11 Biology

Digestion

anatomy of stomach

Stomach: Stomach is a J-shaped bag-like structure. It is situated in the upper left portion of the abdominal cavity. There are three major parts in the stomach, viz. the cardiac, fundic and pyloric regions. The oesophagus opens into the cardiac region of stomach. The pyloric region opens into the first part of small intestine.


Small Intestine: Small intestine is a long and highly coiled structure. It is divided into three regions, viz. duodenum, jejunum and ileum. The duodenum is U-shaped, jejunum is somewhat coiled and the ileum is highly coiled. The opening of the stomach into the duodenum is guarded by the pyloric sphincter.

Large Intestine: Ileum opens into the large intestine. The large intestine is somewhat shorter than the small intestine but has larger diameter. The large intestine consists of caecum, colon and rectum. Caecum is a blind sac which hosts some symbiotic microorganisms.

Appendix: This is a narrow finger-like tubular projection situated at the junction of small and large intestines. The vermiform appendix arises from caecum. Appendix is a vestigial organ.



Colon: Colon is divided into three parts, i.e. an ascending, a transverse and a descending part. The descending colon opens into the rectum. The rectum; in turn; opens out through the anus.

transverse section of small intestine

Wall of Alimentary Canal: The wall of the alimentary canal is made of four layers, viz. serosa muscularis, sub-mucosa and mucosa. Serosa is the outermost layer and is made up of a thin mesothelium with some connective tissues. The epithelium of visceral organs is called mesothelium. Muscularis is formed by smooth muscles which are usually arranged into an inner circular and outer longitudinal layer. An oblique muscle layer may be present in some regions. The sub-mucosal layer is formed of loose connective tissues.



DIGESTIVE GLANDS

Salivary Glands: There are three pairs of salivary glands, viz. parotids, sub-maxillary/submandibular and sbublinguals. The parotids are present in the cheek. The sub-maxillary/sub-madnibular is present in the jaws. The sublinguals are present below the tongue. The salivary glands secrete salivary juice into the buccal cavity.

Liver: Liver is the largest gland in the human body. It weighs about 1.2 to 1.5 kg in adult human. Liver is situated in the abdominal cavity, just below the diaphragm. There are two lobes in the liver. The functional and structural units of liver are the hepatic lobules. Hepatic cells are arranged in the form of cords in the hepatic lobules. Each lobule is covered by a thin connective tissue; called the Glisson’s capsule. Hepatic cells produce bile. The bile passes through the hepatic ducts and is stored and concentrated in the gall bladder. The gall bladder is a thin muscular sac, situated above the liver. The duct of gall bladder (cystic duct); along with the hepatic duct forms the common bile duct.

The bile duct and the pancreatic duct open into the duodenum as the common hepato-pancreatic duct. The hepato-pancreatic duct is guarded by a sphincter called the sphincter of Oddi.

Pancreas: Pancreas is a compound gland, i.e. both exocrine and endocrine. It is an elongated organ. This is situated between the limbs of the U-shaped duodenum. The exocrine portion of the pancreas secretes and alkaline pancreatic juice which contains various enzymes. The endocrine portion of the pancreas secretes hormones; insulin and glucagon.