Homeostasis in mammals

Biology A2 · Homeostasis. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Homeostasis is keeping the internal conditions of the body, such as temperature, water potential of the blood and blood glucose concentration, within narrow limits. This keeps enzymes and cells working well.

Control is by negative feedback. A receptor detects a change away from the set point. The nervous or endocrine system carries the information to an effector (a muscle or gland), which brings the value back.

The kidney removes urea and controls the water content of the blood. Each nephron filters the blood and then takes back the useful substances. The pancreas controls blood glucose with two hormones, insulin and glucagon, that have opposite effects.

Rules to remember

Urea is made in the liver by deamination: the amino group of an excess amino acid is removed, forms ammonia, and this is converted to urea
Ultrafiltration: high blood pressure in the glomerulus (afferent arteriole wider than efferent) forces water and small solutes into the Bowman's capsule; the basement membrane holds back blood cells and large proteins
Selective reabsorption in the proximal convoluted tubule: all glucose and amino acids, most water and ions; glucose enters by cotransport with Na⁺; the cells have microvilli and many mitochondria
Low water potential of blood → osmoreceptors in the hypothalamus → ADH released from the posterior pituitary → more aquaporins in collecting duct membranes → more water reabsorbed → small volume of concentrated urine
Insulin (from β cells): more GLUT4 proteins in muscle cell membranes and glucose → glycogen in liver and muscle cells, so blood glucose falls
Glucagon (from α cells) on liver cells: receptor → G-protein → adenylyl cyclase → cyclic AMP (second messenger) → protein kinase A → enzyme cascade → glycogen broken down to glucose
Test strip: glucose oxidase turns glucose into gluconic acid and hydrogen peroxide; peroxidase uses the hydrogen peroxide to make the chromogen coloured. A biosensor gives an electric current and a number instead

Common mistake

ADH is made in the hypothalamus but released from the posterior pituitary gland. Also, glucagon does not enter the liver cell: it binds to a receptor on the cell surface and cyclic AMP carries the signal inside.

Worked example

A person eats a very salty meal and drinks nothing. How does the body respond, and what happens to the urine?

The salt lowers the water potential of the blood, and osmoreceptors in the hypothalamus detect this.
More ADH is released into the blood from the posterior pituitary gland.
ADH makes collecting duct cells add more aquaporins to their membranes, so more water moves out of the urine by osmosis and back into the blood.

Answer: A small volume of concentrated urine is produced, and the water potential of the blood rises back towards the set point.

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