There are six main stages of the digestive system, as whole food is turned into energy for the body, and in chronological order they are:
Ingesting food is the first stage, and this occurs at the mouth. All food enters the body through this orifice, which can act as storage while food waits to be digested, with the stomach also being used for storage. People can only eat a few times a day due to the storage size.
Approximately seven litres of fluid are secreted by the digestive system throughout the day, consisting of saliva, mucus, hydrochloric acid, enzymes and bile.
Dry food is moistened by saliva, which also starts off the digestion of carbohydrates. Mucus helps lubricate and protect the GI tract, while hydrochloric acid chemically digests food, killing bacteria in the process, to protect the body.
Enzymes break down carbohydrates, lipids and protein, as well as other large macromolecules. Then, because digestion is easier with lipids in the form of tiny globules, bile emulsifies large masses of them.
Mixing and movement
The three main processes which are used to move food around and mix it, are swallowing, peristalsis and segmentation.
Swallowing is when food leaves the mouth, goes through the pharynx, and ends up in the esophagus. When the food is in the mouth, muscles move it around and push it to the back, ready to go to the pharynx.
Peristalsis is when food that has been partially digested is moved along the GI tract by muscular waves. The food doesn’t travel very far, and it takes a lot of waves for it to move from the esophagus to the end of the GI tract.
Segmentation is when short segments of the small intestine contract. This increases the contact between food and the walls of the intestine, and mixes the food more, resulting in more nutrients being absorbed.
In order for large pieces of food to be reduced to their chemical components, mechanical digestion and chemical digestion need to occur.
These processes happen at the same time, with mechanical digestion being responsible for physically breaking down large pieces of food into small pieces. This occurs through chewing and the muscular mixing of food (within the stomach and intestines), further helped by bile which is produced by the liver, breaking fats down into tiny globules.
Chemical digestion breaks down large, complex molecules into smaller molecules, which can be absorbed easier. The majority of chemical digestion takes place in the small intestine, due to the pancreas secreting pancreatic juice, which is very strong. The pancreatic juice can digest carbohydrates, lipids, nucleic acids and protein.
Food enters the duodenum – the first and shortest section of the small intestine – from the stomach as partially digested food, called chyme. Chemical digestion then occurs in this area so that when it leaves, to move through the small intestine, it has been reduced to its building blocks. These are amino acids, fatty acids, monosaccharides and nucleotides.
Having now been broken down thoroughly, into those building blocks, the penultimate stage – absorption – occurs.
Starting in the stomach, water and other simple molecules are absorbed into the bloodstream, although the majority of the absorption actually occurs in the small intestine. This is because the folds of the walls increase the contact between the intestine and the digested food.
The excretion of waste is the final stage of the digestive process. Called defecation, substances which cannot be digested are removed from the body, to prevent them from building up in the gut.
Defecation needs to occur regularly to prevent indigestible substances building up, and a conscious part of the brain is responsible for choosing when to defecate.