B3.3 Keeping internal conditions constant
3.1 Controlling internal condition * Keeping the conditions in the in the body within a narrow range is called homeostasis. * Temperature, blood glucose, water and ion content and levels of waste products must all be controlled * Waste products that have to be removed include; * CO2 (produced by respiration) removed by the lungs * urea (produced in the liver when amino acids are broken down) removed by the kidneys and temporarily stored in the bladder * Water and ions enter the body when we eat and drink. If the water or ion content in the body is wrong, too much water may move into or out of the cells. This could damage or destroy the cells
3.2 The human kidney * The kidneys filter the blood, excreting substances you do not want and keeping the substances that the body needs * A healthy kidney produces urine by: * first filtering the blood, * reabsorbing all the sugar, * reabsorbing the dissolved ions that the body needs, * reabsorbing as much water as the body needs, * releasing urea excess ions and water in the urine * The urine is temporarily stored in the bladder before being removed from the body
3.3 Dialysis – an artificial kidney * A dialysis machine carried out the same job as the kidneys. The blood flows between partially permeable membranes * The dialysis fluid contains the same concentration of useful substances as the patients blood, e.g. glucose ad mineral ions. This means that the useful substance don’t diffuse out of the blood so they do ot need to be

reabsorbed. Urea diffuses out from the blood into the dialysis fluid * Dialysis needs to be carried out at regular intervals * If the patient were to have a successful kidney transplant, the person will not need further dialysis
3.4 Kidney transplants * Kidneys may be obtained from a victim of a fatal accident or sometimes from living donor * The new kidney must be a very good ‘tissue match’ to prevent rejection * There are proteins called antigens on the surface of cells. The recipient’s antibodies may attack the antigens of the donor organ because they recognise them as being foreign * Following the transplant the recipient must take immunosuppressant drugs to prevent rejection. However this leaves the patient vulnerable to common infections * There are also the risks from the operation to take into account
3.5 Controlling body temperature * Human body temperature must be kept at about 37˚C so that the enzymes will work efficiently. The core body temperature must be kept stable * Body temperature is monitored by the thermoregulatory centre in the brain. This centre has receptors which detect the temperature of the blood going through the brain * Temperature receptors in the skin also send impulses to the brain giving information about the skin temperature * The skin looks red when we re hot due to increased blood flow * When it is hot, more water is lost from the skin (as sweat) so more water must be taken in with drinks and food to balance the loss

* If the core temperature rises: * Blood vessels near the surface of the skin dilate allowing more blood to flow through the skin capillaries. Energy is transferred by radiation and the skin cools * Sweat glands produce more sweat. The energy required for the water to evaporate comes from the skin’s surface, so we cool down * If the core temperature falls: * Blood vessels near the surface of the skin constrict and less blood flows through the skin capillaries. Less energy is radiated * We ‘shiver’. Muscles contract quickly. This requires respiration and some of the energy released warms the blood

3.6 Treatment and temperature issues * When patients have kidney failure, the doctors need to consider: * the general health of the patient * how long the patient has