D3.3 HomeostasisIB Biology SL: Revision notes

Homeostasis is the maintenance of the internal environment of an organism: variables are kept within preset limits despite fluctuations in the external environment. In humans, homeostatic variables include body temperature, blood pH, blood glucose concentration and blood osmotic concentration.

In negative feedback, a change away from the set point is detected by receptors and triggers responses by effectors that reverse the change. It works whether the value is above or below the set point, returning it towards the set point. Positive feedback would amplify any change, moving the variable further away, so it is unsuitable for homeostasis.

Endocrine cells in the islets of Langerhans in the pancreas monitor blood glucose.

• High glucose: β cells secrete insulin, carried in the blood to target cells in the liver, muscle and adipose tissue. Cells take up glucose; liver and muscle convert it to glycogen; adipose cells make fat.
• Low glucose: α cells secrete glucagon, which stimulates the liver to break down glycogen and release glucose.

Hormones reach all cells in the blood but only cells with the matching receptors respond.

Type 1 diabetes: the immune system destroys the β cells, so little or no insulin is secreted. It usually starts in childhood; risk factors are mainly genetic. It cannot currently be prevented and is treated with insulin injections or pumps, plus glucose monitoring.

Type 2 diabetes: insulin is secreted but target cells become less responsive (insulin resistance). Risk factors include obesity, lack of exercise, diet high in sugar and fat, age and family history. It can often be prevented or controlled by weight loss, exercise and diet, with drugs if needed.

Peripheral thermoreceptors in the skin detect external temperature and send impulses to the hypothalamus, which also monitors blood temperature and compares it with the set point. The hypothalamus sends nerve impulses to effectors and releases TRH, causing the pituitary gland to secrete TSH, which stimulates the thyroid to secrete thyroxin. Thyroxin raises the basal metabolic rate and heat production. Effectors include skeletal muscle (shivering) and brown adipose tissue.

Birds and mammals regulate body temperature by physiological and behavioural means (e.g. seeking shade).

Too cold:

• Vasoconstriction of skin arterioles reduces heat loss.
• Shivering releases heat from muscle respiration.
• Uncoupled respiration in brown adipose tissue: protons leak back across the inner mitochondrial membrane, so energy is released as heat, not ATP.
• Hair erection traps an insulating layer of air (small effect in humans).

Too hot:

• Vasodilation of skin arterioles increases heat loss.
• Sweating: evaporation of sweat removes heat.