Digestion. The digestive system.




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The digestive system

There are a number of digestive problems which can be relieved with medicines. But first let’s look at how the normal digestive system works.

Picture 13. The digestive system. Roll over the red and blue blobs to find out more.

Picture 14. peristalsis in action.
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Picture 16. Structure of a villus, showing the lacteal and the capillaries.


Moving through the gut
	Waves of muscular contractions called peristalsis moves food along the digestive system. Circular muscles in the wall relax in front of the bolus of food and contract behind it. This pushes the food forward.

Absorption in the small intestine

Once the food has been digested it needs to be absorbed into the blood so that it can be transported around the body. The process of absorption happens in specialised structures called villi. These are tiny finger-like projections that push out into the digested food passing along the small intestine. They give a large surface area for the absorption to take place and make it more efficient. Each villus contains a good network of capillaries to pick up the nutrients and the cells are specialised to quickly absorb molecules by diffusion, osmosis and active transport.

Picture 15. Villi increase the surface area for absorbtion.
Courtesy of Dr Wagner, University of Delaware.

Amino acids and glucose enter the bloodstream directly. Fatty acids and glycerol are first absorbed into a part of the lymph system called the lacteal. This transports the fatty acids and glycerol up to the neck and then into the bloodstream at a vein that runs under the collar bone.

Your food needs to be broken up. Teeth are neatly designed for biting food and chewing it into smaller pieces. Your muscular tongue moves the food round, mixing it with saliva. The saliva softens the food so it can be swallowed. Enzymes in the saliva begin the process of digestion.

When you swallow food, the muscles in your gullet (called the oesophagus) contract, to push the food down into your stomach.

Waves of muscular contractions called peristalsis moves food along the digestive system. Circular muscles in the wall relax in front of the bolus of food and contract behind it. This pushes the food forward.

Most of the chemical breakdown of food happens in the stomach and small intestine. Enzymes are secreted and mixed with the food. The enzymes break down large molecules into smaller ones so the food becomes soluble. Acid in the stomach helps with this breakdown.

Food passes out of the stomach into the small intestine. The small intestine (or gut) is a tube about 10 metres long. Food is pushed along it by the gut muscles. Circular muscles behind the food contract, pushing the food onward. This is called peristalsis.

Because some of the food is now soluble, it can pass through the wall of the small intestine and into the bloodstream. This is called absorption. This soluble food can be used by body cells, for energy and growth. Undigested food moves on inside the small intestine.

A normal diet contains a lot of water - 5 to 7 litres per day. This is mainly absorbed through the large intestine into the bloodstream.

The remaining undigested food has to be got rid of. Solid waste is stored in the rectum and passed out (defaecated) through the anus. We have to learn to control this defaecation.

Digestion means breaking down insoluble food into small soluble molecules which can be absorbed into the bloodstream. Once it has been absorbed, food can be used to do the things needed to keep the body going. For example, it can produce and store energy and build new proteins.

The digram shows the main parts of the digestion system. Roll your cursor over the red spot to see what they are. Roll your cursor over the blue spots to find out what happens to food at each stage.