How does the plate with steak, chips and salad become tissues of our body? The digestive system takes us halfway and the circulatory system does most of the rest.
Living systems constantly exchange energy. Physiological processes – anabolic, catabolic and homeostatic – require energy. Ultimately, the energy comes from the light energy that plants use to transform carbon in the atmosphere into biological substances in the process of photosynthesis. We humans get our energy by consuming these biological substances either directly or consuming other organisms that do. The digestive system takes apart the biological substances (food) stepwise and transforms it into a form (nutrients) that human cells can use.
The alimentary canal or the gastrointestinal tract (GI tract) is a tube through which ingested substances, e.g. food, are pushed along for physical and chemical processing.
The digestive system’s anatomy is comparable to that of an industrial smelter. Some structures bring in raw materials, and other structures extract, process and ship out substances. The body uses both mechanical (in the mouth and stomach) and chemical mechanisms to break down the raw materials. The processes are organized linearly – substances keep moving along in one direction at a steady pace.
Food begins its journey through the digestive system in the mouth, also known as the oral cavity. Inside the mouth are many accessory organs that aid in the digestion of food—the tongue, teeth, and salivary glands. Teeth tear and grind food into smaller pieces, which are moistened by saliva before the tongue and other muscles push the food into the pharynx. The saliva starts the digestion of complex carbohydrates, but no protein digestion is done in the mouth.
Pharynx and Esophagus
The pharynx or the throat leads to the esophagus, the tube that extends from the mouth to the stomach. At the end of the esophagus is a muscular ring called the lower esophageal sphincter or cardiac sphincter. The function of this sphincter is to close the end of the esophagus and trap food in the stomach.
After passing through the bottom sphincter of the esophagus, the food drops into the stomach, the widest part of the alimentary canal. Food remains in the stomach two to six hours, during which time it is churned in an acidic substance, gastric juice. The digestion of protein and fats is started, separating them into their basic components – amino acids and fatty acids. As the food is churned it turns into an oatmeal-like paste called chyme. The chyme moves on into the small intestine.
The small intestine is a long muscular tube, up to 3 meters. The intestine specializes in the import and export of biological substances of many kinds. The processes of chemical digestion run furiously. The carbohydrates, proteins and fats are broken down into molecules such as glucose, amino acids, fatty acids and glycerol. The capillary beds that line the intestine define the interface of the digestive and circulatory systems. By the time food leaves the small intestine, around 90% of all nutrients have been extracted from the food that entered it.
The large intestine is a thick tube about 1.5 meters long and is positioned around the small intestine. The large intestine absorbs water and contains many symbiotic bacteria that aid in the breaking down of wastes, to extract some small amounts of nutrients.
Aging and the Digestive System
Aging has limited effect on the digestive system. Essentially, normal digestion and absorption occur in the elderly. However, there are some changes in the digestive system that parallel the age-related changes seen in the other systems. Like other systems, the rate of new cell growth declines and tissues become more susceptible to damage.
Due to a decrease in smooth muscle tone along a majority of the aging GI tract, food moves through the system more slowly as the contractions necessary for the movement and breakdown of food become weaker. Constipation could become a problem along with hemorrhoids.
National Digestive Diseases Information Clearinghouse (NDDIC).