Homeostasis

Homeostasis is the maintenance of a constant internal environment despite changes outside. Cells only work within narrow limits, so the body must hold steady its temperature, water and salt balance, blood glucose and pH. Most of this is done by negative feedback: a change is detected, and the body responds to reverse it, bringing the level back to normal.

The kidneys filter the blood, removing the toxic waste urea (made in the liver from excess amino acids) and adjusting the water and salt content. They filter small molecules out of the blood, then reabsorb what the body needs — glucose, useful salts and much of the water — leaving urine.

Balancing water is osmoregulation: when you are short of water, the hormone ADH makes the kidneys reabsorb more, giving less, more concentrated urine; when you have plenty, they reabsorb less, giving more, dilute urine.

Mammals hold a steady body temperature (about 37 °C in humans) — thermoregulation. When too hot, the body sweats (evaporation cools the skin) and blood vessels near the skin widen (vasodilation) to lose heat. When too cold, it shivers (muscle activity releases heat) and the skin vessels narrow (vasoconstriction) to conserve heat; hairs also stand up to trap an insulating layer of air.

Blood glucose is controlled by two hormones from the pancreas. When it rises (after a meal), insulin makes the liver and muscles take up glucose and store it as glycogen, lowering it. When it falls, glucagon makes the liver release glucose, raising it. In diabetes, this control fails — often because too little insulin is made — so blood glucose can rise dangerously high.

If the kidneys fail, wastes such as urea build up in the blood to dangerous levels. Two treatments exist. In dialysis, the patient's blood is passed through a machine where it flows next to a partially permeable membrane bathed in dialysis fluid; wastes and excess salts diffuse out into the fluid while useful substances (kept at the right concentration in the fluid) stay in. It works, but must be repeated several times a week. A kidney transplant is a longer-term cure, though donor organs are scarce and the body may reject the new kidney.

In diabetes the body cannot control blood glucose. In Type 1, the pancreas makes little or no insulin (often from childhood), and it is treated with insulin injections and a controlled diet. In Type 2, the body stops responding properly to insulin (linked to diet, weight and age), and it is managed mainly by diet, exercise and sometimes medication. Untreated, high blood glucose damages blood vessels, nerves, the eyes and kidneys over time.