DIGESTIVE SYSTEM

Digestive system is the food processing system of human body.
The whole digestive system is in the form of a long, hollow, twisted and turned tube, called the alimentary canal, which starts from the oral cavity and ends at the anus. The overall process of digestion and absorption of food occurs in this tube.
The tube is divided into different parts on the basis of structure and function of each part. These parts are described below.

Parts of digestive system:

Human digestive system consists of the two categories of parts.

The first category consists of those organs that are directly involved in the process of digestion and absorption.

The second category consists of those organs that aid the process of digestion and absorption of food by producing chemical substances or by some other way, but are not directly involved in the process of digestion and absorption. The first category of organs may be called as “necessary organs” and the second category as “accessory organs” but it should be kept in mind that without the aid of accessory organs, the process of digestion is seriously impaired.
When the accessory organs fail to perform their functions completely, the process of digestion may also completely stop.

Necessary Organs Of the digestive system:

1.ORAL CAVITY
2.ESOPHAGUS
3.STOMACH
4.SMALL INTESTINE
5.LARGE INTESTINE
6.ANUS

Accessory Organs of the digestive system:

1.LIVER AND GALL BLADDER
2,PANCREAS
3.SALIVARY GLANDS
4.TEETH
5.TONGUE

Mouth

In the human body, the mouth (oral cavity) is a specialized organ for receiving food and breaking up large organic masses. In the mouth, food is changed mechanically by biting and chewing. Humans have four kinds of teeth: incisors are chisel-shaped teeth in the front of the mouth for biting; canines are pointed teeth for tearing; and premolars and molars are flattened, ridged teeth for grinding, pounding, and crushing food.

In the mouth, food is moistened by saliva, a sticky fluid that binds food particles together into a soft mass. Three pairs of salivary glands—parotid, submaxillary, and sublingual—secrete saliva into the mouth. The saliva contains an enzyme called amylase, which digests starch molecules into smaller molecules of the disaccharide 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 (throat) and is forced through the opening to the esophagus.

Esophagus

The esophagus is a thick-walled muscular tube located behind the windpipe that extends through the neck and chest to the stomach. The bolus of food moves through the esophagus by peristalsis: a rhythmic series of muscular contractions that propels the bolus along. The contractions are assisted by the pull of gravity.

Stomach

The esophagus joins the stomach at a point just below the diaphragm. A valvelike ring of muscle called the cardiac sphincter surrounds the opening to the stomach. The sphincter relaxes as the bolus passes through and then quickly closes.

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. Pepsinogen is converted to the enzyme pepsin in the presence of hydrochloric acid. Hydrochloric acid is secreted by parietal cells in the stomach lining. The pepsin then digests large proteins into smaller proteins called peptides. To protect the stomach lining from the acid, a third type of cell secretes mucus that lines the stomach cavity. An overabundance of acid due to mucus failure may lead to an ulcer.

Small intestine

The soupy mixture called chyme spurts from the stomach through a sphincter into the small intestine. An adult’s small intestine is about 23 feet long and is divided into three sections: the first 10 to 12 inches form the duodenum; the next 10 feet form the jejunum; and the final 12 feet form the ileum. The inner surface of the small intestine contains numerous fingerlike projections called villi (the singular is villus). 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, a large, glandular organ near the stomach. In addition, bile enters the small intestine from the gallbladder 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 lipase can act, the large globules of fat must be broken into smaller droplets by bile. Bile is a mixture of salts, pigments, and cholesterol that is produced by the liver and stored in the gallbladder, a saclike structure underneath the liver.

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.

Most absorption in the small intestine occurs in the jejunum. The Most absorption in the small intestine occurs in the jejunum. 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 facilitated diffusion 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.

Absorption is completed in the final part of the small intestine, the ileum. Substances that have not been digested or absorbed then pass into the large intestine.

Large intestine

The small intestine joins the large intestine in the lower-right abdomen of the body. The two organs meet at a blind sac called the cecum and a small fingerlike organ called the appendix. Evolutionary biologists believe the cecum and appendix are vestiges of larger organs that may have been functional in human ancestors.

The large intestine is also known as the colon. It is divided into ascending, transverse, and descending portions, each about one foot in length. The colon’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. Feces consist of nondigested food (such as cellulose), billions of mostly harmless bacteria, bile pigments, and other materials. The feces are stored in the rectum and passed out through the anus to complete the digestion process.

Functions of digestive system:

As stated above, digestive system is the food processing system of human body. The food taken by human beings is digested into simpler molecules that can be absorbed into the blood and utilized for various functions of human body. As the digestive system is the only route of intake of nutrients (in normal conditions), therefore necessarily all the nutrient requirements of human body are fulfilled by this system alone. If the digestive system is impaired for some reason, health of the affected individual will seriously decline.

The function of the digestive system is digestion and absorption.

Digestion is the breakdown of food into small molecules, which are then absorbed into the body.

The digestive system is divided into two major parts:

The digestive tract (alimentary canal) is a continuous tube with two openings: the mouth and the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Food passing through the internal cavity, or lumen, of the digestive tract does not technically enter the body until it is absorbed through the walls of the digestive tract and passes into blood or lymphatic vessels.

Accessory organs include the teeth and tongue, salivary glands, liver, gallbladder, and pancreas.

The FUNCTION of digestive system involves the following seven processes:

Ingestion is the process of eating.

Propulsion is the movement of food along the digestive tract. The major means of propulsion is peristalsis, a series of alternating contractions and relaxations of smooth muscle that lines the walls of the digestive organs and that forces food to move forward.

Secretion of digestive enzymes and other substances liquefies, adjusts the pH of, and chemically breaks down the food.

Mechanical digestion is the process of physically breaking down food into smaller pieces. This process begins with the chewing of food and continues with the muscular churning of the stomach. Additional churning occurs in the small intestine through muscular constriction of the intestinal wall. This process, called segmentation, is similar to peristalsis, except that the rhythmic timing of the muscle constrictions forces the food backward and forward rather than forward only.

Chemical digestion is the process of chemically breaking down food into simpler molecules. The process is carried out by enzymes in the stomach and small intestines.

Absorption is the movement of molecules (by passive diffusion or active transport) from the digestive tract to adjacent blood and lymphatic vessels. Absorption is the entrance of the digested food (now called nutrients) into the body.

Defecation is the process of eliminating undigested material through the anus.