In vertebrates, the small intestine is the part of the gastrointestinal tract (gut) following the stomach and followed by the large intestine, and is where the vast majority of digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to describe the entire intestine. This article is primarily about the human gut, though the information about its processes is directly applicable to most mammals. (A major exception to this are cows; for information about digestion in cows and other similar mammals, see ruminants.)
Size and divisions
The small intestine in an adult human measures on average about 5 meters (16 feet), with a normal range of 3 - 7 meters; it can measure around 50% longer at autopsy because of loss of smooth muscle tone after death. It is approximately 2.5-3 cm in diameter.
The surface of the small intestine is increased by its special structure, and it is about 200-250 square meters.
The small intestine is divided into three structural parts: ..
The three sections of the small intestine look similar to each other at a macroscopic level, but there are some important differences.
The parts of the intestine are as follows:
Digestion and absorption
Food from the stomach is allowed into the duodenum by a muscle called the pylorus, or pyloric sphincter, and is then pushed through the small intestine by a process of muscular-wavelike contractions called peristalsis.
The small intestine is where most chemical digestion takes place. Most of the digestive enzymes that act in the small intestine are secreted by the pancreas and enter the small intestine via the pancreatic duct. The enzymes enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients. The hormone secretin also causes bicarbonate to be released into the small intestine from the pancreas in order to neutralize the potentially harmful acid coming from the stomach.
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