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The whole point of the digestive system is to 'digest', or break down, your food from large molecules to small molecules that your cells can use for things such as respiration.
Each of the three main food types are broken down from large molecules into smaller ones. To do this the digestive system makes use of various enzymes.
A good way to understand the whole process is to think of the food molecules starting off as a train made up of connected carriages.
The enzymes secreted by the digestive system then uncouple the carriages so that they can be taken elsewhere.
The following table shows what the main food types are digested into:
|Starting food:||Type of enzyme used:||Products made:|
|Carbohydrates (starch)||Carbohydrase||Simple sugars, like glucose|
|Fats||Lipase||Fatty acids and Glycerol|
Right! Now we can start to think about food!
The start of the process of digestion occurs in the mouth using the teeth and tongue.
There are four different types of human teeth:
- Incisors for cutting.
- Canines for piercing.
- Pre-molars for cutting and crushing.
- Molars for crushing and grinding.
All teeth have the similar features.
After the food has been mechanically broken up it is also mixed with saliva which moistens it and adds the enzyme salivary amylase which begins to digest starch.
The tongue helps to form the food into a small, moist ball called a bolus, which can be easily swallowed.
The bolus is squeezed down the oesophagus (gullet) by wavelike contractions of the surrounding muscle. This is called peristalsis.
Peristalsis occurs throughout the length of the digestive system.
In the stomach, the food is mixed up with hydrochloric acid, which acidifies the food, helping to soften it further and kill any nasty bugs. The acid conditions also allow a protease enzyme called pepsin to start to act on any proteins in thefood.
The stomach continually churns up the food so that it is in a nice sloppy mess!
A ring of muscle called a sphincter is relaxed to allow the food out of the stomach. Next it passes into the small intestine, or if you want the fancy names, the duodenum and the ileum (both parts of the small intestine). In the duodenum the food is mixed with bile, which is made in the liver but stored in the gall bladder. This emulsifies any fats in the food, breaking them up into small globules and allows lipase enzymes to attack them.
The pancreas secretes alkali, protease, carbohydrase and lipase enzymes into the duodenum. These further digest all the food types into their smaller molecules.
Within the ileum, the main part of the small intestine, more enzymes act on any remaining large molecules.
Then we are ready for the next bit...
By now, all that is left of your delicious food is a very watery mess. But here is where you get all the goodness out of it. It is all quite absorbing!
Lining the inner walls of the ileum are millions of tiny finger-like projections called villi. Each villus has the same basic structure.
It has a central lacteal into which fat droplets pass and are carried away to the liver. Also, a capillary network can remove sugars and amino acids into the blood stream. The villus is surrounded by a very thin layer of epithelial cells which allow for easy absorption.
The process of absorption is basically to get small molecules out of the digestive system and into the blood stream. From here, all the useful small molecules can be taken around to be used in each cell of the body.
To keep things moving smoothly, there are goblet cells which secrete mucus. Also, the villi wave about amongst all the lovely mush of food. The rhythmic movements of peristalsis keep the food moving along through the intestine.
By the time we get to the first bit of the large intestine, the colon, the only thing left to be absorbed is most of the water. Only waste material including the roughage is left behind within the intestine. Scientifically, we refer to all of this unpleasant material as faeces, although you may know a few other names!
The faeces are stored in the rectum, the last part of the large intestine, until that call of nature called defaecation occurs. Then the faeces are allowed to pass out of the sphincter at the anus. That's going to the toilet!
By now you should be flushed with success having digested all that lot!