Human Digestion and Nutrients - Essay Example

The small intestine consists of the jejunum, duodenum, and ileum and the large intestine consists of ascending colon, transverse colon, descending colon, and rectum. The wall of the gastrointestinal tract (GIT) is composed of muscular layers which allow two types of movements: the mixing movements and the propelling movements. Mixing movements are found in small tracts of the GIT and allow rhythmic contraction of the wall while the propulsive movements are wavelike and are also called peristalsis, pushing the mass of food into the next part of the GIT (Boland, Golding, & Singh, 2014).

The mouth is the first place of food digestion and breaks food into smaller proteins. Saliva also contains the enzyme salivary amylase which breaks the food starch into smaller molecules. In the oral cavity, lingual lipase is also secreted which has a very small impact on fat digestion. Saliva mixes with the food adding mucus to it forming the food bolus. The food bolus is transferred into the esophagus, which propels the bolus through peristaltic movements into the stomach. The stomach produces hydrochloric acid which is important to kill several pathogenic organisms in the food and also aids in protein digestion by converting pepsinogen into pepsin. Pepsin breaks the protein into smaller chains so that it can be absorbed. Gastrin is also a part of the stomach secretions and aids in stomach motility and also stimulates the secretion of other gastric secretions. Intrinsic factor is a key component in the absorption of vitamin B12, which occurs in the small intestine, terminal ileum. Without intrinsic factors, absorption of vitamin B12 will not be possible (Insel, Ross, McMahon, & Bernstein, 2014).

Most of the digestion in the small intestine occurs in the duodenum which receives the secretions from the gall bladder, pancreas, and also from the intrinsic duodenum glands. Pancreas secretes major enzymes for the digestion of fats, carbohydrates, and proteins. Pancreas also secretes bicarbonate secretions to aid in digestion. The presence of bile in the duodenum stimulates the secretion of bile from the gall bladder into the duodenum. Cholecystokinin is the signaling hormone for this purpose. Bile acts as an emulsifier of fat and keeps the lipid molecules in the watery chyme. In the small intestine, all the major nutrients are broken down into small units which can be absorbed easily also in the small intestine. Vitamins, minerals, and cholesterol are absorbed undigested.  The large intestine is the major site of propulsive movements and mixing of food, along with the activity of the gut bacteria producing vitamin K and various gases. No further digestion of food takes place in the large intestine (Insel, Ross, McMahon, & Bernstein, 2014). Carbohydrates are digested by the enzymes amylase and glucosidases, which break the carbohydrates into monosaccharides. The disaccharidases are present in the brush border of the small intestine aiding in their conversion to monosaccharides. Lipids are digested by lipases such as cholesterol esterases, phospholipases A2, and pancreatic lipases (Boland, Golding, & Singh, 2014).