Thermoregulation Control of an organism’s temperature within a functional range

What is thermoregulation ? Definition: The mechanisms and control systems used by the body to balance thermal inputs and thermal losses so as to maintain constant core temperature –Ie. the control of an organisms temperature.

Homeotherms & Poikilotherms Not all animals can regulate their body temperature Homeotherms – animals that maintain a stable internal T, regardless of external conditions Poikilotherms – animals with a body T that varies a lot in response to external conditions. Endotherms – animals that use internal mechanisms to regulate body temperature Ectotherms – animals that use behavioural mechanisms to regulate body temperature.

Human Thermostat Hypothalamus (CNS) –Usually at 37 degrees Celcius –Diurnal variation – varies throughout day Max: 10am – 6 pm Min: 11pm – 3am

Thermoregulation Increased body temperature activates mechanisms promoting heat loss Lowered body temperature activates mechanisms promoting enabling the accumulation or production of heat Such a system is called a feedback control system, meaning that the consequences of the process dictate how it will go on further

Body Temperature If too HIGH –Increase metabolism  exhaust resources –Shortness of breath if blood too warm –Upper limit ~ 45 degrees –Rigor at 50 degrees – x-bridge cycle can’t operate, can’t hydrolyze ATP-release. If too LOW –No set value –Lose consciousness, brain conserving energy –15 degrees self-anesthetized

Heat Relative heat loss: –Feces & urine 3% –Respiration 20% –Skin 77% Natural Generation: –Muscles –Chemical changes

4 Ways to Lose Heat Conduction Convection –Blood Cell  ECF  Blood Evaporation –sweat Radiation

Thermoregulation Description Three main effector mechanisms:  1 st acts on blood vessel diameter  Vasodilation & vasoconstriction  2 nd is provided by metabolic effectors, which are substances produced by the body to increase its activity  Thermogenesis (heat production), thyroid gland  Muscles, digestion, organs  3 rd is provided by the sweat glands

Effects of Temperature Changes on Regulatory Activity

Thermoregulation Feedback Loop

What influences body temperature ? Exercise Time of the day Temperature of the environment Digestion Level of water consumption

What influences body temperature ? Body temperature: varies in the narrow range between 36.5 and 37.5 C Slightly increases during the day (reaches a peak between 6-10 pm and a low between 2-4 am) Fever → reaches a peak in the evening, and decreases during the night so that in the morning,even a very sick person may have almost nomal temperature

What influences body temperature ? Physical activity: increases body temperature Marathon runners ( body temperature can be as high as C)

Feedback mechanisms When the surroundings are cold or when the body is resting: The CNS (brain) constricts the blood vessels underlying the skin Vasocontriction occurs and the blood vessels become narrower Less blood flows underneath the skin This reduced hear loss by →conduction, radiation and convection Body temperature returns to normal

Thermoregulation and human health Thermoregulation → utmost importance in maintaining health Core temperature changes of the order of 3 C will not interfere with physiological functions but any variation outside that range has very serious effects Examples: at 28 C→ The muscles can no longer work, at 30 C → confusion occurs and the body can’t control its temperature, at 33 C → loss of consciousness, at 42 C → brain damage, at 44 C → death occurs (proteins denature)

Common diseases and disorders Fever-increase in body core temperature → not an illness but a natural reaction to a number of illnesses Hyperthermia-overheating of the body →caused by external factors ( hot environment, or a hot bath) Hypothermia-a low body temperature → exposure to cold weather or low body temperature by decreased metabolism Hypothyroidism-thyroid hormones are below normal-one can not regulate body temperature efficiently