Physiological Homeostasis Chapter 33

Internal Environment  Millions of cells in a body make up a community  Different parts of the body dependent on each other  All the cells and the fluid around them makes the internal environment  This environment must be controlled  E.g. water and solute concentrations, blood sugar, & temperature

Homeostasis  Maintaining the internal environment within limits  Changes from the norm are detected by receptors  These send out a nerve/hormone signal to effectors  Effectors then cause a response  Response counteracts the original change from the norm  This is negative feedback control

Homeostasis in the human body  Osmoregulation  - maintaining water, salt and ion concentrations  - kidneys act as effectors  - Water levels detected by osmoreceptors in the hypothalamus  - Triggers the release of Anti Diuretic Hormone (ADH) by the pituitary  - Low water conc.= lots of ADH  - ADH increases tubule and collecting duct permeability to water  - increased water reabsorption  - opposite occurs with high water conc.

Homeostasis in the human body  Blood sugar level  - Cells constantly use blood sugar  - Glucose only supplied when food is eaten  - glucose supply has to be closely regulated  - Glucose stored as glycogen in the liver  - Blood sugar level detected by the Islets of Langerhans (in the pancreas)  - If level is high, insulin is produced  - Insulin activates an enzyme (glucose converted to glycogen)  - If level is low, glucagon is produced  - Activates a different enzyme (glycogen converted to glucose)  - In an emergency, adrenaline released by the adrenal glands  - It inhibits insulin secretion, promotes glucagon secretion

Diabetes Mellitus  When insulin secreting cells are non-functional  Insufficient (or no) insulin produced  Blood glucose levels can rise (2-6 times more)  Glucose often excreted in the urine (unable to be reabsorbed)  Cells use glucose inefficiently  Fat stores depleted to produce more sugar  Can lead to weight loss and tissue wasting  Now successfully treated by controlled diet and insulin injections/implants

Control of body temperature  Ectotherm  - animal unable to regulate own body temperature  - ‘cold blooded’  Endotherm  - animal that can regulate it’s own body temperature  - high metabolic rate generates lots of heat  - Role of hypothalamus  Acts as a thermostat  Receives impulses from thermoreceptors  Also has central thermoreceptors –senses blood temp.  - this reflects core temp.  Sends nerve impulses to effectors

Control of body temperature  Role of skin  Correcting overheating:  - arterioles near skin dilate  - more blood flow to surface  - heat lost by radiation through the skin  - water in sweat converted to water vapour  Correcting overcooling:  - arterioles constrict near skin  - less blood flow, less heat radiated  - sweating rate reduced  - erector muscles contracted  - hairs (or feathers) raised to trap a layer of air – insulation  - shivering & increased metabolic rate also occur

Voluntary Control  If body temp changes, message sent to cerebrum  Makes person ‘feel cold’ or ‘hot’  Appropriate action taken e.g. extra or less clothing, hot or cold drink

Importance of homeostasis  Maintains internal environment at steady optimum state  Body can function even if environment changes  Can only work up to a point  In extreme situations homeostasis cannot keep up  Can lead to death