1. The problems posed by nutrients are breaking down the food products which contain them and getting them into the circulation for distribution to the parts of the body where they can be used. Food first enters the mouth, where it is broken down by the teeth, and partially dissolved by the saliva (Applegate, 2000, 331-338). It is then passed through the pharynx and down the esophagus to the stomach where digestion begins. The churning actions of the stomach wall break down food particles, and hydrochloric acid, activates pepsinogen to produce the protein-digesting enzyme pepsin. Gastrin is secreted to regulate gastric activity, mucus is secreted to protect the stomach wall, intrinsic factor is secreted to aid in vitamin B12 absorption, and the bolus of chyme is passed on into the small intestine.
The small intestine is designed to finish the process of digestion begun in the stomach, absorb the nutrients, and pass the residue on to the large intestine (Applegate, 2000, 338). The mucosa and submucosa show circular folds known as plicae circulares which increase the surface area for absorption, and finger-like projections of mucosa called villi project from the circular folds, further increasing surface area. The surface epithelium has microvilli on the surface, called a brush border, which increases the surface area for absorption even more.
The first part of the small intestine is called the duodenum, and it receives the chyme from the stomach, as well as secretions from the liver and pancreas (digestive enzymes) (Applegate, 2000, 338-339). It has mucous glands in the submucosa. The second part of the small intestine is the jejunum, and the third portion is the ileum. These two areas are not distinctly separated, and there is a gradual decrease in the number and length of the villi and an increase in the number of goblet cells beginning in the jejunum and ending in the ileum. Intestinal glands secrete large amoun...