Digestion is the process of breakdown of complex food materials into simpler compounds with the help of enzymes.

With the entry of food into the alimentary canal, the salts and water are found absorbed in it. But other particles composing the food are so large that they cannot be absorbed directly. It is necessary to break down them to fine particles for diffusion. This process is called digestion and is brought about by enzymes.



It is defined as a process that involves a chemical breakdown of complex food materials into simpler compound by enzymatic activity which can be readily used by the animal through absorption and assimilation.

Kinds of digestion:

1. Intracellular digestion:

It is the digestion carried out in the food vacuoles of the organism. It is called as intracellular digestion. It is seen in lower invertebrates.

2. Intercellular or extra cellular digestion:


By this process, digestion occurs inside the digestive tac.

Mechanical aspects:

1. Movement of food mass:

The food is propelled inside the digestive tract by muscular activities called peristalsis. It is defined as “a wave of muscular contraction proceeded by wave of relaxation which causes the contents of the hollow tube (gut) to be passed forwards.” By this process the bolus is pushed forward and also triturated.


Digestion in the alimentary canal:

Digestion in Mouth:

The food undergoes various operations inside the mouth such as –

(i) Moistened for easy chewing


(ii) Masticated with the help of grinding teeth, tongue and check muscles.

(iii) Tased by tastebud.

(iv) Partially digested by digestive enzyme and made into a bolus.

(v) Finally swallowed.



There are four pairs of salivary glands such as carotid. Infraorbital, sublingual and submandibular. They secrete saliva.


Reflex action and nervous stimulus activates salivary glands to secrete saliva.


Constituents of Saliva:

1. It is slightly acidic or pH is 6.7

2. Water is 99.5%

3. Inorganic constituents – Nacl, potassium chloride, calcium phosphate eetc.

4. Organic constituents – Mucin, ptyalin or salivary amylase and maltase.

Digestive Function:

(1) Ptyline acts on soluble starch and convert it into maltose or dextrin. (2) Maltose converts maltose into glucose (in man). (3) Degulatition: The bolus is pushed by tongue against hand palate into pharynx. The pharyngea muscles contract to push the food into oesophagus inside, where peristalsis occurs till it reaches the stomach. The chardiac sphincter avoids regurgitation.

(ii) Digestion in Stomach (protein)

The food is churned thoroughly by the wave like contraction and expansion of the uscles.

It stores food for 3-4 hours to avoid continuous feeding.

Chemical digestion:

Activator: The mucous membrane of stomach secretes gastrin, which activates gastric glands to secrete gastric juice. It also stimulates the secretion of insulin and glucagons of pancreas.

Composition of gastric juice:

1. It contains 99.5% of water.

2. The inorganic salts like sodium chloride, potassium chloride, cal-chloride, cal-phosphate, mag-phosphate and bicarbonates are present.

3. Organic materials such as mucin, pepsinogen, protein and gastric lipase are present.

4. Other substances, HCl (0.4% – 0.5%) is also present.

Action of HCl acid:

1. It stops the action of ptyalin.

2. It kills bacteria and germ.

3. It softens and breaks down the cementing materials between the cells.

4. It makes the medium acidic.

5. It regulates the opening and closing of pylorus.

6. It activities pepsinogen into pepsin and prorennin into rennin.

Action of Prorennin:

This inactive enzyme (found in young mammals) is converted into active rennin by HCl. It curdles milk, separating casein which is converted into paracasein, n the presence of calcium. This paracasein is digested by pepsin.

Action of Pepsinogn :

The inactive pepsinogen is activated by HCl into active pepsin. Pepsin converts more pepsinogen automatically into pepsin. It converts proteins and paracasein into peptones and proteoses.

Action of gastric-lipase:

It splits fat into fatty acid and glycerol.


This grinded semi-digested acidic food is called as chime. It passes through the pylorus in spurts into duodenum.

Digestion in small intestine:

It is divided into two parts. The food moves by segmentation contraction.

Digestion in dueodenum: The entry of acidic chye stimulates, duodenal mucosa to secrete, three hormones as activator.

(i) Enterogastrone: It slows gastric contraction and stops secretion of gastric juice.

(ii) Cholecytokinin: It stimulates gallbladder to secrete bile. It partially stimulates pancreas.

(iii) Secretin: It stimulates pancreas along with pancreozymin.

Composition of Pancreatic Juice:

1. Its pH is 8-8.3

2. It is alkaline.

3. It contains inorganic salts like icaronates of Na, K, Ca, Mg, Zn, etc.

Organic constituents are trypsinogen and chymotrypsinogen, Amylopsin, lipase.


1. The inactive trypsinogen and chymorypsinogen is converted into activetrypsin and chymotrypsin by enterokinase. Trypsin and chymotrypsin acts on rotein, peptones and proteoses, converting them into di and tri peptides.

