Tissues, Organs and Organ Systems
Multicellular organisms are composed from a large number of cells. As particular cells develop they can differentiate in terms of their features and, in doing so, adapted to a particular function. Tissues are aggregations of similar cells. Organs, on the other hand, are aggregations of tissues which work together to form a specific physiological function. Organs, in turn, form systems with other organs thereby creating an organism.
Animal organs
Animals are big multicellular organisms. They’re composed of systems which are able to exchange different materials. As a multicellular organism develops its cells differentiate so that they’re able to perform specific functions.
Tissues
Tissues are groups, or aggregations, of cells which are similar in both structure and function. The table below explains three different types of tissue found in animals.
| Tissue type | Function |
| Muscular tissueGlandular tissue
Epithelial tissue |
This is able to contract and by doing so the muscle can moveThis can produce a range of substances including hormones and enzymesThis covers certain parts of the body |
Organs
Organs are composed of a range of tissues. An organ doesn’t have to be composed of the same type of tissue: it might be made up of several different types. The stomach is one example. This organ contains:
- muscular tissue which allows it to move and churn up food and other substances
- glandular tissue which produces digestive juices needed to break down food
- epithelial tissue which works as a barrier, covering both the outside and inside of the stomach
Organ systems
Organ systems are made up of groups of organs. Each organ then carries out a specific function but all of them work together towards one goal. An example of an organ system is the digestive system. This system can be found in humans and other mammals and involved the exchange of substances with the environment.
The digestive system is made up of:
- glands, like the salivary glands and the pancreas, which create digestive juices
- the stomach and small intestine where the process of digestion takes place
- the liver which produces bile, important for digesting fats
- the small intestine which absorbs soluble food
- the large intestine which absorbs any undigested food and then produce the waste product faeces
NOTE: You should be able to recognise the organs of the digestive system on a diagram.