Functional regions of digestive system
In general the digestive system of metazoans is divided into 4 major functional regions of:
- Reception
- Conduction and storage
- Digestion and absorption
- Conduction and formation of faeces
The region for reception is associated with devices for mastication or chewing of food (like teeth); for paralysing the struggling prey (toxic enzymes from saliva); initiating digestion and lubricating the food with mucous. The oesophagus of chordates and some invertebrates serves to conduct the bolus (mass of chewed food) by peristaltic movement from buccal cavity. In some animals this region has a crop for storage. The crop in birds is also used to ferment mildly or digest food. This is later regurgitated by parent birds for their nestlings. The storage region allows the animals to store food and use it when it is not easily available. For example, leaches take in infrequent large meals of blood and digest it slowly over a month.
The herbivore animal spends hours masticating the food it takes in hurridly and stores it in its stomach for further use. In the third region or digestive region the enzymes reduce the food to a form that can be absorbed by the body of the organism. As the food is digested, the absorbable food is passed to the blood stream and the unabsorbed material is stored briefly in the final section of the alimentary canal where further removal of excess water and, consolidation of undigested material into faeces takes place, before it is expelled out of the body. In vertebrates this function is carried out in the large intestine. In higher vertebrates, each area of the gut is specialised for a certain activity, digestive enzyme are produced in glands as well as in the wall of the gut. Absorption occurs in the intestine predominantly.