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One of the important differences between living and non-living things is that the living things are capable of actively responding to changes in their environment. These environmental changes act as stimuli which bring about a definite response from the organism.

Proper coordination of the different parts of the body is essential for proper responsiveness. If all the cells in the body respond in the same way to a particular stimulus, the organism could have elicited only a few responses. Running, for example, needs certain muscle fibres to contract and certain other muscle fibres to relax at the same time. The liver has to provide additional materials to be oxidized in order to generate extra energy. The cells need extra amounts of oxygen for their metabolism which generates additional amounts of carbon dioxide to be breathed out. Thus, for a concerted activity or response to be elicited there is a need for coordination of the functions of the various organ systems, organs, tissues and cells.

Apart from the need for coordination of the functions of various systems in the body to fulfill the needs of day-to-day life, there is need for coordination of processes like development, growth and differentiation in an organism. In short, all activities of an organism involve coordinated functioning of various organs and organ systems which enable it to respond to various external and internal stimuli. The functions of individual cells, tissues and organ systems in one part of the body are related to the activities of the other parts of the body and this requires a form of communication among them.

Animals in general, have two different, but related systems of internal communication: (1) a somewhat slower system called the endocrine system and (2) a very fast acting nervous system. The nervous system consisting of specialized cells called neurons, conducts electrochemical impulses from one part of the body to another. These impulses are generally of brief duration and the nervous responses are usually brief. The endocrine system consists of specialised glands called endocrine glands which secrete the chemical substances called hormones. The hormones released into the blood stream or other body fluids reach every cell through the circulatory system and elicit a response in distantly located organs and tissues of the organism. Usually the response to a hormonal stimulus is a change in the metabolism of cells and these changes may persist for long periods of time.

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