Digestion Explained

Digestion involves taking in large pieces of food and breaking them down into molecules small enough to be absorbed into your blood stream. Food also contains indigestible material which passes through the digestive system and exits the body - this bit is called egestion.
Digestion starts in the mouth where large pieces of food are physically broken down by chewing. The physical breakdown of food is called mechanical digestion. But digestion involves more than just the physical breakdown of food.
When you eat food, you consume a lot of relatively large molecules like starch, proteins and lipids. These molecules must be chemically broken down using enzymes.
Remember that enzymes are molecules in living organisms that help reactions happen faster.
As we’ve mentioned chewing is an example of mechanical digestion. The chemical breakdown of food, for example using enzymes, is known as chemical digestion.
The first stage of chemical digestion is also in the mouth. Here the salivary glands release saliva to mix with the food. Saliva contains an enzyme called amylase which breaks down starch into maltose.
From the mouth food is passed down the oesophagus. Food moves down the oesphagus because of waves of muscle contractions. Think of it a bit like squeezing toothpaste through the tube. This is known as peristalsis.
Next is the stomach. When it reaches this point the food is physically churned up by the muscle contractions of the stomach wall. This is another example of mechanical digestion. There is also chemical digestion in the stomach: here the enzyme pepsin is found which breaks proteins into peptides (which will later be broken down further into amino acids). Hydrocholic acid in the stomach provides the low (acidic) pH for pepsin to function and it also destroys pathogens present in your food.
The partially digested food, now known as chyme, passes from the stomach to the small intestine. The first section is called the duodenum and here a material called bile is added. Bile is produced in the liver and stored in the gall bladder before entering the small intestine. It has two main functions: it is alkaline and so helps neutralise stomach acid, and it emulsifies lipids. This means it takes large lipid globules and breaks them down into smaller droplets. This provides a larger surface area for enzymes to work.
Also in the duodenum pancreatic juice, produced in the pancreas, is added. Pancreatic juice contains enzymes to further chemically break down food. It contains amylase to break down starch into maltose, trypsin to break down protein into peptides, and lipases to break down lipids into fatty acids and glycerol. The walls of the small intestine then release maltase which breaks maltose into glucose molecules, and peptidases which split peptides into amino acids.
Now that the food has been broken down into smaller molecules, these molecules can be absorbed into the blood in a section the small intestine called the ileum. The inside of the ileum is lined with many little projections called villi. (By the way, it’s “one villus”, “many villi” - villi is the plural). These villi increase the surface area of the small intestine’s inner wall to maximise the rate of absorption of digested food. Each villus is also lined with microvilli to further increase surface area.
Most food molecules are absorbed into blood capillaries in the villi but the products of lipid digestion are absorbed into a structure called the lacteal.
The remaining material now enters the colon, a section of the large intestine. Here most of the water is reabsorbed into the blood stream. Passed on to the rectum is the material that couldn’t be digested such as cellulose, along with bacteria and cells from the lining of the digestive system. This semi-solid material, known as faeces, is stored in the rectum until it exits the body through the anus.
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Natty Roadser - JR Tundra
Noble Dub - Silent Partner

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