The Gastrointestinal (GI) Tract
The digestive system, also known as the gastrointestinal (GI) tract, is made up of a long, multifaceted tube whose ultimate purpose is to break down ingested food particles into usable nutrients and absorb them into the body. The organs that make up the digestive system are:
Each one of these components has a very specific job it must perform in order for healthy functioning of the body.
The mouth is responsible for the initial mechanical degradation of nutritious food. There is also a small amount of enzymatic degradation, as the salivary glands release amylase, which breaks down complex carbohydrates into simple sugars. Once the food has been sufficiently ground up and moistened, it is moved to the back of the mouth and swallowed.
The esophagus has the simple task of moving swallowed food from the mouth-pharynx area into the stomach. It accomplishes this through peristalsis. This is the muscular movement of food through a wave effect that effectively forces food towards the muscle. Peristalsis is also responsible for allowing humans to swallow while hanging upside-down.
The stomach is the next stop on the gastrointestinal tract. It is important to note the junction between the esophagus and the stomach is controlled by a muscular structure known as the sphincter. This ringlike muscle keeps the path to the stomach closed so that the highly degradative acids and enzymes in the stomach do not harm the esophagus. Once the peristalsis wave gets to the sphincter, however, it relaxes and allows the food to pass through.
Here, the stomach muscles further crush the food while the enzymes that the stomach lining secretes degrade the present proteins. The most notable enzyme here is pepsin, which must reside in an extremely acidic environment to act properly. In order to achieve the low pH pepsin requires, the stomach secretes hydrochloric acid (HCl) into the lumen (empty space). The benefits of this are two-fold: not only does it promote pepsin activity, but it also effectively kills most biological organisms that have survived the mastication (chewing) and peristalsis processes.
The small intestine is actually quite large, stretching up to twenty feet of length in humans, and is divided up into three total sections:
- duodenum (9-10 inches)
- jejunum (8 feet approx)
- ileum (11-12 feet)
After the food has been digested to a certain degree, it is referred to as chyme. This thick semifluid collection of partially digested food is ejected from the stomach through the pyloric sphincter into the first section, called the duodenum.
This section is the shortest but most important part of the small intestine. It performs the most chemical digestion than any other part of the digestive system. It causes the release of a myriad of digestive enzymes, including amylase, trypsin and lipase. It is also responsible for signaling important biological factors such as bile, which helps with the digestion of fats, and chemicals like bicarbonate, which raises the pH of the previously acidic chyme to a suitable level for trypsin’s function.
The jejunum and ileum sections of the small intestine follow the duodenum and are designed to have incredibly high surface-areas. These compose of the majority of the length of the small intestine and allow for extremely high rates of absorption of digested material.
The final part of the digestive system is the large intestine. This section of intestine is much shorter than the small intestine, but larger in diameter. The primary functions of the large intestine are to reabsorb water and indigestible food matter such as vitamins, as well as to pass along waste material for eventual elimination in the rectum.
Many types of bacteria live within the large intestine and are helpful to the human host. Some produce Vitamin K and Biotin that is readily absorbed and can make a significant contribution if that particular person’s vitamin intake is low. However, some bacteria can be a nuisance by producing gas that causes flatulence.