Digestion Process

Before you take a bite of your favorite food, your mouth is already watery and filled with saliva containing enzymes and other chemical molecules ready to help in the mashing and dissolving of your food. Of course, one is more focused in the delicious taste of the aliment, than in what is occurring within your body. Let’s analyze the process of nutrition and digestion that takes place when we eat.

Perhaps you didn’t know this, but there are different modes of ingestion (taking food in) which are used by different organisms to adapt to their body structures and environments. There are suspension feeders, substrate feeders, fluid feeders, and bulk feeders, this last one pertaining to human beings and other familiar animals. Bulk feeders eat large pieces of food and they either chew it or swallow it whole.

In our mouth, our saliva begins to break down carbohydrates with a special enzyme called amylase. The instant we swallow our food it will go through the pharynx (throat), pass through the esophagus, and land on the stomach to initialize the real process of digestion.

Inside the stomach, a gastric juice will be secreted in its main chamber (lumen) where the food and the juice will be churned into chyme. The lumen is lined with epithelial cells specialized to secrete a mucus that protects your stomach from damaging itself with the extremely acidic substance it forms. The epithelial gastric chief cell in the stomach will release pepsinogen into the lumen. Then, the gastric juice contains parietal cells that release hydrogen and chloride ions into the lumen. Hydrogen and chloride will combine to form hydrochloric acid, which is so acidic that it can dissolve metal. This acid will kill almost all bacteria and denature most proteins.

The epithelial gastric chief cell in the stomach releases pepsinogen into the lumen. After this, the pepsinogen mixes with the hydrochloric acid and becomes pepsin. Pepsin is an enzyme that will digest proteins by breaking their peptide bonds. In this way, proteins are converted into smaller polypeptides that will travel to the small intestine.

Within the small intestine, polypeptides will be disaggregated into amino acids. The first several inches of the small intestine are known as the duodenum. This part of the intestine basically receives chyme mixed with secretions from other organs to continue the digestion process. Specifically speaking, the pancreas delivers bicarbonate to neutralize acid, trypsin and chymotrypsin to break down proteins, and the enzyme amylase to break down starches and carbohydrates. On the other hand, the liver gives bile to start breaking down fats.

Lined with villi, the nutrients in the small intestine will be absorbed into the cells of the villi by passive and active transport. Capillaries in the core of the villi will lead nutrients to the blood and carry them to the liver. Detoxifying the blood, the liver will then distribute it to the heart. Special enzymes in the small intestine will convert fat into water-soluble compounds. These compounds are absorbed by the villi and passed into the lymphatic system.

Then comes the large intestine formed by colon, appendix, and cecum. 90% of the water from chyme, digestive juices, ingestion, etc. is absorbed by the colon. Many bacteria live in the colon and are responsible for producing foul-smelling gas, most of them are the known as “Escherichia Coli”. Our feces or “poop” are about 50% of bacteria in weight. Finally, the appendix seems to play a minor role in the immune system.

In short, but this is how food becomes poop for the toilet and nutrients for the body.