Sat May 05, 2012 By: Rajat Jain

Describe the process of human digestion ? Point By point

Expert Reply
Sun May 06, 2012

The major organs or structures that coordinate digestion within the human body include the mouth, esophagus, stomach, small and large intestine, and liver.


Mouth- The mouth is a specialized organ for receiving food and breaking up large organic masses. In the mouth, food is changed mechanically by biting and chewing. Food is moistened by saliva. The saliva contains an enzyme called amylase, which digests starch into maltose. During chewing, the tongue moves food about and manipulates it into a mass called a bolus. The bolus is pushed back into the pharynx and is forced through the opening to the esophagus.

Esophagus- The bolus of food moves through the esophagus by peristalsis

Stomach - The stomach is an expandable pouch located high in the abdominal cavity. Layers of stomach muscle contract and churn the bolus of food with gastric juices to form a soupy liquid called chyme. The stomach stores food and prepares it for further digestion. In addition, the stomach plays a role in protein digestion. Gastric glands called chief cells secrete pepsinogen, which is converted to pepsin in the presence of hydrochloric acid. The pepsin then digests large proteins into smaller proteins called peptides. To protect the stomach lining from the acid stomach cells secretes mucus that lines the stomach cavity.

Small intestine: The chyme enters from stomach to the small intestine. The three parts of small intestine are duodenum,  jejunum and ileum. The inner surface of the small intestine contains numerous fingerlike projections called villi. Each villus has projections of cells called microvilli to increase the surface area.

Most chemical digestion takes place in the duodenum. In this region, enzymes digest nutrients into simpler forms that can be absorbed. Intestinal enzymes are supplemented by enzymes from the pancreas. In addition, bile enters the small intestine from the gall bladder to assist in fat digestion.

The enzymes functioning in carbohydrate digestion include amylase (for starch), maltase (for maltose), sucrase (for sucrose) and lactase (for lactose).

For fats, the principal enzyme is lipase. Before this enzyme can act, the large globules of fat must be broken into smaller droplets by bile.

Protein digestion is accomplished by several enzymes, including two pancreatic enzymes: trypsin and chymotrypsin. Peptides are broken into smaller peptides, and peptidases reduce the enzymes to amino acids.

Nucleases digest nucleic acids into nucleotides in the small intestine also.

The products of digestion enter cells of the villi, move across the cells, and enter blood vessels called capillaries. Diffusion accounts for the movement of many nutrients, but active transport is responsible for the movement of glucose and amino acids. The products of fat digestion pass as small droplets of fat into lacteals, which are branches of the lymphatic system.

Substances that have not been digested or absorbed then pass into the large intestine.

Large intestine - The large intestine's chief functions are to absorb water and to store, process, and eliminate the residue following digestion and absorption. The intestinal matter remaining after water has been reclaimed is known as feces. The feces are stored in the rectum and passed out through the anus to complete the digestion process.

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