Homeostasis: how animals keep their internal environment stable

How the assessment works

• This is an internal standard: you research one control system in one animal, often the human, and write a report or annotated diagram explaining it.
• Choose from body temperature, blood glucose, water balance, blood pressure or respiratory gases, and include the purpose, the parts, how they interact and what happens when the system is disrupted.
• A single specific example, such as a swimmer in cold water or a person with type 1 diabetes, gives your explanation something concrete to work through.

Homeostasis and negative feedback

• Cells are bathed in tissue fluid, the internal environment, and their enzymes and membranes work properly only within narrow limits of temperature, pH, glucose and water.
• Homeostasis keeps each variable within a normal range around a set point, such as a core temperature of about 37 °C.
• In negative feedback a change away from the set point triggers responses that reverse the change, so the variable swings slightly above and below the set point rather than staying perfectly still.
• Positive feedback does the opposite and amplifies a change, as when oxytocin strengthens contractions during birth; it ends with an event, not a stable state, and in a homeostatic system it usually means breakdown.

The parts of a control system

• The stimulus is a change in the variable; receptors detect it, such as thermoreceptors in the skin and in the hypothalamus itself.
• The control centre compares the input with the set point and sends signals out by nerves (fast, short-lived) or hormones (slower, longer-lasting); for temperature, water and blood glucose it is the hypothalamus or the pancreas.
• Effectors, muscles and glands, carry out the response, and the changed variable is detected again by the receptors, closing the loop.
• Many systems have separate effectors for each direction, such as insulin and glucagon, which gives finer control than switching one effector on and off.

Body explorer

Endotherms and ectotherms

• Endotherms, birds and mammals, generate most of their body heat from metabolism and keep a high, steady core temperature, so they can stay active in cold air or water.
• The cost is food: an endotherm needs several times the energy of an ectotherm of the same mass, much of it simply to make heat.
• Ectotherms, such as tuatara, skinks and fish, take heat from their surroundings and regulate mostly by behaviour: basking on sunny rocks, pressing against warm surfaces, retreating to burrows.
• Tuatara are active at body temperatures as low as about 5 °C, lower than most reptiles, which suits cool southern islands but means they grow and reproduce very slowly.

Thermoregulation in a mammal

• Too hot: the hypothalamus triggers sweating, so evaporation removes heat, and widens (vasodilates) the arterioles supplying the skin, so more warm blood flows near the surface and loses heat by radiation.
• Too cold: skin arterioles narrow (vasoconstrict), keeping warm blood in the core, skeletal muscles shiver to release heat from respiration, and hair erector muscles raise the hairs to trap air.
• Brown fat releases heat directly because its mitochondria let protons leak back across the inner membrane without making ATP, and thyroxine raises metabolic rate over longer cold spells.
• Kororā (little penguins) cut heat loss in cold water with countercurrent exchange: arteries to the flippers and feet run beside veins coming back, so heat passes into the returning blood instead of the sea.

Try it: negative feedback

Blood glucose: insulin and glucagon

• Blood glucose is normally kept at about 4 to 8 mmol/L: too low and the brain, which relies on glucose, is starved; too high and water is drawn out of cells by osmosis and proteins are damaged.
• After a meal beta cells in the islets of the pancreas detect the rise and release insulin; it makes muscle and fat cells move GLUT4 transporter proteins into their membranes so glucose enters, and makes liver cells store glucose as glycogen.
• Between meals alpha cells detect the fall and release glucagon, which makes the liver break glycogen down to glucose and make new glucose from amino acids, releasing it into the blood.
• The pancreas is both receptor and control centre here, and the two hormones act as an antagonistic pair.

Try it: up or down?

Water balance: ADH and the kidney

• Osmoreceptors in the hypothalamus detect when the blood becomes too concentrated, after sweating hard or eating salty food.
• The posterior pituitary releases antidiuretic hormone (ADH), which travels in the blood to the collecting ducts of the kidney.
• ADH makes collecting duct cells insert aquaporin water channels into their membranes, so more water moves back into the blood by osmosis and a small volume of concentrated urine is made; thirst rises too.
• When the blood is too dilute, ADH release falls, aquaporins are removed, and a large volume of dilute urine is made.

When the system breaks down

• In type 1 diabetes the immune system destroys the beta cells, so no insulin is made; glucose stays high after meals, cells switch to burning fat, and acidic ketones build up, which can be fatal without insulin injections.
• In hypothermia, such as a tramper soaked and caught in wind on the Tongariro Alpine Crossing, heat loss outruns heat production and the core drops below 35 °C; judgement fails, and below about 32 °C shivering stops and enzymes slow.
• In heat stroke, during long exertion in hot, humid weather, sweat cannot evaporate fast enough and dehydration cuts sweating, so the core rises above about 40 °C.
• In both temperature cases the extremes drive positive feedback: a cold body makes less heat and a hot one makes more, so the change speeds up instead of correcting.