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Digestion


Digestive System

the digestive tract or alimentary canal

The diagram to the left is of the alimentary canal also known as the digestive tract and also shows other organs of the digestive system like the liver.

After being swallowed, the food travels down the Oesophagus or esophagus, this is continually being damaged by the friction of food, so the epithelium is a few cells thick and secretes mucas to lubricate the food's passage. The muscularis externa is also much larger to give more force to peristalsis.

The next place it enters is the stomach this is a temporary store, mixes the contents up and also is the site for a bit of digestion. The mucosa are folded to allow for expansion as more food is added. The muscularis externa is larger still here to allow the food to be churned up.

Into the small intestine opens the pancreatic and bile ducts: opening into the duodenum. This then leads into the ileum. The bile salts neutralise the stomach acid as well as emulsify fats. Also here, the mucosa has thin projections of villi which greatly increase the surface area for maximum absorption.




Food is moved thorugh the digestive system by a process called peristalsis. This is where the muscles along the gullet squeeze before the food to push it along, just like when you squeeze the toothpaste tube from the bottom to get it out.

peristalsis

Enzymes

For more information about enzymes see Enzymes

Enzymes are what digest, or break up the food. They are a biological catalyst: speeding up the break-down of the different components of food on a chemical level. Here are a few and what they do:

digestive enzymes

Enzymes are sensitive to temperature and pH, these must be at an optimum level so they work best. For example, protease works best in an acidic environment because it is found in the stomach, but then the alkaline bile neutralises and emulsifies this so lipase works best in neutral conditions.

Human Gut Wall

Below is a digram of the human gut wall, on the right are labelled the different layers that exist.

The lumen is not really a layer of the gut but the space which the food goes through. You have to imagine the structure below curled into a circle; as this is what it looks like in the body.

The first actual layer is the mucosa, it has a layer of epithelium, made of epithelial cells, which have projections called villi.

Below this is a muscle layer, known scientifically as the muscularis externa, it is reponsible for peristalsis which moves food through the digestive tract.

the human gut wall

In the diagram you should also notice the capillaries, part of the blood network which takes absorbed food away. And also the lacteals which are a special type of lymph gland which take fats away in the lymphatic system.

The Villi

By the time everything reaches here, the food has been digested into small enough particles that it can pass through the alimentary tract lining and be absorbed into the blood.

The food products pass into the blood stream through villi: these are small foldings of the small intestine that cover on its internal surface.

diagram of a villus

It is important to know there are a number of factors improving efficiency.

The villi on their own increase the surface area, but the cells which make up the surface of the villus have their own small projections called microvilli (see diagram) these further increase the surface area which means that the digestion products can be absorbed more quickly.

The villus has a supply of blood vessels this means substances absorbed can be transported to where they are needed more directly.

The surface of the villus is only one cell thick, so products pass more easily through it.

Digestion of Triglycerides

To learn more about enzymes see the page enzymes.

Triglycerides are a type of lipid; here you will learn how the body breaks down this molecule.

Digestion begins in the duodenum where bile enters from the liver, bile salts make the big blobs of fat into small micelle droplets which massively increases the surface area and makes digestion much easier.

Also in the duodenum, pancreatic lipase this breaks the triglyceride into fatty acid and glycerol. When these substances get into the ileum they diffuse into the epithelial cells ... and then are put back together again!

These resynthesised lipids make proteins called chylomicrons, these enter the lacteals and travel through the lymphatic system, making it milky. They travel in this system until entering the blood stream at the vena cava where they are stored as fat tissue.

Updated: 1 January 2012