Revision:Digestive System

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The Digestive system - A-level Biology Revision notes

What is the digestive system?

The digestive system consists of a tube which leads from mouth to anus. The digestive system is made up of the oesophagus, the stomach, small intestine and large intestine. Each of these parts has a different function and a different structure. Food and fluids which are ingested are pushed through the digestive system by a process of peristalsis as demonstrated bellow.

File:Peristalsis.JPG

The muscles above the bolus in the digestive tract contract while those in front of it relax, this gradually causes the movement of the bolus through the digestive system.

What is the structure of the digestive tract?

The general structure of the digestive tract goes as follows:

• Mucosa – This is involved in the secretion of digestive juices and the absorption of digested food. There are often folds or villi which increase the surface area. Coating the mucosal layer is a thin epithelial layer

• Submucosa - This contains blood and lymph vessels as well as nerves. This is where the secretory glands are located.

• Muscle layer - – This is made up of circular muscle which contracts the lumen and longitudinal muscle which shortens the gut.

• Serosa- This is a tough layer of connective tissue which holds the structure together.

Parts of the Digestive System

Stage 1 - The Mouth

Food first enters the digestive system in the mouth. Here is it initially broken down both mechanically (through chewing) and chemically through the enzymes in saliva such as amylase which breaks down starch.

Stage 2 - The Oesophagus

This is a tube which connects the mouth to the stomach, food travels down this tube by peristalsis. There are a few mucus secreting glands but only small villi.

Stage 3 - The Stomach

The stomach is effectively a bag of muscle which can expand there are three layers of muscle which churns the contents of the stomach. Food is stored in the stomach for a few hours during which time it becomes a liquid known as chime. The mucosa of the stomach wall has no villi but does have structures which are known as gastric pits which are associated with gastric glands. The gastric glands secrete hydrochloric acid to kill bacteria. The enzymes pepsin and rennin are secreted to digest proteins. The chime is gradually released into the small intestine gradually through a large muscular sphincter known as the pyloric sphincter.

Stage 4 - The Small Intestine

The small intestine is made up of three parts the duodenum the jejunum and the ileum. In the duodenum digestion continues and two separate secretions assist this. The pancreases secretes pancreatic juices which contain carbohydrases, proteases and lipases. Bile is secreted from the liver and contains bile salts which aid lipid digestion and sodium hydrogen carbonate which neutralises the stomach acid. The jejunum and the ileum are very similar they are where final digestion occurs, glands in the mucosa and sub mucosa secrete enzymes, mucus and sodium hydrogen carbonate.

Stage 5 - The Large Intestine

The large intestine is made up of the caecum, appendix, colon and rectum. There are numerous villi and mucus secreting glands. Water is reabsorbed from the large intestine and semi solid faeces are formed.

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Created from class notes by randdom.