Digestion

The following article is a summary of the main processes that occur in the digestion of food. It explains what happens along the pathway of the digestion.

What does digestion do?

The food we eat contains various substances, not all of which are needed in the human body. Therefore, the food needs to be broken down so the necessary particles can be separated from the waste and broken down to be absorbed. There are three main types of food molecules that are broken down in digestion: starch, protein and lipids (oils and fats). Digestion would occur naturally at body temperature, but this would take a very long time. The process is sped up by enzymes such as amylase, protease and lipase.

Mouth

Digestion begins in the mouth, where the food is broken down into smaller pieces and mixed with amylase, which is secreted from the salivary glands to begin the breakdown of starch. At this stage, the pH is generally around 7.

Stomach

The broken down food goes through the esophagus to the stomach, where it is stored. However the stomach is not only for storage, it is also the organ in which the breakdown of proteins begins. In the wall of the stomach, there are gastric pits from which hydrochloric acid and pepsinogen are excreted. There are also cells that produce mucus, to protect the cells in the stomach wall from auto-digestion. Pepsinogen is an inactive form of pepsin, if the cells in the gastric pits were to produce active pepsin on the spot, it would cause auto-digestion. Instead, the pepsinogen becomes active pepsin after mixing with the hydrochloric acid in the lumen of the stomach, where it is used to begin the breakdown of protein. The hydrochloric acid also kills any microorganisms that may have entered the digestive system. The lumen of the stomach is a highly acidic environment, with a pH of around 1.5. The contents of the stomach (mainly acid and broken down food) is called chime.

Small Intestine: Duodenum

The next stage of digestion takes place in the duodenum, the first section of the small intestine. Here, the chime is mixed with bile from the gall bladder to balance the pH so it is ideal for enzyme activity. It is mainly in the duodenum that protein, starch and lipids are broken down into amino acid, glucose, fatty acid and glycerol. This is the last stage of the actual breakdown of molecules into component parts.

Enzymes

Small intestine: ileum

The ileum is the second section of the small intestine, which follows the duodenum. In the ileum, the process of absorption begins. The wall of the ileum is made up of villi and microvilli to increase the surface area for the maximum possible absorption of molecules. Villi are “finger-like” projections on the wall of the ileum. Each villus is covered with microvilli, which again increases the surface area. The villi are very rich in capillaries, which absorb mono saccharides (glucose) and amino acids. The villi also contain lacteals, which are connected to the lymphatic system. The lacteals absorb fatty acids and glycerol.

Large intestine

Following the small intestine is the large intestine (colon). This is where water is reabsorbed. What remains is waste material (feces) which is egested through the anus.