The intestine is the digestive system in most vertebrates. It follows directly behind the stomach. In the intestine, the final digestion of the food taken, the absorption of nutrients, as well as the removal (evacuation) of undigested substances are carried out.
In some animals, the digestive tract consists only of this section. At the same time, the length of the intestine depends on the type of animal, the characteristics of the food it uses and age.
The most primitive closed structure of this section of digestion is observed in flatworms and gastrointestinal (colonial cnidaria). The latter have a single gastrovascular cavity, through which a metabolism occurs between the individual that feeds and other members of the colony. In flatworms, there is one hole for eating and removing waste.
Invertebrates (insects, mollusks) have an intestine consisting of three sections (anterior, posterior and middle intestines). The mouth, goiter, pharynx, esophagus are included in the front part, the anus and rectum are located in the back part, in the middle - the intestines, malpighian vessels, stomach. Some coral polyps have an intestinal evertible. Digestion is thus carried out externally.
The larvae of the lamprey-tortoises and lancelet have a fairly rectum and small intestine. The organ begins immediately after the oropharyngeal cavity. The length of the intestines of adult lampreys is small (no more than the body itself). The organ begins immediately after the esophagus, has a valve in the form of a spiral. Due to this helical longitudinal fold, the area of absorption and digestion increases. A spiral valve is also present in sturgeons and sharks.
In many vertebrates, the length of the intestine allows you to divide it into departments - the rectum, colon and small intestine. The secrets of the pancreas and liver are removed to this organ. Pyloric appendages (blind processes) are located at the beginning of the intestines of bone fish. Their number may be different. Amphibians have a duodenum, and the rectum opens into the cloaca.
Land herbivorous reptiles have a well-developed cecum. It is located on the border of the thick and thin section. The intestines of the (thin) birds are several times longer than the body. Two blind outgrowths separate it from the posterior section, which opens into the cesspool. There is no rectum or colon.
Herbivorous animals have a fairly long intestines. At the same time, the blind and thick departments are well developed. The length of the colon in rodents can reach 53% of the total length of the entire intestine. In predators, it is much shorter. For example, a sheep has an intestine whose length exceeds the body length by 29-35 times, a wild boar - by 14, a horse - by 12, and a wolf - by 6. The thick section is inhabited by the symbiotic microflora, which is most developed in herbivores.
The intestines in humans are the largest organ of the digestive system. It starts from the stomach and ends with the anus (anus). The food mass is promoted through peristalsis (wave-like contractions). In the digestion process, microorganisms located in the organ and its microflora participate. In this section of the gastrointestinal tract, not only digestion is carried out, but also the absorption of beneficial elements from food, the synthesis of individual hormones, and immune processes.
In life, the length of the intestines in humans (adults) is about four meters. At the time of birth, she is about three meters, which is six times more than the growth of a newborn on average. The length of the human intestine after death increases and is about seven to eight meters. This is due to the relaxation of muscles after death.