Various digestive secretions and their actions in the gastrointestinal tract

The alimentary canal is otherwise called the gastro-intestinal tract. It extends from the mouth to the anus. The alimentary canal consists of the following main parts.

1. Buccopharyngeal cavity (mouth, buccal cavity and pharynx).

2. Oesophagus

3. Stomach

4. Sall intestine (duodenum, jejunum, ileum etc.)

5. Large intestine (caecum, vermiform appendix, colon,rectum and anus)

Digestive secretions in the gastro-intestinal tract:

1. Secretion of saliva in Buccopharyngeal cavity:

In the buccal cavity food is mixed woth saliva secreted by salivary glands. Presnce of smooth food increases salvation and rough food decreases salivation. Saliva is secreted by three pairs of salivary glands – 1. Partoid, 2. Submandibular, 3. Sublingual glands.

Saliva contains 99% water, 0.2% inorganic salts, mucous and enzymes. It has a pH-value between 6.0 – 7.4 and this slightly acidic. The enzymes present in the saliva are salivary amylase called ptyalin and a bacteriolytic enzyme called lysozyme.

Action of Saliva:

(i) Ptylin acrs on starch and converts it first into dextrins, shorter polysaccharides and then to diasaccharide, maltose.

(ii) Saliva lubricates the mouth and moistens the food. It makes mass of food suitable for swallowing.

(iii) Moistening of buccal cavity helps in movement of tongue. This facilities speech.

(iv) Moistening of taste buds stimulate the sense of taste.

(v) The antibacterial enzyme “lysozyme” dissolves the cell wall of many bacteria and finally kills them. That is why mammals lick their wounds to heal them.

2. Secretion of Gastric Juice stomach:

Soon after the food reaches the stomach a hormone called gastrin is secreted un the mucous membrane of the pyloric region of the stomach. Ot os then circulated in the blood to the rest of the stomach, where it stimulates the production of gastric Juice.

The gastric Juice contains 90% waer, small quantities of HCl (0.4%) and enzymes namely rennin, pepsin and gstric lipase.

Action of Hydrochloric acid:

(i) It converts the medium to become acidic (previously the medium was alkaline because of the saliva in the buccal cavity)

(ii) It prevents the action of ptyalin.

(iii) It softens the food.

(iv) It regulates the opening and closing of pyloric sphincter.

(v) It converts the inactivated propepsin or pepsinogen and prorenin to active pepsin and rennin respectively.

Action of Rennin:

Rennin is found in the gastric Juice of young mammals only. It is secreted as prorennin and after being converted to active rennin by the HCl, it changes milk into curd i.e. converts soluble casinogen (the protein of milk) into insoluble casein. Further it acts on casein in the presence of calcium ions and converts it into insoluble calcium paracasein i.e. curd (Solid). This curding is necessary because milk is to be retained in the stomach for a long period and to be acted upon thoroughly by the enzyme pepsin.

Prorennin – HCl – Rennin

(proenzyme)

Milk protein (casein) – Rennin – Curd (calcium paracasein) Ca⁺⁺

Action of Pepsin:

Like rennin, pepsin is secreted in the inactive state called propepsin or pepsinogen and is converted to active pepsin by HCl. It acts on proteins in the acidic medium and changes them into peptones and proteoses.

Propepsin Proenzyme or pepsinogen – HCl – Pepsin

Proteins – Pepsin – Peptones + Proteoses

Calcium paracasein – Pepsin – Peptones + Proteoses.

Action of gastric lipase : Gastric lipase is secreted in very small amount and it is also weal in action. It acts on fats and break them into fattyacids and glycerol. This enzyme is destroyed by alkali or high acidity.

Fats – gastric lipase – attyacids + glycerol.

3. Secretion of pancreatic juice and bile into the Duodenum:

In the duodenum the semidigested food of the stomach, the chyme, is subjected to two juices – pancreatic juice from the pancreas and bile from the liver.

Action of pancreatic juice:

Pancreaticjuice contain enzymes for the digestion of proteins, carbohydrate and fat. It also contains large amount of bicarbonate ions which neutralizes the acidity of the chime.

The proteolytic enzymes found in pancreatic juice are (i) trypsin (ii) chymotrypsin (iii) carboxypolypeptidase (iv) ribonuclease and (v) deoxyribonuclease. The first three proteolytic enzymes are secreted in inactive forms as trypsinogen, chymotrypsinogen and procarboxypolypeptidase. The trypsinogen become activated by an enzyme called enterakinase present in the intestinal juice secreted by intestinal mucosa. Trypsin once formed converts the other inactive trypsinogen, chymotrypsinogen and procarboxy polypeptidase to theis respective active forms.

Trypsinogen  – Enterakinase – Trypsin

Peptones and proteoses – trypsin – Peptides

Chymotrypsinogen – trypsin – Chymotrypsin

Procarboxypeptidase -Trypsin- Carboxypeptidase

Peptones and proteoses – Chymotrypsin – peptidases

Peptides – Carboxypeptidase – smaller peptides + amino acids

DNA – Deoxyribonuclease – Deoxyribonuclease

RNA – Ribonuclease – Ribonucleotides

The carbohydrate digesting enzyme i.e. pancreatic amylase hydrolyses starch, glycogen and other crbohydrates (except cellulose) into diasaccharides and trisaccharides.

Starch and Glycogen – Pancreatic amylase – Maltose + Isomaltose + Limit dextrin.

The enzymes for fat digestion are pancreatic lipase or steapsin.

Fats – pancreatic lipase – fattyacids + glycerol

Action of Bile secreted from liver:

Bile is a yellowish-green alkaline fluid. The chief constitutes of bile are the bile pigments and the bile salts. The two chief bile pigments are billirubin and biliverdin which areformed by the breakdown of haemoglobin of RBC. The bile salts include sodium glycoholat and sodium taurocholate.

Bile does not contain any digestive enzymes. The bile salts helps in emulsifying fats in the duodenum. Emulsification is aprocess of breaking down of large fat globules into very small fine droplets. The innumerable droplets of fats provide a large surface area for their hydrolysis by lipase. Being alkaline (pH 8.0 to 8.6) it neutralizes the acidity of the chime.

4. Secretion of intestinal juice in the ileum:

The intestinal juice is also called as succus entericus. When bile and pancreatit juice enters the duodenum at the sametime succus entericus secreted by the intestinal mucosa in the ileum.

Intestinal juice contains mucin and several enzymes such as erepsin or peptidase, lipase, invertase, maltase, lactase, isomaltase, limit dextrinase, nucleotidase and nucleosidase. These enzymes complete the digestion of all kinds of food substances.

The intestinal juice also contains enterakinase.

Peptides – Peptidase – Amino acids

Remaining emulsified fat – lipase – fatty acid + Glycerol

Sucrose – Invertase – Glucose + Fructose

Maltose – maltose – Glucose

Isomaltose – Isomaltose – Glucose

Limit dextrins – Limit dextrinase – Glucose

Nucleotides – Nucleotidase – Nucleotides + Phosphates

Nucleotides – Nucleotidase – Nitrogen bases + sugars