Laryngopharynx – lowest part of pharynx.
Function serves as passage for the food from the oral cavity to the oesophagus.
Oesophagus - 22-25 cm narrow, muscular tube, extends posteriorly and carries food into stomach by peristalsis.
Stomach –is a wide, "J" shaped, bag like, distensible, muscular sac.
Function – food reservoir and mechanical churning and subsequent digestion of food.
Small intestine – narrow tube, longest part (6 mts in an living adult), has 3 parts :
Duodenum – 'U' shaped, widest, shortest part (25 cm), receives hepato - pancreatic duct{joint duct from liver (hepato) and pancreas}.
Jejunum – Long , coiled, middle part.
Ileum - Longest part , highly coiled, opens into large intestine.
Functions – Completion of digestion, absorption of digested food.
Large intestine – shorter (1.5 meters) and wider than small intestine, has 3 parts –
Caecum –A small blind sac coming off the colon at the latter‟s junction with the ileum.
Vermiform appendix –A short (8 cm), slender, worm – like projection in caecum (vestigial organ in humans).
Colon – Long, sacculated, has 4 regions – ascending colon, transverse colon, descending colon, sigmoid colon.
Rectum – Leads into an anal canal which opens out at the anus guarded by two anal sphincters.
Digestion of food :
Starts in the oral cavity in humans, continues in the stomach and is completed in small intestine.
Buccal cavity -
Salivary glands : secrete salivary juice/ saliva into the buccal cavity and salivary amylase or ptyalin (pH= 6.8) and lysozyme (kills bacteria).
Mucus in saliva lubricates and adheres masticated food particles into a bolus which is swallowed and conveyed through pharynx and oesophagus by successive waves of muscular contractions (peristalsis) into the stomach.
Stomach – Gastric glands –secrete acidic gastric juice (pH = 1.8) from 3 types of gland cells:
(1) Oxyntic cells / parietal cells – secrete hydrochloric acid which kills microbes and provide acidic medium for activation of pepsin enzyme.
(2) Chief cells/ peptic cells / zymogen cells – secrete gastric enzymes as proenzymes – pepsinogen, prorennin.
(3) Mucous cells / Goblet cells – secrete mucus.
Stomach stores food for 4-5 hours and mixes the food thoroughly with acidic gastric juice by churning movements of its muscular wall and is called chyme.
Small intestine
Chyme gets mixed with three digestive juices which function in alkaline medium. –
(1) Bile (from liver),
(2) Pancreatic juice (from pancreas),
(3) Intestinal juice (from intestine).
Liver – largest gland in our body, weighing 1.2-1.5 kg in an adult human, located in upper right side of abdominal cavity just below the diaphragm, dark red and spongy.
Gall bladder- A thin walled muscular, situated below right lobe of liver. Bile juice is temporarily stored in Gall Bladder.
Bile – yellowish green, alkaline fluid containing bile salts (help in emulsification of fat), sodium bicarbonate, glycocholate, taurocholate, bile pigments (bilirubin and biliverdin), cholesterol, phospholipids etc. Bile juice does not contain any enzyme, yet essential for digestion because of its ability to emulsify fats.
Pancreas - is an elongated, compound (having both endocrine and exocrine parts) gland situated between the limbs of the „U‟ shaped duodenum.
Acini - Exocrine part. Secrete an alkaline pancreatic juice containing sodium bicarbonate, proenzymes: trypsinogen, chymotrypsinogen and procarboxypeptidases, and other enzymes like – pancreatic amylase, pancreatic lipase and nucleases. Islets of Langerhans - Endocrine part. Secretes hormones – insulin and glucagon.
Intestinal glands – numerous, microscopic glands present in the mucosa of small intestine. Two types – crypts of Lieberkuhn and Brunner’s glands.
Intestinal juice/succus entericus (pH = 8.3) - Mixture of secretions of crypts of Lieberkuhn – secrete enzymes (aminopeptidases, dipeptidases, intestinal amylase, maltase, sucrase, lactase, intestinal lipase, nucleotidases, nucleosidases and enteropeptidases/ enterokinase etc.) and mucus. Brunner‟s glands – secrete alkaline watery fluid and mucus.
Mucus glands – Mucosal epithelium of entire digestive tract has goblet cells which produce mucus (lubricates food and digestive tract, protects underlying cells from digestive enzymes).
Duodenum – receives bile juice and pancreatic juice, creates alkaline medium for the activity of pancreatic enzymes.
Large intestine – Though no digestion takes place in large intestine but absorption of water & formation of faces occurs here.
Digestion of fat :
Action of bile
Emulsification of fats- Breaking of large fat droplets into many small droplets which provides larger surface area for lipase activity.