Other protein splitting enzyme:

Carboxypepidase: It acts on di and tri peptides and converts them into gree-aminoacid.

Role of amylase: Amylase hydrolyses starch and carbohydrates into maltse.

Amylase à Starch à maltose

Role of maltose: It is present in taces and converts maltose into glucose.

(Pancreatic amylase is not present in human infants upto the age of 6 months).

Role of Ribonuclease: It hydrolyses RNA into nucleotides.

Composition of Bile:

1. It is alkaline

2. pH is 8-8.6.

3. Watr content is nearly 86%.

4. Mineral salt includes chlorides, carbonates, bicarbonates and phosphates of odium, potassium and calcium.

5. Bile salts like sodium taurocholate and sodium glycocholate reduce surface tension and emulsify fat- into fine droplets of one diameter.

6. Bile salts absorbs fat soluble vitamins like A, D, E and K.

7. Bile pigments comprises of Bilivardin and Bill rubin.

8. Bile has no enzyme.

Role of Lipase:

It hedrolyses emulished fat into fatty acids and glycerol.

Digestion in Ileum:

The intestinal gland secretes its secretion through crypts of Liberkuhn.

Stimulatio of intestinal secretion:

Duocrinin and enterocrinin stimulates the flow of intestinal juice or succus entericus.


1. It is alkaline.

2. It contains water and mucous.

3. It has chloride, bicarbonate, and phosphate of sodium, potassium, calcium and Mg.

Important enzymes and their functions:

1. Enterokinase converts inactive trypsinogen into active trypsin in duodenum.

2. Erepsin or amino peptidase converts di and tri-peptidase into amino acids.

3. Invertase acts on sucrose and converts it into glucose and fructose.

4. Maltose – Maltose – Glucose

5. Lactase – Lactose – Glucose and galactose.

6. Intestinal lipase – Emulsified fats – fatty acids & Glycerol.

7. Phospholipase – Phospholipids – Glycerol, fatty acids and phosphoric acid.

8. Phosphatase – Organic phosphate – free phosphate.

9. Polynucleotidase – nucleoacid – nucleicsides

10. Nucleosidase – nucleosides – Nitrogen bases, pentose sugar and phosphate.

This completely digested fod is called Chyle.

Digestion in Caecum:

The cellulases secreted by symbiotic bacteria in the caecum, converts cellulose into maltose.

Function of Large intestine:

1. It secretes mucin.

2. It absorbs water, drug and minerals.

3. It excretes excess of calcium, iron and bismuth.

4. It decomposes cellulose.


1. Mouth – There is no absorption except some drugs.

2. Stomach absorbs water, glucose, alcohol etc.

3. Small intestine.

The plicasemilunaris contains large number of villi which has capillaries of hepatic portal system and extension of lymphatic vessels (lacteal).

(i) The glucose, fructose etc are absorbed by the blood.

(ii) The amino acids are absorbed by blood through active transport mechanism.

(iii) The digested fats forms micells.

(iv) It is resynthesized with the help of bile salts into triacylglycerides and fat soluble by lacteals. It passes through thoracic duct into subclarian vein.

(v) The vitamins like B complex except B12 and C, which are water soluble and fat soluble vitamins like A,D, E and K are absorbed in small intestine.

(4) Large Intestine:

Maunly water is absorbed in the large intestine along with some medicines.


1. Glucose is absorbed and stored as glycogen by a process of glycogenesis with the help of insulin. Excess of glucose is stored as fat.

2. Aminoacids are required for building protoplasm and proteins.

The aminoacids are delaminated with the formation of urea and the rest produce energy by the Kreb cycle.

3. Fat is stored as adipose tissues and in the form of liquids. It helps to construct nuclear and plasma membrane.

Protection against self-digestion:

1. Mucous forms a chemical barrier.

2. The alkaline mucous neutraliises acidity.

3. Cll membrane has low permeability to hydrogen ion.

4. Most enzymes are secreted in inactive form.

5. Antienzymes are produced by epithelial cell to counteract enzymatic action.

6. Extracellular passages are absent.

7. Epithelial mucous cells are continuously replaced in every three days.

Constitution of faeces:

It is semisolid, brow paste consisting of water (67.70%) cellulose, insole and other protein residue, microorganisms, cells and mucous.


The walls of the rectum up pelvis, diaphragm and abdominal muscles contract to expel the faeces.


The rabbit eats up its own night faeces, which is semi solid and black, containing digested cellulose. IT helps also in the absorption of vitamins produced by bacterial action.


The protein metabolism is controlled by hormones. Ammonia is formed in deamination of aminoacids. It is harmful and its removal from the body is necessary. The extra carbohydrates and proteins are converted into fat in the liver. They remain stored in this form and are used at the time of need. Some energy is released which combines with neutral phosphate to make it powerful. It probably combines with ADP to from ATP which is used for various activities of the body.