Our body is composed of billions of cells. Structurally and functionally similar cells get organized in the body into different tissues. Different tissues constitute the organs, and different organs constitute systems. The various organ systems of the body are digestive system, respiratory system, circulatory system, etc. in order to perform a particular function the component organs of each system are dependent on each other. They work in harmony and in a co-ordinate way for the individual as a whole. In the absence of such co-ordination and organism cannot do many things that it normally does?
For example, when we run, our muscles require greater energy which can be produced when there is a greater supply of oxygen, and this calls for an increase in the rate of breathing. When we stop running, our muscles do not need much energy. Consequently, there is no need for extra oxygen and the rate of breathing returns to the normal level. When we eat, our hand brings the food to the mouth, the teeth and jaw muscles help in chewing the food and the sallva helps in digestion. All these activities are co-ordinate and well-organized.
There are mainly two controlling mechanisms in our body: the neural control, brought about by nerves, and the chemical control achieved with the help of hormones. These two types of control help the body to keep it in a steady state. This conditions is termed homeostasis (homeo means same and stasis means status or condition). Thus control and co-ordination help in maintaining a state of stability between the internal conditions of an organism and the external environment.
For example, rise in temperature in the environment causes sweating in human beings and prompts the animals such as insects, toads, lizards, etc., to move towards the shade. The evaporation of the sweat cools our body. Since the body loses water by perspiration, we feel thirsty and drink more water on a hot day. If organisms do not react to counteract rise in temperature, dehydration will take place and their internal structures will be damaged. In this manner, suitable adaptive responses enable an organism to live through changes in the environment.