Digestion is a complex process that involves many parts of the human body, and is responsible for much of life as we know it. Human digestion may be explained in a step by step process from the moment food enters the mouth, to the moment it exits the human body, completing the digestive route.
First, food enters the mouth and is broken down through both mechanical and chemical digestive pathways. Mechanically, it is cut down by the grinding and crushing action of the teeth in the mouth, while chemically food is broken down by the enzyme salivary amylase, secreted in saliva. This enzyme breaks down some carbohydrates.
Next, food particles that have been rolled into a bolus by the tongue in the mouth travel down the pharynx, down the esophagus, into the stomach. They pass the epiglottis, a flap that helps direct food, and the cardiac sphincter, a muscular flap that prevents the stomach material from entering the esophagus.
Once in the stomach, more chemical and mechanical digestion occurs. The crushing and breaking action of the stomachs contractions with HCl help not only break down food, but also lend a hand in converting pepsinogen to the enzyme pepsin. This enzyme in the stomach takes the roll of the chemical digester, and breaks certain peptide bonds, thereby breaking up proteins in the stomach. Partly digested food from the stomach now travels to the small intestine, through the pyloric sphincter, and is know as chyme.
Once in the small intestine, a large amount of chemical breakdown of the chyme occurs, thanks to the myriad of enzymes and substances secreted by the various organs surrounding the small intestine. The pancreas, gall bladder, and liver all play a role in this chemical digestive process. The small intestine is divided into three sections, these being the duodenum, the jejunum, and the ileum, all of which are important in the digestive process. Many different types of enzymes are utilized in this process, all of which play different roles in breaking down different types of fats, proteins, and carbohydrates. Chemical absorption takes place in the small intestine as well, with vitamins and minerals being absorbed through the walls of the organ.
Finally, the remaining digested substances enter the last part of their journey, the large intestine. Divided into three portions, the cecum, colon, and rectum, the large intestine provides an important absorptive region for water and other material from the substances passing through, although this must be careful balanced. Too much water absorption may lead to constipation, while too little may lead to diarrhea and dehydration. The remaining waste exits the human body through the rectum and out the anus, where the wastes are expelled. Source: MCAT Kaplan Biology Review book