- 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.
- 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.
- 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.