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The transport system in animals is called the circulatory system. The materials are transported from one part of the body to another by a mass flow system which is the circulatory system. The animals, particularly the more advanced forms, have a higher metabolic rate. This means that they require more nutrients and oxygen at a faster rate. They also produce more wastes that have to be removed from the cells in less time. The circulatory system has to ensure that the materials reach all the cells of the body and in time. This system should also collect the materials from the cells and transport them wherever required. Thus, the movement of materials into and out of the system should be possible. The need to meet all these requirements has resulted in a mass flow system through blood vessels and body spaces.
Materials to be transported include digested food, respiratory gases, hormones, excretory products, etc. The digested food includes sugars like glucose, amino acids, fatty acids and their derivatives. Blood which is the vascular tissue in man is over 90% water. Thus, only the water-soluble substances are transported through this fluid. For the transport of fatty substances, body uses another fluid called the lymph.
Thus, there are two circulatory systems:
- Blood circulatory system
- Lymphatic circulatory system
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