1. Receptors are groups of specialised cells. They can detect changes in the environment, which are called stimuli, and turn them into electrical impulses. Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus.

4. Homeostasis means keeping a constant internal environment. t is carried out around the whole body. Homeostasis reaches from every cell up to whole organs and systems. Why do we need it? If there was not a constant internal environment, our enzymes would not work properly. That would mean that nothing would operate correctly and we would die. What has to be controlled? All of our cells are bathed in a watery solution, which is formed by some of our blood plasma which is allowed to leak out of our blood. This carries away any waste materials back into the blood. The balance of things in this tissue fluid is critical for the cells and the whole body. There are basically 6 things that are essential for health and that must be controlled: 1. Carbon dioxide Extra carbon dioxide must be removed, otherwise the body becomes too acidic. It is lost mainly in the air we breathe out, but a small amount is lost in the urine. 2. Urea This is the waste chemical produced when we digest amino acids in the liver. It is poisonous and so must be removed. This is done mainly through the urine, although some is lost through our sweat. 3. Ions If the right balance of ions is not kept, our cells can become shrivelled, swollen or even burst! Important ions include sodium, potassium, hydrogen and phosphate. These are controlled through our urine and the amount of water we drink. We also lose some, like sodium ions, through our faeces and our sweat. 4. Sugar Having enough glucose for respiration plus adequate stores of glycogen is critical. If the blood glucose level falls too low we will die. 5. Water Seventy percent of our body mass is water. Without keeping the right amount of water we would die. The kidney is the key means of this control (see later). 6. Temperature The enzymes that control every chemical reaction in our body work best at their optimum temperature of 37 degrees Centigrade. If our body cells get too hot or too cold they would die. So would we! A cool way to remember these 6 things is by learning this... When (Water) Shall (Sugar) I (Ions) Clean (CO2) The (Temperature) Utensils (Urea) Which organs are involved? The following diagram shows the organs that have roles in homeostasis: Through the hypothalamus and pituitary glands the brain has a long-lasting and powerful effect on the body by involving hormones. The hypothalamus monitors water, temperature and carbon dioxide content of blood. The pituitary gland secretes a number of hormones, a key one is ADH which is important in regulating the water content of the body. The liver helps to control glucose content of the body by storing it as glycogen. It is also involved in temperature regulation, acting as the body's furnace by increasing the rate of respiration when we are cold. The lungs are involved by getting rid of carbon dioxide from the body. The pancreas is involved in maintaining a constant amount of glucose in the body through the actions of glucagon and insulin. The muscles of the body can help to maintain a stable body temperature as muscular activity and shivering help to generate heat. The kidney are involved in controlling the amount of water in the body. The skin is the largest organ and has a central role in maintaining a constant temperature