Whatever food we eat is in complex form, which is not absorbed by the body as such, first, it has to be broken down in the simplest form which could be easily absorbed in the bloodstream; this process of chemical and mechanical breakdown of food is simply called ‘digestion’.
Digestion of food starts of course with the process of ‘ingestion’ that is taking food in the mouth, then food is chewed by the teeth, with the help of tongue and saliva called ‘mastication’, the process of digestion starts with the mouth itself with the salivary amylase present in saliva which is secreted by salivary gland. Here digestion of carbohydrate starts and the rest of the foods are also broken into smaller particles and mixed with saliva called ‘bolus’. This ‘bolus’ then goes to the stomach via esophagus by its special movement called peristalsis, hydrochloric acid kills most contaminating microorganisms and begins the mechanical breakdown of some food (e.g., denaturation of protein), and chemical alteration of some. The hydrochloric acid also has a low pH, which allows enzymes to work more efficiently. After some time (typically an hour or two in humans, 4–6 hours in dogs, somewhat shorter duration in house cats, …), the resulting thick liquid is called chime. A chime will go through the small intestine, where 95% of absorption of nutrients occurs, through the large intestine with waste material eventually being eliminated during defecation. This process is done by a particular system in the body called the gastrointestinal system or tract which stretches from the mouth to anus.
The final result of the digestive enzyme described is to reduce the foodstuff of the diet to forms that can be absorbed and assimilated. These end products of digestion are, for carbohydrates, the monosaccharides (principally glucose); for proteins, the amino acids; and for triacylglycerol, fatty acid, glycerol, and monoacylglycerols.
Before nutrients can be absorbed, food particles must be digested – converted into usable compounds by acids and enzymes that are released as the food moves from point to point along the tract. Different kinds of food are treated in different parts of the system – some sugars are partially digested in the mouth, while proteins begin to be broken down in the stomach; most of the digestion and absorption, however, takes place in the small intestine. The passage from one section to another is controlled by the one-way valves known as sphincters.
To move the food along this processing line, the esophagus, stomach, and intestines employ to squeeze the tube shut as rapidly as 12 times a minute. These contractions move in waves down the tube, squeezing the food ahead in much the same way fingers squeeze toothpaste out of a tube.
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