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Physiological Homeostasis


Internal environment The concentration of water, sugar and other chemicals, as well as the body’s temperature must be maintained at a constant level to allow the body’s metabolism to function correctly.


Physiological homeostasis is the maintenance of the body’s internal environment within certain tolerable limits despite changes in the body’s external environment (or changes in the body’s rate of activity).


Negative Feedback Control Changes in factors affecting the body’s intenal environment (i.e. deviations from a norm or set point) are detected by receptors. These send out nervous or hormonal messages which are received by effectors. These bring about responses to return the system back to it’s set point.


Negative Feedback Control


e.g. 1 Osmoregulation This is the means by which the body maintains the correct concentration of water (and salts and ions). The hypothalamus in the brain acts as the receptor in this system The kidneys act as effectors in this system.


If the water concentration of the blood decreases (e.g. due to lack of drinking water, sweating, eating salty foods), osmoreceptors in the hypothalamus detect this. These trigger the release of much antidiuretic hormone (ADH) by the pituitary gland.


On arriving at the kidneys, ADH increases the permeability of the tubules and collecting ducts to water and so more water is reabsorbed, reducing the volume of urine produced and returning the blood water concentration back to normal.


If water concentration of the blood increases, less ADH is produced and so the tubules and collecting ducts of the kidneys become less permeable to water so volume of urine produced increases.


Osmoregulation


e.g.2 Blood Sugar Regulation 

Glucose required for oxidation in respiration for energy Only available when food is eaten so a regular supply is required by a homeostatic mechanism Glucose is converted to glycogen for storage in the liver


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A rise in blood sugar level is detected by cells in the pancreas called the Islets of Langerhans

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These receptor cells produce insulin which converts glucose for storage: glucose

insulin

glycogen


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If blood sugar drops below the set point, different cells in the Islets of Langerhans detect this and produce glucagon This breaks down glycogen to release glucose: glucagon

glycogen

glucose


“Blood sugar low, glucose gone, What you need is glucagon. To turn glucose into glycogen, What you need is insulin” Top tip: This is one of the few instances in biology where spelling is crucial – be sure to spell glycogen and glucagon correctly


Adrenaline  In an emergency when the body needs additional glucose quickly for the “fight or flight” response, the adrenal glands release adrenaline.  This overrides the normal homeostatic system by inhibiting the secretion of insulin and promoting the breakdown of glycogen.  When the “emergency” has passed, adrenaline secretion returns to minimum and blood sugar level is corrected by the normal homeostatic control


Diabetes mellitus  Some or all of a sufferers insulin-secreting pancreas cells are non-functional.  This causes an abnormally high blood glucose level which can be detected in the urine due to non-absorption by the kidneys  Nowadays successfully managed by regular injections of insulin and a controlled diet.


e.g.3 Thermoregulation 

An ectotherm is unable to regulate it’s own body temperature and so it’s body temperature reflects that of the external environment e.g. snakes, lizards An endotherm is able to control it’s internal temperature e.g. mammals, birds. Endotherms have a high metabolic rate and a homeostatic control system for temperature.


The hypothalamus in the brain is the receptor i.e. temperature-monitoring station in this system. It reacts to nerve impulses from thermoreceptors in the skin and acts as it’s own thermoreceptor to changes in blood temperature reflecting changes in core body temperature.


Role of the skin - overheating 1. Vasodilation: Arterioles leading to the skin become dilated allowing a large volume of blood to pass through the capillaries near the surface of the skin and lose heat by radiation. 2. Increased rate of sweating: Heat energy can also be lost by the body in converting sweat to water vapour.


Role of the skin – overcooling 1. Vasoconstriction: Arterioles leading to the skin become constricted so little blood flows to the skin surface. Shunt vessels also become involved in diverting blood from the skin. 2. Decreased sweating


3. Contraction of erector muscles: Birds and mammals have muscles in the skin which contract, raising the hairs or feathers creating an insulating layer of air. 4. Increase in shivering and metabolic rate


Top Tip & Handy Hint: Make sure you know where levels of conditions are monitored: Water & Temperature = Hypothalamus Blood sugar = Pancreas


33 Physiological Homeostasis  
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