Food taken in during nutrition is absorbed and assimilated to different cells of the body. But to release the energy present in food, oxygen is needed. This oxygen is taken in during respiration and carried to all cells of the body so as to oxidise the food and release energy. The transportation system carries digested food to different body tissues, respiratory gases between lungs and tissues and waste materials for excretion. The metabolic waste materials formed during the various activities of the body have to be removed through the excretory system. All these life processes have to be well coordinated with each other, for which we need the nervous and hormonal system.
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