- In unicellular organisms, all functions like digestion, respiration and reproduction are performed by a single cell. In multicellular animals, a group of similar cells along with intercellular substances perform a specific function; such an organization is called tissue.
- Tissues are organized in specific proportion and pattern to form an organ. Examples- stomach, lung, heart and kidney.
- When two or more organs perform a common function by their physical and/or chemical interaction, they together form organ system. Examples- digestive system, respiratory system, etc.
- Cells, tissues, organs and organ systems exhibit division of labor for the survival of the whole body.
Fig. digestive system of human
- Animal tissues are broadly classified into four types