Tissues, organs and systems

Multicellular organisms are built up in levels of organisation:

• Cells — the basic building blocks, some specialised for a particular job.
• Tissues — a group of similar cells that work together to carry out a shared function.
• Organs — a group of different tissues that work together to perform a specific function.
• Organ systems — a group of organs that work together to perform an overall function.

Organ systems then work together to form a whole organism.

The digestive system is an organ system built from organs including the stomach, small intestine, large intestine, liver, pancreas and salivary glands. Each of these organs is built from several tissues:

• Muscular tissue contracts to squeeze food through the gut.
• Glandular tissue produces substances, such as digestive enzymes.
• Epithelial tissue covers the inside and outside surfaces of the organ.

The stomach, for example, uses muscular tissue to churn food and glandular tissue to produce digestive juices, all covered by epithelial tissue.

In the digestive system, each organ has its own part to play:

1. The salivary glands make saliva, which contains amylase to start digesting starch in the mouth.
2. The stomach churns food and makes hydrochloric acid and protease.
3. The pancreas makes digestive enzymes and releases them into the small intestine.
4. The liver makes bile, which is stored in the gall bladder and helps the body digest fats.
5. The small intestine finishes digestion and absorbs the soluble products into the blood.
6. The large intestine absorbs water from the undigested food that is left.

No single organ can digest a meal alone: they work together as one organ system.

Exam questions often ask you to place something at the right level:

To decide, ask what it is made of. One kind of cell working together makes a tissue. Several different tissues make an organ.