Human digestive system consists of two main parts, alimentary canal and digestive glands.

Alimentary canal comprises of following parts

(i) Mouth (ii) Pharynx (iii) Oesophagus (iv) Stomach (v) Small intestine (vi) Large intestine (vii) Rectum (viii) Anus

Digestive glands include

(i) Salivary glands are situated just outside the buccal cavity and secrete salivary juice into it.

(ii) Liver is the largest gland in the body, situated in the abdominal cavity just below the diaphgram and has two lobes. It secreates bile which helps in the digestion of fats.

(iii) Pancreas is the compound organ situated between the limbs of U-shaped duodenum acting as endocrine and exocrine organ. The exocrine portion secretes pancreatic juice where endocrine portion secretes hormones like insulin and glucagon.

Gall Bladder is a pear-shaped sac-like structure, that is attached to the posterior surface of the liver by a connective tissue. The bile secreted by the hepatic cells of liver, passes through the hepatic ducts and gets stored and concentrated in the gall bladder. Non-functioning and removal of gall bladder would be bile to simply run in the continuous stream from the liver into the intestine. It would not be as concentrated as the bile started in gall bladder actually is. The body gradually adjust to it but a low fat diet is recommonded because fat digestion is significantly reduced after gall bladder removal.

(a) Absorption of amino acids and glycerol takes place in the small intestine.

(b) The faeces in the rectum initiate a neural reflex causing an urge for its removal.

(c) Skin and eyes turn yellow in liver infection.

(d) Rennin is a proteolytic enzyme found in infants gastric juice.

(e) Pancreatic juice and bile are released through hepato-pancreatic duct. (f) Dipeptides, disaccharides and glycerides are broken down into simple substances in the region of small intestine called duodenum.

Gastric glands process three major types of cells namely

(i) Mucous neck cells (Goblet cells) These cells are present throughout the epithelium of gastrointestinal tract and are involved in the secretion of mucus.

(ii) Peptic or Chief cells (Zymogenic cells) These cells are usually basal in location and are involved in the secretion of gastric digestive enzymes such as proenzymes pepsinogen and prorennin.

(iii) Parietal or oxyntic cells These cells are large and most numerous present on the side walls of the gastric glands. They are involved in the secretion of HCl and Castlis Intrinsic Factor (CIF). (factor essential for the absorption of vitamin- $\mathrm{B}_{12}$ in ileum).

The mucus secreted by the goblet cells along with the bicarbonates from the pancreas play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid.