Temperature Regulation

Why temperature regulation?

Why temperature regulation? All organisms are limited by their ability to survive in different temperatures Some, like reptiles and amphibia are poikilothermic Others like humans are homeothermic

Core temperature The core temperature of the human body is 37°C ± 0.6°C The core of the human body includes the organs of the thorax, abdomen and the head This is where the vital organs are located Their enzyme systems must operate in optimum conditions The periphery of the body can withstand some deviation from the core temperature

Core Temperature

Mechanisms of Heat Transfer 1) Radiation, especially infrared 2) Conduction: heat transfer by direct contact conductors – high conductance insulators – low conductance (high resistance)  Convection:  conduction due to movement of a liquid or gas

3) Evaporation 0.58 Cal required to evaporate 1g of H2O from the surface of the skin. heat of vaporization = 0.54 Cal When environmental temperature > body temperature, evaporation is the only mechanism to cool the body.

Balance Body Skin Environment Basal metabolic rate Muscle activity Shivering Vasomotor Sweating Piloerection Conduction Convection Radiation Evaporation Body Skin Environment

Set-point hypothesis So how is it regulated? Reference Response Controlled system Feedback Set-point hypothesis

Response Controlled system Feedback Balance hypothesis warm cool

Anterior Hypothalamus Inputs Outputs Anterior Hypothalamus Central thermosensors warm cool Peripheral Neural sweating shivering vasoconstriction vasodilation Hormonal adrenaline TRH

Central Control of Body Temperature Controlled Variable temperature of body core Set Point 37oC Receptors central thermoreceptors: in hypothalamus of brain peripheral thermoreceptors: in skin (mostly affect behavior, very little role in regulating core temperature) Control Center hypothalamus Effectors dermal blood vessels skeletal muscle endocrine glands brown fat sweat glands

Hypothalamic Control of Body Temperature When core body temperature drops below the set point, the following mechanisms are activated to increase body temperature:  heat loss vasoconstriction [piloerection]  heat production shivering muscle contraction generates heat hormone secretion e.g., thyroxine, epinephrine (adrenaline) activation of brown fat e.g., in human infants mitochondria generate heat instead of ATP

Hypothalamic Control of Body Temperature When core body temperature rises above the set point, the following mechanisms are activated to decrease body temperature:  heat loss vasodilation (inhibit vasoconstriction) sweating  heat production  voluntary activity  fuel intake  thyroxine secretion

Muscles of skin arteriole walls relax NEGATIVE FEEDBACK Blood temperature Muscles of skin arteriole walls relax Core body temperature >37°C Hypothalamus Sweat glands increase secretion nerves Muscles reduce activity Thermoreceptors Thermoreceptors Body loses heat Return to 37°C

Muscles of skin arteriole walls constrict NEGATIVE FEEDBACK Blood temperature Muscles of skin arteriole walls constrict Core body temperature <37°C Sweat glands decrease secretion nerves Muscles shivering Thermoreceptors Hypothalamus Thermoreceptors Body loses less heat Return to 37°C Body gains heat

Babies Babies have an extra mechanism (cant shiver) Brown fat. Suprascapular deposits Rich in Iron containing mitochondria

And the menstrual cycle Rectal temp OC 37.4 36.8 36.2 12 18 6 24 Time Postovulatory Preovulatory

Fever and hyperthermia An elevation of body temperature that exceeds the normal daily variation and occurs in conjunction with an increase in the hypothalamic set point----fever Fever due to a disturbance of thermal regulatory control----hyperthermia

Development of fever Pyrogenic activator Macrophage, monocyte, lymphocyte, etc. Endogenous pyrogen Thermoregulatory Centre Mediators in regulation of body temperature Rise in set point fever

Pyrogenic activator A substance that can manufacture and release endogeneous pyrogen which produces a fever. 1．Exogenous pyrogen bacteria virus other microorganism 2. internal metabolic product antigen-antibody complex steroid inflammatory substance

Fever monocytes and phagocytes release endogenous pyrogen (Interleukin-1, IL-1). The anterior hypothalamus is sensitive to IL-1 Hypothalamic sensitivity to temperature is altered.

The body temperature then becomes regulated at a new, higher level. So… The body temperature then becomes regulated at a new, higher level. There is some evidence that the raised body temperature enables the fight against the infection.

In acute infection, the ability to mobilise fat stores is inhibited. But... Every 1OC rise in temperature increases basal metabolic rate and oxygen consumption by about 14%, In acute infection, the ability to mobilise fat stores is inhibited.

Consequently Skeletal muscle is broken down and the amino acids are used in gluconeogenesis. This can be debilitating.

In addition to the increased demand for energy And even worse In addition to the increased demand for energy Temperatures (above 42OC) damage nerve cells impair thermoregulation have more serious consequences.