TEMPERATURE REGULATION Dr Mangala Gunatilake Dept. of Physiology

Objectives : Classification of living organisms Body temperature Variations of body temperature Heat Balance – Heat loss/ Heat production Regulation of body temperature Fever & Hypothermia

Homeothermic animals Body temperature Poikilothermic animals Thermoregulatory mechanisms are rudimentary eg. Fish, reptiles & amphibians Birds and mammals including humans can maintain a constant body temperature irrespective of ambient temperature Invertebrates cannot adjust their body temperature

Ambient Temperature Oral Temperature (36.3 – 37.1 °C) Axillary Temperature Surface (skin) Temperature (29.5 – 33.9 °C) Rectal Temperature Core Temperature (37.8 °C) Temperature of deeper tissues & organs

Variations of Body Temperature Physiological variations * Species 1.Age 2.Sex 3.Diurnal variation 4.After meals 5.Exercise 6.Sleep 7.Emotional excitement 8.Menstrual cycle

Pathological variations 1. Hyperthermia * Infections * Hyperthyroidism * Lesions of brain 2. Hypothermia * Hypothyroidism * Lesion in posterior hypothalamus

Heat Balance Heat Production (Heat Gain) Heat loss Basic Metabolic activities Assimilation of food Muscular activities Radiation of heat from the environment Role of Hormones Determines body temperature

Mechanisms of Heat Loss from the body * Conduction to air (15%) * Conduction to solid objects (3%) Air currents (* Convection) * Evaporation (22%) * Radiation heat waves (60%) Walls * Defecation * Urination

Panting – eg. In dogs – as they do not have sweat glands, there is evaporation of water from the surface of the tongue and dribbling of saliva with rapid and shallow breathing. * Fluffing of feathers in birds Reduce conduction of heat from the * Erection of fur in hairy animals body as the thickness of layer of air * Horripilation in humans between the body surface and the layer of fur or feathers will increase. So, more heat will be preserved in the body.

Regulation of Body Temperature There are reflex and semireflex thermoregulatory responses which include following responses. 1. Autonomic eg. shivering 2. Somatic eg. Stamping of feet, dancing up & down in a cold day 3. Endocrine eg. Increase secretion of catecholamines & TSH 4. Behavioural eg. Moving into a hot room and wearing suitable cloths when exposed to cold weather

Regulation of Body Temperature Posterior Hypothalamus Anterior hypothalamus Cold Hot By reflex responses integrated at the Hypothalamic level Integration is done by the hypothalamus in response to information from sensory receptors in the skin, deep tissues, brain etc. In these places there are more cold receptors than hot receptors. There are hot sensitive neurons There are cold sensitive neurons

There are threshold temperatures for each of the main temperature regulating responses, and when threshold is reached, the response begins. Eg. Threshold for sweating & vasodilation - 37 °C Threshold for vasoconstriction °C Threshold for shivering °C

Exposure to ‘cold’ Stimulation of Heat Gain center (Posterior hypo.) 1.Shivering 2.Hunger 3.Increased voluntary activity 4.Increased secretion of catecholamines Increase Heat production Decrease Heat loss 1.Cutaneous vasoconstriction 2.Curling up 3.Horripilation

Exposure to ‘Hot’ Stimulation of Heat Loss center (Anterior hypo.) 1.Anorexia 2.Apathy & inertia Decrease Heat production Increase Heat loss 1.Cutaneous vasodilatation 2.Sweating 3.Increased respiration

Fever Toxins from bacteria Inflammation Pyrogens released from the degenerating tissues Monocytes, Macrophages, Kupffer cells Cytokines (act as Endogenous pyrogens) eg. IL1 Activates the preoptic area of the hypothalamus Release of ‘Prostaglandins’ Temperature set point is raised Heat conservation & increase in heat production Fever

Beneficial effects of fever (Hyperthermia): 1. It occurs as a response to infection 2. Rise in temperature inhibits the growth of microorganisms 3. Antibody production is increased 4. Slows growth of some tumors Anyway, very high temperatures more than 40 °C are harmful. If it is for long periods, it can cause permanent brain damage, heat strokes and even death.

Hypothermia : When skin or blood is cooled to lower the body temperature in humans * Metabolic & physiologic processes will slow down * Respiration and heart rate are slow * Blood pressure is low * Consciousness is lost Hypothermia is useful to do open heart surgeries and to perform brain operations.

Thermogenic function of Brown Fat Brown Fat More abundant in infants Located - between the scapulas at the nape of the neck along the great vessels in the thorax and abdomen

Brown Fat White Fat Fat cells contain several * Fat cells contain only a small droplets of fat single large droplet of fat Fat cells as well as blood * principal sympathetic vessels have an extensive innervation is solely sympathetic innervation on blood vessels Fat cells contain many mitochondria

In ‘Brown Fat’, * Oxidative phosphorylation is present which generates ATP * Uncoupled oxidative phosphorylation which does not generate ATP. So more heat is produced.

For more information………. Section on ‘Temperature Regulation’ in Ganong & Guyton