- Week 1, Digestion

This week in Biology class we learned about the process of digestion and all the "aspects" involved in such. To digest something is to break down nutrients such as proteins, carbohydrates and lipids. The process of digestion is a series of chemical reactions where ingested food is broken down into smaller and smaller forms. For example, proteins, carbohydrates and lipids are broken down into amino acids, monosaccharides and fatty acids, respectively, with the help of enzymes.
When enzymes are involved in the digestion process of nutrients, it is called a chemical digestion. On the other hand, nutrients such as vitamins, minerals and water don't need to be digested, once they are absorbed by the human body.

The digestion process starts with ingestion, which takes place in the mouth. When food is eaten, teeth break it into smaller molecules, process also known as mechanical digestion. Mechanical digestion may also occur inside the mouth due to the enzyme salivary amylase (present on the saliva produced by the salivary glands), which can break down starch. Nevertheless, after this first breakage of food, it moves to the esophagus and forward with peristalsis; involuntary contractions that push food down. Peristalsis can also be considered a part of mechanical digestion, once it squishes and breaks food.

Passing through the muscular tube which is the esophagus, food goes to the stomach, where both mechanical and chemical digestions will occur. Such digestions are reflected on the mashing and mixing of food and in the pepsin digestion of proteins. Additionally, the stomach is filled with HCl, a gastric acid which helps the breakage of nutrients, activates the protease pepsin and acts as a barrier against germs. In order to avoid the leaking of such acid, the walls of the stomach are filled with mucus. After food passes through the stomach, it goes to the liver, where the gallbladder is responsible for the storage of bile, which separates fat into smaller parts, hence, allowing its future absorption. Followed by the liver, there is the pancreas, the most active site for chemical digestion. The pancreas produces all types of enzymes, such as amylase, lipase, trypsin, and dipeptidase, and it also produces insulin, that controls the amount of sugar in the blood. The rich juices from both the pancreas and the gallbladder are later received by the deodenum located in the small intestine. The small intestine has villi, finger-like extensions, which greatly increase the surface area for absorption of molecules such as glucose, amino acids and fatty acids. When such nutrients are absorbed, they are distributed to the cells through the blood, also due to villi. Villi are made of lacteal, small vessels of lymphatic system, and capillary beds, small vessels of circulatory system. In this way, nutrients that are absorbed easily go to the blood, circulatory system, and are directed to a wide variety of body cells.

Finally, whatever was not absorbed by the small intestine is taken to the large intestine. As mentioned before, water was not digested, however, it is only absorbed at the end of the process, in the large intestine. This second intestine also allows bacteria to produce vitamins B and K, and acts in pre

paration for undigested, or unabsorbed, food to be eliminated. In the large intestine such waste material is compacted and is sent to the rectum, which will later release it through the anus.

A nice and explanatory video on digestion can be seen if clicked here.