Читать книгу King's Applied Anatomy of the Abdomen and Pelvis of Domestic Mammals онлайн
26 страница из 32
1 Transport
2 Physical treatment
3 Chemical treatment
4 Absorption.
2.2.1 Transport of food
Food transport, i.e. the active passage through the digestive tract, is promoted by the musculature that is located throughout the entire length of the digestive tract. There are two muscular coats comprising an inner circular and an outer longitudinal layer, together with a variable layer of oblique fibres. Contractions of the muscle layers promote the propulsion of food through the gastrointestinal tract, and segmental contractions ensure mixing of the intestinal contents.
gap junctions
boluses,
peristalsisbolusmucus
2.2.2 Physical treatment of food
chyme
2.2.3 Chemical treatment of food
Water, salts and vitamins can be absorbed directly by the appropriate lining cells of the intestines, whereas enzymes and certain other substances are needed for the prior breakdown of proteins, carbohydrates and fats. These enzymes are produced and secreted by cells within the digestive tract, that is, by cells lining the stomach and intestines, and by glands external to the digestive tract. The latter comprise the salivary glands, liver and pancreas, which are derived embryologically from the developing digestive tract and are important sources not only of enzymes but also of other substances such as bile salts that are involved in the emulsification of fats. The production of enzymes to facilitate the breakdown of food in the digestive tract represents a marked degree of specialisation on the part of the cells concerned, since the enzymes are produced in large quantities and are then secreted to act beyond the confines of the producer cells. All body cells produce a variety of enzymes, but in general these act intracellularly.