Digestion - How it all works

By Amanda Whitford

The different organ systems in our body work together to carry out the various essential functions that keep us alive and healthy. One of the most important functions is the ability to process the food we eat into available nutrients and to absorb these for use in other parts of our body.

Digestion begins in the mouth. When we chew food we are breaking it into smaller pieces, making it easier to swallow and increasing its surface area so the various digestive enzymes have easier access. Saliva contains small amounts of enzymes which start to break down carbohydrate. With the help of the saliva and the tongue, the food is formed into an easy to swallow ball which then makes its way down the oesophagus and into the stomach when we swallow.

Here there is a very different environment. The stomach has a pH of 2 so the environment is very acidic compared to the mouth. This acidity serves different purposes: it helps to protect us from illness, as many harmful things like certain bacteria cannot survive these acidic conditions and it also activates enzymes that help breakdown food components, like proteins, into smaller pieces. Gastric juice from the stomach mixes with the food and the strong muscles in the stomach move the food around helping to break it down ready for the intestines. Some protein and fat is broken down in the stomach, but not much carbohydrate digestion occurs here.

Once the food in the stomach has been sufficiently pummelled, it is released in small amounts at a time into the small intestine. Here it mixes with pancreatic juice which, as the name suggests, comes from the pancreas. This contains bicarbonate to neutralise the acidity from the stomach, and digestive enzymes to breakdown carbohydrate, protein and fat. Bile from the liver and bile duct acts as an emulsifier for fat droplets (it dissolves them into smaller droplets) allowing a bigger surface area for digestion. The breakdown of these dietary components is important as our bodies can’t absorb them in the form that we swallow them in.

The small intestine is our primary site of digestion and absorption and about 90% of these processes occur here. The small intestine is long with many folds, meaning there is a large surface area for this absorption to occur. Smooth muscle found in the lining moves the food through the intestine with the cells of the small intestine absorbing nutrients on its way. Anything unable to be digested and absorbed enters the large intestine which has important roles of its own including the absorption of water and some vitamins, and producing faeces that can be excreted.

You may be wondering why there are vitamins left at this point. This is due to the presence of millions and millions of gut bacteria and their ability to ferment products in the large intestine that we are unable to digest and/or absorb. One of the particularly important benefits that this gives us is the production of several B vitamins and Vitamin K which the large intestine can then absorb.

The remaining product of these digestive and absorptive processes can then be stored in the rectum for elimination from the body as faeces.

This simple overview barely touches on all the processes that are involved to extract the nutrients that we need from food. Multiple organs work together communicating and functioning in a highly coordinated manner. The food we eat can have a big effect on all of these processes. Many of the nutrients extracted from our food are essential for the proper functioning of our digestive system. If diet is inadequate, then these processes can be disrupted and this may cause problems for our health.

We truly are what we eat and what we absorb.


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