D3.3 Homeostasis

Your internal environment is astonishingly steady. While the world outside swings between freezing nights and baking afternoons, the temperature of your core barely shifts from 37 °C, and the glucose concentration of your blood is held within a narrow band whether you have just eaten a meal or fasted overnight. This active maintenance of a stable internal environment is called homeostasis, and it is what frees enzymes and cells to work reliably regardless of external conditions. The central idea for D3.3 is that almost all homeostatic control depends on negative feedback: a change in a variable triggers a response that reverses that very change.

Negative feedback: the universal control loop

Homeostasis works by keeping each controlled variable close to a set point. A general control loop has three parts: a receptor (or sensor) that detects the value of the variable, a control centre that compares it with the set point, and an effector that produces a corrective response. The information flows in a loop, so the outcome of the response feeds back to the receptor.

The defining feature is negative feedback: when a variable rises above the set point, the response lowers it; when it falls below, the response raises it. Because the correction always opposes the original deviation, the variable oscillates gently around the set point rather than drifting away. This is fundamentally different from positive feedback, where a change amplifies itself — useful in special cases such as childbirth or the action potential, but destabilising if used for routine regulation.

Common exam trap: homeostasis does not hold a variable perfectly constant. It allows small fluctuations and continually corrects them, so describing the value as fixed loses marks — say it is kept within narrow limits.

Thermoregulation and blood glucose

Body temperature is monitored by thermoreceptors, with the hypothalamus acting as the control centre. If the core overheats, the hypothalamus triggers vasodilation of skin arterioles (more blood flows near the surface, losing heat) and sweating (evaporation removes heat). If the core cools, it triggers vasoconstriction, shivering (muscle contraction releases heat) and contraction of hair erector muscles. These responses are opposite-acting effectors driven by the same control centre — a textbook negative-feedback example.

Blood glucose concentration is regulated by two hormones from the pancreas. After a meal, rising glucose is detected by β (beta) cells of the islets of Langerhans, which secrete insulin. Insulin makes liver and muscle cells take up glucose and convert it to glycogen (glycogenesis), lowering blood glucose. When glucose falls, α (alpha) cells secrete glucagon, which stimulates the liver to break glycogen back down to glucose (glycogenolysis), raising blood glucose. Insulin and glucagon are antagonistic: each opposes the other to hold glucose near its set point.

Diabetes mellitus

The syllabus expects you to distinguish the two main forms of diabetes mellitus, a condition in which blood glucose control fails.

• Type 1 diabetes usually begins in childhood. The body’s own immune system destroys the insulin-secreting β cells, so little or no insulin is produced. It is treated by regular monitoring of blood glucose and injection of insulin, especially around meals.
• Type 2 diabetes usually develops later in life and is strongly associated with diet, obesity and physical inactivity. Insulin is still produced, but target cells become less responsive to it (insulin resistance). It is often managed first through diet and exercise, and later with medication.

In both forms, blood glucose can rise so high that the kidney cannot reabsorb all of it, so glucose appears in the urine — a classic diagnostic sign. Untreated, the resulting swings in glucose damage blood vessels and nerves over time.

Osmoregulation and the kidney

The kidney maintains the water and solute balance of the blood, a process called osmoregulation. The functional unit is the nephron. Blood is filtered under pressure in the glomerulus, useful substances are reabsorbed along the tubule, and the remaining fluid forms urine.

Water balance is fine-tuned by antidiuretic hormone (ADH). Osmoreceptors in the hypothalamus detect the water potential of the blood. When the blood is too concentrated (for example after sweating), more ADH is released from the pituitary gland. ADH makes the walls of the collecting duct more permeable to water, so more water is reabsorbed back into the blood and a small volume of concentrated urine is produced. When the blood is too dilute, ADH release falls, less water is reabsorbed, and a large volume of dilute urine is produced. This is once again negative feedback — the response always opposes the change in blood concentration.