D3.3 HomeostasisIB Biology HL: Revision notes

Homeostasis is the maintenance of the internal environment within preset limits despite external change. Human homeostatic variables include body temperature, blood pH, blood glucose and blood osmotic concentration.

Negative feedback reverses any deviation, above or below the set point. Positive feedback amplifies change, so it cannot keep a variable stable.

Pancreatic islet cells monitor glucose. High glucose: β cells secrete insulin; target cells in liver, muscle and adipose tissue take up glucose and make glycogen or fat. Low glucose: α cells secrete glucagon; the liver breaks down glycogen. Hormones travel in the blood; only cells with receptors respond.

Type 1: autoimmune destruction of β cells, no insulin; treated with insulin injections, not preventable. Type 2: insulin resistance of target cells; risk factors are obesity, inactivity, diet, age and genes; prevented or managed by diet, exercise and weight loss, then drugs.

Peripheral thermoreceptors send impulses to the hypothalamus, which also senses blood temperature. It controls effectors directly and via the pituitary (TSH) and thyroid (thyroxin, raising metabolic rate).

• Cold: vasoconstriction, shivering, uncoupled respiration in brown adipose tissue, hair erection.
• Hot: vasodilation, sweating.

Birds and mammals also use behaviour, e.g. seeking shade or huddling.

Excretion is removal of waste products of metabolism, such as urea. Osmoregulation is regulation of osmotic concentration, measured in osmol L⁻¹ (blood plasma about 0.29 osmol L⁻¹).

In the glomerulus, high blood pressure forces plasma through the capillary wall, basement membrane and podocyte slits into the Bowman's capsule: ultrafiltration. Small solutes (glucose, urea, ions) pass; proteins and cells do not. In the proximal convoluted tubule, all glucose and amino acids and most ions and water are reabsorbed, largely by active transport. Toxins and unwanted solutes stay in the filtrate and are excreted in urine.

The ascending limb of the loop of Henle actively transports sodium ions out of the filtrate into the medulla. It is impermeable to water, so the medulla develops a high osmotic concentration. There is a limit to this active transport, which sets the maximum concentration of urine.

Water leaves the collecting ducts by osmosis into the salty medulla. Osmoreceptors in the hypothalamus detect plasma osmotic concentration:

• Too high (dehydration): more ADH is secreted by the pituitary; vesicles containing aquaporins fuse with collecting duct membranes; more water is reabsorbed; urine is small in volume and concentrated.
• Too low: less ADH; aquaporins are taken back into intracellular vesicles; urine is dilute.

Arterioles dilate or constrict to redirect blood as activity changes.

• Vigorous activity: cardiac output rises; flow to skeletal muscles rises greatly; gut and kidney flow fall; skin flow rises for heat loss.
• Wakeful rest: gut and kidneys receive a large share.
• Sleep: muscle flow falls; gut and kidneys maintained.
• Brain: flow stays nearly constant in all states because neurons need a continuous supply of glucose and oxygen.