Groups of cells with a similar structure together with the non-living material (inter-cellular substance) between them form many a type of tissues.
Tissues performing a common function or functions are grouped into organs, e.g. heart is one of the vital organs of the body. Team-work is an essential ingredient for survival in the living body. A 'system' is a group of organs working together to perform a series of functions.
The skeletal system, the muscular system, the skin etc. are some of these. The other important systems of the body which merit more detailed treatment are:
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Nervous system
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Circulatory system
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Respiratory system
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Digestive and excretory system
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Endocrine system.