Digestive Glands

The digestive glands associated with the alimentary canal include the salivary glands, the liver and the pancreas. Saliva is mainly produced by three pairs of salivary glands, the parotids (cheek), the sub[1]maxillary/sub-mandibular (lower jaw) and the sub-linguals (below the tongue).
These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity. Liver is the largest gland of the body weighing about 1.2 to 1.5 kg in an adult human.
It is situated in the abdominal cavity, just below the diaphragm and has two lobes. The hepatic lobules are the structural and functional units of liver containing hepatic cells arranged in the form of cords.
Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule. The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder.

The duct of gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct.
The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct which is guarded by a sphincter called the sphincter of Oddi.
The pancreas is a compound (both exocrine and endocrine) elongated organ situated between the limbs of the ‘C’ shaped duodenum.
The exocrine portion secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, insulin and glucagon.
These include salivary glands, liver and pancreas.

Salivary glands

These are 3 pairs in humans i.e the parotids (cheek), the sub-maxillary/sub-mandibular (lower jaw) and the sublinguals (below the tongue). These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity.

DIGESTION AND ABSORPTION | Alimentary canal | Small Intestine | Large Intestine

Liver

It is largest gland of the body(1.2 to 1.5 kg), situated in the abdominal cavity, just below the diaphragm and has two lobes.

The hepatic lobules are the structural and functional units of liver containing hepatic cells arranged in the form of cords.
Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule. The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder.
The duct of gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct.
The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct which is guarded by a sphincter called the sphincter of Oddi.

Pancreas

It is a compound (both exocrine and endocrine) elongated organ situated between the limbs of the ‘U’ shaped duodenum. The exocrine portion secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, insulin and glucagon.

DIGESTION OF FOOD

(accomplished by mechanical and chemical processes)

Buccal cavity performs two major functions, mastication of food and facilitation of swallowing.

The teeth and the tongue with the help of saliva masticate and mix up the food thoroughly. Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus.
The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or deglutition. The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis.

Saliva contains electrolytes (Na⁺, K⁺, Cl–, HCO–) and enzymes, salivary amylase and lysozyme.
The chemical process of digestion is initiated in the oral cavity by salivary amylase. About 30 per cent of starch is hydrolysed here by this enzyme (optimum pH 6.8) into a disaccharide – maltose.

Lysozyme acts as an antibacterial agent that prevents infections.

Stomach mucosa has Gastric glands having three major types of cells namely –

(i) Mucus neck cells which secrete mucus;

(ii) Peptic or chief cells which secrete the proenzyme pepsinogen

(iii) Parietal or oxyntic cells which secrete HCl and Castle’s intrinsic factor (essential for absorption of vitamin B12).

Stomach stores the food for 4-5 hours. The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme. The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme pepsin which converts proteins into proteoses and peptones (peptides).
The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid. HCl provides the acidic pH (pH 1.8) optimal for pepsin.
Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins. Small amounts of lipases are also secreted by gastric glands.

Small intestine receives 3 digestive juices i.e Bile juice, Pancreatic juice and Intestinal juice.

Pancreatic juice and bile are released through the hepato-pancreatic duct.

The pancreatic juice contains inactive enzymes – trypsinogen, chymotrypsinogen, procarboxypeptidases, and amylases, lipases and nucleases.
Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the pancreatic juice.

The bile released into duodenum contains bile pigments (bilirubin and biliverdin), bile salts, cholesterol and phospholipids but no enzymes. Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases.

The intestinal mucosal epithelium has goblet cells which secrete mucus. The secretions of the brush border cells of the mucosa alongwith the secretions of the goblet cells constitute the intestinal juice or succus entericus. This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc.
The mucus alongwith the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities. Sub-mucosal glands (Brunner’s glands) also help in this.
Proteins, proteoses and peptones in the chyme on reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice as given below:

Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides

Fats are broken down by lipases with the help of bile into di-and monoglycerides

Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides

The enzymes in the succus entericus act on the end products of the above reactions to form the respective simple absorbable forms. These final steps in digestion occur very close to the mucosal epithelial cells of

the intestine.

The breakdown of biomacromolecules mentioned above occurs in the duodenum region of the small intestine. The simple substances thus formed are absorbed in the jejunum and ileum regions of the small intestine. The undigested and unabsorbed substances are passed on to the large intestine.

No significant digestive activity occurs in the large intestine.

The functions of large intestine are:

(i) absorption of some water, minerals and certain drugs;

(ii) secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it for an easy passage.

The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine through ileo-caecal valve, which prevents the back flow of the faecal matter. It is temporarily stored in the rectum till defaecation.

The activities of the gastro-intestinal tract are under neural and hormonal control for proper coordination of different parts.
The sight, smell and/or the presence of food in the oral cavity can stimulate the secretion of saliva.
Gastric and intestinal secretions are also, similarly, stimulated by neural signals.
The muscular activities of different parts of the alimentary canal can also be moderated by neural mechanisms, both local and through CNS.
Hormonal control of the secretion of digestive juices is carried out by the local hormones produced by the gastric and intestinal mucosa.