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Digestive System

Digestive system is the collection of organs that work in co-operation to break down food into small and absorbable substance.
It consist of organs such as the mouth, oesophagus, the stomach, intestines, liver, etc., called the alimentary canal.

Farm animals show variation in the general structure and functions of their stomach. While cattle, sheep and goats have their stomach modified into four chambers, (polygastric animals or ruminants), domestic fowls, pigs, rabbits and even man have a simple stomach. These animals are called monogastric animals.

Digestion in Monogastric Animals e.g. pig and rabbit.

Mouth: Here, the food is chewed and masticated, mixed with saliva, rolled into a bolus and sent through the oesophagus to the stomach by peristaltic movement. During its stay in the mouth, the food reacts with an enzyme called ptyalin or salivary amylase which converts starch to maltose. Ptyalin works best in alkaline medium.

Oesophagus: Digestion in its real sense does not take place in the oesophagus. It merely acts as a passage. When food enters the oesophagus, its muscular walls will begin to contract and expand in a form of movement called peristaltic movement. This pushes the food into the stomach.

Stomach: Here, gastric juice is secreted; gastric juice contains the enzymes pepsin, rennin and hydrochloric acid. Pepsin acts on protein and converts it into proteases and peptones. Renin coagulates milk into curdle milk that is, caseinogens to insoluble casein. Hydrochloric acid (HCL) stops the effect of the ptyalin from the mouth. The food which is churned into a thick paste called chyme is then passed into the duodenum.

Duodenum: This is the first region of the small intestine. As food enters the duodenum, the walls of the intestine secrete the hormones, which stimulates the secretion of digestive juice from the liver and the pancreas.

The liver secretes bile (greenish liquid containing alkaline), which emulsifies fats. The pancreas on the other hand secretes pancreatic juice made up of the following enzymes:

(a) Amylase which converts starch to maltose.
(b) Lipase converts emulsified fats (globules) into fatty acids and glycerol. Thus, fat digestion is completed here.
(c) Trypsin converts peptones to polypeptides.

The food now becomes watery and is called chyme and is passed into the ileum.

(a) Peptidase – converts peptides to amino-acids.
(b) Sucrase – converts sucrose into glucose and fructose.
(c) Lactase  � converts lactose into glucose and galactose.
(d) Maltase – converts maltose into glucose.

At this juncture, the digested material in form of glucose, amino acid, fatty acid and glycerol are absorbed into the blood stream through the villi of the ileum. The villi are tiny finger-like projections on the surface of the ileum.

The other food materials not digested or absorbed are passed to the colon or large intestine. The colon is concerned with the re-absorption of water into body. The semi solid materials left are called faeces and consist of undigested food, dead bacteria, mucus, dead cells, surplus bile and other secretions. This is then passed through the rectum to the anus and voided.

Digestive system of a rabbit

Digestive System of a bird

Birds, unlike rabbits or pigs have no teeth, lips or cheeks and as such gather their food with the help of a beak and a tongue which has a projection for forcing in food through the oesophagus to the crop.

Crop: This is used temporarily to store bulky foods so that they can be acted upon by bacteria. The food which is by now moistened and fermented is passed to the proventriculus.

Proventriculus or first stomach: Here, gastric juice is secreted which contains hydrochloric acid pepsin, which will act on the food accordingly. The food then moves to the gizzard.

Gizzard: The gizzard acts as teeth by grinding the food eaten by the bird. Bile juice from the liver is mixed with the food here. The food now enters the duodenum, ileum and later the colon.

At the junction of the small and large intestines, there is a pair of blind tubes called caeca (single caecum). The caecum harbours micro-organisms which help in the digestion of cellulose into glucose. The faeces is egested through the cloaca to the exterior via the vent.

image_thumb3Digestive system of a bird

Digestive system of a ruminant

Farm animals like sheep, goat, cow, deer have complex stomach structure. This is because their stomach is divided into four compartments namely the rumen, reticulum, omasum and abomasums which is the true stomach.

The ruminants gather their food (mainly rough herbage), swallow it in a hurry without chewing. The food passes to the rumen through the oesophagus. The food is temporarily stored in the rumen which is the first stomach and it is richly supplied with bacteria and protozoan which act on the food and through which they can synthesise their amino acids. Thus it is not necessary to supplement ruminants with amino acids. The food then passes to the reticulum (second stomach).

The food stays in the reticule for a short period until its appetite is satisfied. When the ruminant is resting, it brings back the partially digested food (cud) to the mouth by anti -peristaltic movement called regurgitation.

The food which by now is pulpy is re-swallowed into the omasum which is also known as the manyplies or psalterium. From here, the food again passes to the abomasums which functions as the true stomach in monogastric animals. The food then leaves here to the small intestine where further digestion and absorption occur as in the monogastric animals. The undigested and unabsorbed food substances pass through the anus as faeces.

Differences Between Monogastric and Ruminant Digestive System



Alimentary canal of a ruminant animal


Absorption of food

After digestion of food, absorption takes place. This occurs in tiny finger-like structures in the surface of the ileum called the villi. Fats and oils are absorbed at the lacteal, sugar is absorbed at the arterile end while the amino acids are absorbed at the venule end. After absorption, excess sugar is converted into glycogen in the liver and later to fats; whereas proteins are converted into urea. When absorption is complete, assimilation takes place.

Structure of the villus