People maintain normal body temperature despite variations in both their metabolic activity and Ambient temperature Homeothermic animals (hot blooded) Animals with body temperature changes with environmental temperature are called heterothermic animals (cool blooded)
The Core temperature: - Temperature of the internal organs like the brain, and organs in the thoracic and abdominal cavities Shell temperature: - Temperature of the body surface (the skin)
Body temperature ( C) 37 -- 35 -- 33 -- 31 -- Thermoneutral Temperature zone Rectal temperature ( C) Skin temperature ( C) I 23 I 25 I 27 I 29 I 31 I 33 I 35 Ambient temperature ( C)
The core temperature is precisely regulated The shell temperature varies substantially depending on the temperature of the surroundings The shell/ skin temperature is important for heat loss from the body
Core temperature is usually recorded from the outer ear or mouth Core temperature can also be recorded form the -axilla -rectum (mouth tem + 0.6 C )
Core temperature shows diurnal variation (24h) within the limit of 1 C -Lowest in the early morning (4 am) -Increases to a maximum in late afternoon or early evening (2-4 pm) Increases by 0.5 C in pregnant females and with ovulation Can increased during heavy exercise to values above 37.5 C
Is an important homeostatic mechanism Thermoregulation is: To maintain core body temperature constant within a narrow range (± 0.6 C)despite changes in environmental temperature
For a constant internal (core) body temperature: Rate of heat gain = Rate of heat loss
Consequences of Increased core temperature: The rate of Enzyme activity within cells At a temperature above 42 C enzymes and other cellular proteins will starts to denature and cell damage will occur Temperature above 43 C is incompatible with life
Consequences of decreased core temperature: The cells will function more slowly But Temperature regulation and consciousness are impaired at temperature below 33 C and this might lead to death
The body gain heat by 1- Heat production: Metabolism, SDA of food, muscle contraction, and non-shivering thermogenesis 2- From the environment : if the ambient temperature is wormer than the body The body loses heat to the environment
Heat exchange between the body and the environment can be achieved by: 1- Radiation : loss or gain of heat in the form of infrared rays (thermal energy) 2- Conduction: transfer of heat between objects in direct contact
3- Convection: The removal of heat from the body by a current of air (or water) air in direct contact with the skin is wormed by conduction then it moves away from the surface of the skin and its replaced by un heated air
4- Evaporation: Results in heat loss from the body Two types: 1. Insensible evaporation 2. Evaporation of sweat
1. Insensible evaporations Evaporation of water from the skin, mucosa of the mouth and lungs can not be controlled
2. Evaporation of sweat: The only mean by which the body can lose heat when the surrounding temperature is higher than skin and body temperature In hot climates sweating is essential to prevent the increase in core body temperature to dangerous levels It can be regulated according to body and environmental temperature
Components of the thermoregulatory response: Hypothalamic thermoregulatory center Afferent input from thermoreceptors Efferent output to effectors systems Sympathetic and somatic efferent Effector systems: 1. Cutaneous circulation 2. Sweat glands 3. Skeletal muscles 4. Endocrine glands (adrenal, thyroid gland) 5. Behavioral response
The temperature regulating centre in the hypothalamus is made of two centres 1-Heat loss centre located in the Anterior hypothalamus -it controls the heat loss mechanisms (sweating)
2-Heat production and conservation centre in the Posterior hypothalamus - controls the heat gain and conservation mechanisms)
The hypothalamus receives afferent input from: 1. Peripheral cutaneous thermoreceptors located in the skin 2. Central thermoreceptors located in the Anterior hypthalamus
The set point is a critical level of temperature at which the thermoregulatory mechanisms try to maintain the core temperature It is controlled by the hypothalamus The set-point fore core body temperature is normally at 37 C
The hypothalmus behaves as a thermostate ( set at the set-point ) that maintain a balance between heat loss and heat production Impulses from peripheral and central thermo receptors are transmitted to the posterior hypothalamus The information is compared to the set point Any deviation from the set point will initiate a temperature regulatory change
Deviation above the set-point (increased core temperature) promote heat loss and decrease heat production Deviation below the set point (decreased core temperature) promotes heat gain mechanisms and heat conservation mechanisms
Response include increasing heat production, decreasing heat loss and behavioural responses: 1- Heat production can be increased by 1. Shivering: rhythmic contractions of muscles 2. Increased sympathetic activity: metabolic effect 3. Increased thyroid gland activity: metabolic effect
2- Heat loss can be reduced by: 1. Cutaneous vasoconstriction (hands and feet) 2. Piloerection : skin hair stand on end effective in furry animals not in humans 3. Inhibition of sweating 3- Behavioural adjustments to sensation of cold ex: go to a worm room, wear heavy clothes
The response includes increased heat loss, decreased heat gain, and behavioural responses: 1-Heat loss is increased by: 1. Cutaneous vasodilation= inhibiting the sympathetic outflow to the skin 2. Sweating= evaporation heat loss
2-Heat production is reduced by 1. Inhibit shivering 2. Decreased activity of the thyroid gland 3- behavioural adjustments for sensation of warmth: decrease activity, wear light clothes, find cooler area or fanning
Increased Environmental Temperature Core temperature above the set point Mechanisms for Heat loss Vasodilation Sweating -Evaporation Decreased metabolism Inhibition of shivering Behavioural Decreased Environmental Temperature Core temperature below the set point Mechanisms for Heat gain and conservation Vasoconstriction Shivering Increase metabolism, NSTG Catecholamines and thyroid Inhibition of sweating Behavioural
Fever (Pyrexia) is abnormal elevation of body temperature Fever is due to elevation in the thermo-regulatory set point Caused by pyrogens
Pyrogens are substances that can increase body temperature Pyrogens include: 1. Cytokines (from white blood cells, IL-1, IL6, TNFα) 2. Prostaglandines (PGE2)
Pyrogens induce fever by elevating the hypothalamic set-point temperature The hypothalamic centre sees the normal core temperature as decreased The heat generating mechanisms are activated (shivering and vasoconstriction) and the subject feels cold (Chills) Body temperature rises to the new set-point
When the pyrogen effect is removed the set point will return to the normal value When the set point is back to normal the new body temperature is seen as too high The heat loosing mechanisms are activated (Sweating and vasodilation) and the subject feels warm
The effect of the prostaglandins is to increase the set point Aspirin (Cyclooxygenase inhibitor) inhibit the synthesis of prostaglandins and prevent the pyrogens from increasing the set-point
Heat stroke Heat exhaustion Hypothermia