tolerance limits, this is the restricted range of conditions where cellular operations effectively work at a consistent rate and maintain life. These conditions include temperature, blood glucose levels, pupil diameter control and many more. The body is made up of many cells. Many specialized cells group up to form a tissue. Tissues group up to form organs, these organs can then connect to form the digestive system. The body is therefore made up of many specialized systems. In general, the combined activities of the specialized systems provide the optimum environment for individual cells. This means the preservation of a constant composition of the fluid that bathes the body cell, tissue fluid. This situation is for health and is called homeostasis. Homeostasis adjusts to the changing external environment, e.g. a cold climate; it is a self-adjusting system, which involves biological feedback. Hormones and the nervous system achieve this. This feedback allows for the maintenance of cells, tissues and the body to be in a state of equilibrium, which results in the sustenance of life. The body has particular features such as the skin and circulatory system, which helps its survival against the environment. The metabolism is able to adjust to internal and external changes that occur. Homeostasis is so important that most disease is regarded as a result of its disturbance, a condition called homeostatic imbalance. As we get older, the body's internal environment becomes progressively less stable. As a result we are more vulnerable to illnesses and they produce the changes we associate with ageing. Homeostatic imbalance takes place when the usual negative feedback mechanisms are overwhelmed and the destructive positive feedback mechanisms take over. An example of this phenomenon is heart failure. Illnesses such as hypertension, diabetes are all a result of failed…