyroidism Hypothyroidism Hyperfunction of the adrenal cortex Hypofunction of the adrenal cortex Hyperfunction of adrenal medulla Hypothermia Fever Undernutrition & starvation Diabetes insipidus Shock Heart failure Blood diseases: e.g., anemia  Body Temperature Normal Body Temperature normally  36.6 – 37.2 oC  Heat Balance The balance between heat production & heat loss determines the body temperature. Normally, the rate of heat production = the rate of heat loss.  HEAT GAIN HEAT LOSS THROUGH 1. BMR 1. Skin (85%) mainly through radiation 2. Skeletal muscular activity 2. Respiratory tract (14%) 3. Food intake 3. Body excreta (1%) 4. Hormones e.g. thyroid hormone 5. External environment especially sun radiation.  Thermoregulatory System It consists of: A. Thermoreceptors: 1. Central thermo-receptors: - located in the hypothalamus. 2. Peripheral thermo-receptors: - located in the skin. A. Temperature regulating center (hypothalamic thermostat): 1- Heat loss center: anterior hypothalamus 2- Heat gain center: posterior hypothalamus  On exposure to hot weather  Body temperature tends to increase  stimulation of heat loss center  A) Increase heat loss by: 1- VD of cutaneous blood vessels Increase heat loss from the skin 2 – Sweating B) Decrease heat production by: 1- Inhibition of muscle tone 2- Anorexia (lost appetite). 3- Inhibition of secretion of thermogenic hormones (catecholamines and thyroid).  On exposure to cold weather  The body temperature tends to decrease  stimulation of heat gain center  A) Increased heat production: 1. Shivering increases heat production rapidly (400-500%). The most potent in heat production. 2. Increased secretion of catecholamines and thyroid  increases cellular metabolism and heat gain 3. Increased sensation of hunger and appetite. B) Decrease heat loss by: 1. Cutaneous VC  decreases heat loss. 2. Curling up in a ball  decreases surface area exposed to the environment decrease heat loss 3. Clothes  insulating against heat loss. NB: Shivering is inhibited by alcohol  Fever (Pyrexia) Def.: Elevation of body temperature due to resetting of hypothalamic thermostat. Causes: 1- Infection. 2- Physical trauma. 3- Stress.  Fever (Pyrexia)  Pathophysiology of fever: Infection  release of interleukins (IL1) from macrophages  synthesis & release of prostaglandins (PG) within the hypothalamus  reset thermostat at a new higher level  fever N.B. Aspirin inhibited PG  decrease fever  Stages of fever: three stages 1- Stage (I) of chill: Resetting of hypo. thermostat to a new level above 37  Hypoth. thermostat feel cold  Decrease heat loss (cut. VC) & increase heat prod (shivering)  pale skin , cold feeling. 2- Steady (II) state of fever: When body temperature reaches the new set point  shivering stop & sensation of warmth develop  warm flushed skin. 3- Stage (III)of defervescence: Removal or treatment of causative factor  set point drop to normal  hypothalamic thermostat evoke mechs which decrease body temperature (cutaneous VD & sweating). Benefits of fever: 1- Increase production of antibodies. 2- Prevent growth of certain microorganisms. 3- Prevent growth of certain tumors. Thank you