Digestion consists of:

- Ingestion (taking large pieces of food into the body)

- Digestion (breaking down of food)

- Absorption (movement of soluble products into cells)

- Assimilation (using absorbed materials)

- Egestion (getting rid of undigested material from a cavity)

There are several parts to the digestive system:

1. Mouth

Teeth and tongue break up food > form a bolus

Saliva dissolves soluble substances

Mucus lubricates

Lysozymes kill bacteria

Amylase digests starch

Bolus is swallowed down pharynx

Food does not enter lungs as epiglottis closes off trachea

2. Oesophagus

Tube that connects mouth to gut

Can be closed (it is soft)

Bolus is forced by ‘peristalsis’ by circular contraction of thick muscles

3. Stomach

Food is stored here for a few hours

Muscle churns food, which is released into the small intestine by the sphincter

Gastric juice (pH1) secretes mucus, pepsin and rennin

4. Small Intestine

contains:

- Duodenum (30cm): most digestion occurs here by pancreatic juice which contains many enzymes.

Bile is released into here to aid digestion (bile salts) and neutralise stomach acid (sodium hydrogen carbonate) for efficient pancreatic enzymes

- Jejunum (2m) and ileum (4m): internal surface area is huge and contains folds that comprise villi and microvilli.

Villi contain many cells, whilst microvilli are small sub-cellular structures.

Food is forced by peristalsis

5. Large Intestine

contains:

- Food can remain here for 36 hours

- Many glands secrete mucus

- Produces faeces (cellulose, cholesterol, bile, mucus, mucosa cells, bacteria and water)

- Sphincter is an involuntary muscle that we learn to control