Osmoregulation
Osmoregulation is the regulation of water concentrations in the bloodstream, effectively controlling the amount of water available for cells to absorb. The homeostatic control of water is as follows: * A change in water concentration leads to active via negative feedback control * Osmoreceptors that are capable of detecting water concentration are situated on the hypothalamus next to the circulatory system * The hypothalamus sends chemical messages to the pituitary gland next to it. * The pituitary gland secretes anti-diuretic hormone (ADH), which targets the kidney responsible for maintaining water levels. * When the hormone reaches its target tissue, it alters the tubules of the kidney to become more / less permeable to water * If more water is required in the blood stream, high concentrations of ADH make the tubules more permeable. * If less water is required in the blood stream, low concentrations of ADH make the tubules less permeable.

Temperature Homeostasis (thermoregulation)

One of the most important examples of homeostasis is the regulation of body temperature. Not all animals can do this physiologically. Animals that maintain a fairly constant body temperature (birds and mammals) are called endotherms, while those that have a variable body temperature (all others) are called ectotherms. Endotherms normally maintain their body temperatures at around 35 - 40°C, so are sometimes called warm-blooded animals, but in fact ectothermic animals can also have very warm blood during the day by basking in the sun, or by extended muscle activity (e.g. bumble bees, tuna). The difference between the two groups is thus that endothermic animals use internal corrective mechanisms, whilst ectotherms use behavioural mechanisms (e.g. lying in the sun when cold, moving into shade when hot). Such mechanisms can be very effective, particularly when coupled with internal mechanisms to ensure that the temperature of the blood going to vital organs (brain, heart) is kept constant. We use both.

In humans, body temperature is controlled by the thermoregulatory centre in the hypothalamus. It receives input from two sets of thermoreceptors: receptors in the hypothalamus itself monitor the temperature of the blood as it passes through the brain (the core temperature), and receptors in the skin (especially on the trunk) monitor the external temperature. Both sets of information are needed so that the body can make appropriate adjustments. The thermoregulatory centre sends impulses to several different effectors to adjust body temperature:

Blood Glucose Homeostasis
Glucoregulation

Glucose is the transport carbohydrate in animals, and its concentration in the blood affects every cell in the body. Its concentration is therefore strictly controlled within the range 0.8 – 1g per dm3 of blood, and very low levels (hypoglycaemia) or very high levels (hyperglycaemia) are both serious and can lead to death.
Blood glucose concentration is controlled by the pancreas. The pancreas has glucose receptor cells which monitor the concentration of glucose in the blood, and it also has endocrine cells (called the Islets of Langerhans), which secrete hormones. The α-cells secrete the hormone glucagon, while the β-cells secrete the hormone insulin. These two hormones are antagonistic, and have opposite effects on blood glucose: * Insulin stimulates the uptake of glucose by cells for respiration, and in the liver it stimulates the conversion of glucose to glycogen (glycogenesis). It therefore decreases blood glucose. * Glucagon stimulates the breakdown of glycogen to glucose in the liver (glycogenolysis), and in extreme cases it can also stimulate the synthesis of glucose from pyruvate. It therefore increases blood glucose.
After a meal, glucose is absorbed from the gut into the hepatic portal vein, increasing the blood glucose concentration. This is detected by the pancreas, which secretes insulin from its β-cells in response. Insulin causes glucose to be taken up by the liver and converted to glycogen. This reduces blood glucose, which causes the pancreas to stop secreting insulin.
If the glucose level falls too far, the pancreas detects this and releases glucagon from its α-cells. Glucagon causes the liver to break down some of its glycogen store to glucose, which diffuses into the blood. This increases blood glucose, which causes the pancreas to stop producing glucagon. Because blood glucose levels are allowed to deviate from the set point by about 20% before any corrective mechanism is activated, glucagon and insulin can never both be produced at the same time.