Homeostasis

• Homeostasis is the ability to keep our internal environment constant.
• Mammals are excellent examples of animals that hold a range of internal parameters constant or within a very narrow range.
• Homeostasis is achieved through negative feedback mechanisms.
• Negative feedback mechanism is a type of control in which the conditions being regulated are brought back to a set value as soon as a deviation from this value is detected.
• Mammals are endothermic organisms (warm blooded), they can control their internal body temperature (core temperature) independent of the external temperature.
• The normal body temperature is 37ºC and is kept constant by the homeostatic feedback mechanisms.
• Heat is transferred between our body and the environment through convection of heat, radiation of heat, conduction of heat, and evaporation.
• The control box for body temperature is found in the hypothalamus and is called the thermoregulatory center.
• The thermoregulatory center has two components: heat loss center and heat gain center.
• The thermoregulatory center receives impulses from warm and cold receptors from the skin (thermoreceptors) and core body temperature receptors in the hypothalamus itself.
• The thermoregulatory center responds by sending impulses through motor neurons to effectors (muscles or glands), the control of body temperature is called thermoregulation.
• Vasodilation is caused by relaxation of smooth muscles in the walls of arterioles (opposite of Vasoconstriction).
• During exercise there is an initial rise in core body temperature due to high rates of respiration & stronger muscle contractions which release heat energy.
• So there is more heat produced than lost.
• Homeostasis by negative feedback restores body temperature to the normal set value after a short duration of strenuous exercise.
• In cases of long duration strenuous exercise, such as in marathon runners, body temperature might rise again due to dehydration which reduces sweating and consequently reduces heat loss by evaporation.
• Heat production becomes greater than heat loss when cooling mechanisms fail.
• Hormones can affect transcription factors.
• In a cell, there are protein called transcription factors that control the transcription of genes.
• Factors that increase the rate of transcription are called activators and those that decrease the rate are repressors.
• Different types of hormones act as DNA transcription factors in different ways, and this is how they make changes in the body.
• Protein hormones, such as adrenaline, insulin and ADH hormones, cannot cross the cell membrane and enter the cell.
• Steroid hormones, such as oestrogen and testosterone, are lipid soluble and can pass through the membrane and act as the internal messenger itself.
• Osmoregulation is the maintenance of the osmotic potential in the tissues of a living organism within narrow limits by controlling water and salt concentrations, it is vital for life.
• By controlling the water potential of the blood (both the water content and the solute concentration), the body can control the water potential of the tissue fluid and so protect the cells from osmotic damage.
• In mammals, the main organ involved in the homeostatic control of the water balance of the body is the kidney.
• The liver is also involved in homeostasis, for example in the breakdown of excess amino acids and the removal of toxins.
• The liver plays an important role in protein metabolism.
• Our body cannot store protein or amino acids.
• Hepatocytes (liver cells) deaminate excess amino acids, remove the amino group and convert it first into ammonia, which is very toxic.
• Ammonia produced by deamination of protein is converted into urea (which is less toxic) by a series of enzyme-controlled reactions called the ornithine cycle.
• The remains of amino acids can be used in cellular respiration or converted into lipids for storage.
• Without the action of the liver, any excess protein we eat would be excreted and wasted.