These notes were made in May, 2011, as preparation for mid year exams.
Homeostasis is the maintenance of the internal environment according to the external environment. It goes around a set point (dynamic equilibrium). It has two types of ‘feedback’ systems – in negative feedback, the stimulus is reduced, in positive, the stimulus is increased.
Heat gain must equal heat loss, as a stable temperature is needed for optimum enzyme activity. When core temperatures are above 42°C (hyperthermia), enzymes denature and death can result. When core temperature is less than 35°C (hypothermia), enzymes become inactive, and eventually death can also result.
Metabolic rate is the rate at which energy is released by food breakdown. Basal metabolic rate is the resting metabolic rate. Energy is released as ATP and heat. Metabolic rate can increase up to 40× when exercising. Stress also increases it (sympathetic nervous system) and a 1°C rise in core temperature can increase it by 10%.
Thermoreceptors, which send information to the hypothalamus, are found in the skin and mucous membrane, and a central thermoreceptor is found in the hypothalamus and detects core temperature.
Losing heat/reducing heat gain - Skin is very effective at losing heat- it loses heat via conduction, convection, radiation and evaporation. Vasodilation is controlled by the parasympathetic nervous system. It causes a rise in skin temperature but heat is lost by conduction, convection, and radiation.
Sweat is released from the exocrine system (sweat glands), controlled by the sympathetic nervous system, and evaporates heat off skin. It contains H20, NaCl, urea, lactic acid, etc.
Evaporation also occurs in hot air from the lungs. It is only effective if the air is dry and hotter than the sweat. It is the only way the body can lose heat in temperatures higher than core temperature.
Metabolic rate is decreased by reducing thyroxine release. Less metabolic reactions produces less heat. We also have certain behaviours when hot – we turn on a fan, or take off hot clothing.
Producing heat/reducing heat loss - Vasoconstriction is controlled by the sympathetic nervous system, and keeps the blood near internal organs, and skin temperature drops. The sympathetic nervous system also stimulates the adrenal medulla to release adrenalin and noradrenalin.
Piloerection (erect hairs) traps hot air like insulation. Shivering is forced muscle contractions (10-20/s) that increase muscle tone in rhythmic contractions.
The hypothalamus releases thyroid releasing hormone, which stimulates the anterior pituitary to produce more thyroid stimulating hormone. This causes the thyroid to produce more thyroxine to increase metabolic rate, therefore increasing heat production.
Heat stroke is when core temperatures continue to rise as heat loss mechanisms become ineffective and cease as the metabolic rate becomes too high to counteract. Heat exhaustion is fluid loss and vasodilation, causing a low blood pressure and fainting. Body temperature is still regulated, and unaffected.
Hyperthyroidism/Hypothyroidism - Hyperthyroidism is increased activity of the thyroid and so the overproduction of thyroxine, and an increased metabolic rate. Symptoms include an enlarged thyroid, rapid heart rate, increased appetite but weight loss, fatigue, sweating and anxiety.
Treatment is surgery to remove all or part of the thyroid, drugs to block thyroid production, or radioactive iodine which is absorbed and kills thyroxine producing cells.
Hypothyroidism is usually caused by a lack of iodine, and is becoming more common. Babies with iodine deficiencies are born with cretinism; they are usually mentally and physically retarded. Symptoms include a goitre, increased fatigue, weight gain, lethargy, and cold intolerance.