Digestive System
The animal’s digestive system is considered one of the diverse and complex systems. Organs related to the digestive system work continuously as a team to fulfil a single task: transforming the raw materials of the food into essential nutrients (e.g.
carbohydrates, fats, proteins, minerals, and vitamins) and energy towards growth, maintenance, and reproduction. Generally, we can divide the digestive system into four main components. (1) Gastrointestinal tract (GIT), which transports food from mouth to rectum; (2) trio organs, i.e. liver, pancreas, and gallbladder, break down the food through enzymes or digestive juices; (3) it is a combination of enzymes, hormones, blood, and nerves, breaks down food, modulates the digestive process, and delivers the final nutrients to respective tissues and cells; (4) mesentery, a membrane that supports and holds the digestive organs to the abdominal wall.Animals can digest food through various mechanical and chemical processes. For instance, complex food is mashed to pieces mechanically through teeth whenever food enters the mouth, whereas saliva carries out chemical digestion. Enzymes in saliva like amylase convert complex carbohydrates to simpler carbohydrates. To depict an overall pipeline of digestion, food is ingested as mentioned; it is converted into smaller pieces and mixed with digestive enzymes and transported through motor or muscular activities of GIT. In addition to protecting and lubricating the gastrointestinal tract, secretions from the salivary gland, stomach, gall bladder, and intestines also assist digestion. Digestive enzymes hydrolyse the nutrients in the stomach like carbohydrates into simpler sugars, fats into fatty acids and glycerol, and proteins into amino acids. The main job of a digestive system does not stop at the stomach. Abovementioned nutrients need to be absorbed into the bloodstream; this process occurs in the small intestine. Specialised structures called villi in the intestine increase the surface area of the intestines. More the number of villi more the absorption of nutrients. The nutrients are absorbed into the villi through diffusion and transported into the bloodstream through capillaries in the villi. Rest that is not absorbed are pushed out of the system through the rectum. The digestion process in animals varies according to species, habitat, nature of feeding, and many more. For example, ruminants (cattle, sheep, goats) get nutrients through the breakdown of cellulose of plant cells. Here animals depend on large populations of bacteria and protozoa to ferment the food and derive the nutrients to the animal’s body. Whereas in simple stomach, animals such as horses and pigs, digestion of food occurs in the stomach with the help of digestive enzymes produced in the stomach. The comparative digestive process has been explained in further chapters.
1.7