The system through which food is taken, digested and absorbed is called the digestive system. The digestive system consists of two parts. Alimentary canal and digestive gland.
Digestive tract
The long tube extending from the mouth to the anus is called alimentary canal. The various parts of the alimentary canal are described below.
1. Mouth: The first part of the alimentary canal is called the mouth. It is located below the nostrils.
Function: It receives food material and passes it into the oral cavity.
2. Buccal cavity: The next part of the mouth is the mouth cavity. It houses the teeth, tongue, subtongue or salivary glands. The mouth is always moist as it is covered by the mucous membrane oral mucosa. The floor of the mouth is made up of the mylohyoid muscle. At the back of the oral cavity is the soft palate which prevents food from entering the nostrils. Here the food stays for 5-30 seconds.
The tongue has 2000-8000 taste buds. There are five types of taste buds. Sweetness, saltiness, sourness or acidity, savory or umami and bitterness. Taste buds are replaced by new formations every 5-10 days. Each taste bud contains 50-100 taste receptor cells. Taste receptor cells are located in the form of papillae.
Oral function
(i) Tongue helps in tasting and swallowing food.
(ii) Teeth participate in cutting, tearing and grinding of food.
(iii) Salivary glands secrete saliva.
(iv) Lysozyme destroys bacteria.
3. Pharynx: About 12.5 cm long and funnel-shaped part after the oral cavity is called pharynx. It contains holes called gallettes.
Function: Food enters the esophagus from the pharynx through the gullet hole.
4. Esophagus: The muscular tube about 23-25 cm long after the pharynx is called esophagus.
Function: Moves food into the stomach through contractions in the process of peristalsis.
5. Stomach: The J-shaped curved fleshy sac located below the diaphragm and above the abdomen is called the stomach. Its wall is very thick and muscular. Its length is 30.5 cm and width is about 15.2 cm. Stomach consists of five parts. Cardiac stomach, fundus, body, antrum and pyloric stomach.
(i) Cardiac Stomach: The part of the stomach where the esophagus opens is called the cardiac stomach. At the junction of the esophagus and the cardiac stomach is a muscular ring called the cardiac sphincter. The ring of muscles prevents food from entering the esophagus.
(ii) Fundus: The convex part on the left side near the cardiac stomach is called fundus.
(iii) Body: The central region of the stomach posterior to the fundus is called the corpus. It contains food.
(iv) Antrum : The posterior part of the corpus is called antrum. Food is temporarily stored in the antrum before entering the intestine.
(v) Pyloric Stomach: The part of the stomach which opens into the duodenum is called pyloric stomach. At the junction of the duodenum and the pyloric stomach is a muscular ring called the pyloric sphincter. The musculature helps the food to enter the duodenum.
Stomach function
(i) Digested, fatty and partially digested food stays in the stomach for 2-24 hours. Cardiac sphincter prevents retrograde flow of food.
(ii) Oeish released from parietal cells destroys microbes, dissolves mineral salts, decalcifies bones and creates an acidic environment.
(iii) Peristalsis is the process of movement of food in the stomach after 15-20 seconds.
(iv) Food is crushed into a soft paste.
(v) Digestive juices and paste mix to form slimy mucus or chyme.
6. Small Intestine: The 6-7 meter long tube from pyelolic sphincter to ileocolic sphincter is called small intestine. Here the food stays for 3-5 hours. It is divided into three parts. Duodenum, jejunum and ileum.
(i) Duodenum: The first and U-shaped part of small intestine is called duodenum. Its length is 25-30 cm.
(ii) Jejunum: The tubular part about 2.5 meters long after duodenum is called jejunum.
Function: It digests and absorbs food.
(iii) Ileum: The posterior portion of the jejunum is called the ileum. Its length is three-fifths of the entire small intestine. At the junction of the ileum and the colon is a muscular ring called the ileocolic sphincter.
Function: It has villus like fingers. Increases villus absorption area.
7. Large intestine: About 1.5 meters long and thick part extending from the end of the ileum to the anus is called the large intestine. Here the food stays for 1.5-2.0 days. The large intestine has three parts.
Cecum, colon and rectum.
(i) Cecum: The first part of the large intestine is called the cecum. It is sac shaped. Its length is 6 cm and width is 7.5 cm. At the bottom of the caecum is a tube-like outgrowth. It is called vermiform appendix. Inflammation of the appendix caused by bacteria is called appendicitis. It is a passive organ.
(ii) Colon: The tubular portion after cecum is called colon. Its length is 101-114 cm. The colon has four parts. Ascending colon, transverse colon, descending colon and sigmoid colon.
(1) Ascending Colon: This part rises upwards from the cecum. Its length is 13 cm.
(2) Transverse Colon: This part lies transversely. Its length is 38 cm.
(3) Descending Colon: The part of the colon that is downward is called the descending colon. Its length is 25 cm.
(4) Sigmoid colon: The part of the colon connected to the rectum that forms a loop-like structure is called the sigmoid colon. Its length is 25-38 cm.
Function: Colon digests and absorbs food.
(iii) Colon: The sac-like part located at the end of the large intestine is called colon. The lower part of the rectum swells to form an ampulla. Its length is 13 cm. It contains annular sphincter or sphincter.
Functions: Faeces are produced in the large intestine, water is absorbed, food fermentation and digestion takes place.
8. Anus: The opening through which the rectum opens to the outside is called the anus. It is enclosed by two membranes called external and internal sphincters. The internal sphincter is composed of a ring of smooth muscle and contracts involuntarily. The external sphincter is made up of smooth muscle rings and contracts optionally.
Function: Opens and closes controlled by anal nerves and participates in defecation.
B. Digestive gland
The glands that digest food by secreting different types of digestive juices are called digestive glands or nutritive glands. The digestive glands of human body are salivary gland, liver gland, pancreatic gland, stomach gland and small intestine gland.