Tuesday, July 28, 2020

Digestion and Absorption - Notes | Class 11 | Part 2: Digestive Glands

16. DIGESTION AND ABSORPTION

II. DIGESTIVE (ASSOCIATED) GLANDS

-   They secrete digestive juices.

-   They include salivary glands, gastric glands, intestinal   glands, pancreas & liver.

a. Salivary glands

-    3 pairs. They are

o  Parotids (2): Largest salivary gland. Seen in cheeks.

o  Submaxillary/submandibular (2): Seen in lower jaw.

o  Sublingual (2): Below the tongue.

-  Salivary glands secrete saliva. It contains 99.5% water, mucin (mucus), enzymes like salivary amylase (Ptyalin) and Lysozyme and electrolytes (Na+, K+, Cl-, HCO3- etc).

b. Gastric glands

Tubular glands found on the inner wall. They consist of

a. Mucus neck cells: Secrete mucus. Mucus and bicarbonates protect the stomach wall (mucosal epithelium) from HCl and prevents autodigestion.

b. Chief (zymogen or peptic) cells: Secrete proenzymes like pepsinogen & prorennin and some lipase.  

c.  Oxyntic (parietal) cells: Secrete HCl & Castle’s intrinsic factor (for absorption of vitamin B12).

Gastric glands secrete acidic (pH 1.8-2.0) gastric juice.

c. Liver

-  Largest gland (1.2 - 1.5 kg). Reddish brown.

-  Situated in abdominal cavity, just below the diaphragm.

- Bilobed (large right lobe & small left lobe). Each lobe is formed of hepatic lobules (structural & functional units).

- A lobule has many hepatic cells arranged as cords around a central vein. They secrete alkaline bile juice.

-  Liver lobule is covered by Glisson’s capsule.

-  Bile is transported from liver to duodenum as follows:

Bile → hepatic duct → gallbladder → cystic duct → common bile duct → common hepato-pancreatic duct → duodenum.


-  Hepato-pancreatic duct is guarded by sphincter of Oddi.

-  Bile has no enzymes but contains bile pigments (bilirubin & biliverdin), bile salts, cholesterol and phospholipids.

d. Pancreas

-    Second largest gland. Seen near duodenal loop.

-    It is a cream-coloured heterocrine gland, i.e. it has both exocrine and endocrine parts.

-  The exocrine part has a pancreatic duct that opens into duodenum along with bile duct (hepato-pancreatic duct).

-   Exocrine part secretes alkaline pancreatic juice. It contains inactive protease enzymes (trypsinogen, chymotrypsinogen & procarboxypeptidases), amylases, lipases & nucleases.


e. Intestinal glands

-  Simple tubular glands. 2 types:

1. Crypts of Lieberkuhn: Consists of mucus-secreting Goblet cells and enzyme-secreting Paneth cells.

2. Brunner (duodenal) glands: Confined to submucosa of duodenum. Secrete mucus only.

-  Intestinal glands secrete alkaline intestinal juice (succus entericus). It contains enzymes (maltase, lactase, sucrase, dipeptidase, lipases, nucleotidases, nucleosidases etc).  

-  The bicarbonate and mucus provide alkaline medium and protect intestinal mucosa. 

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