The small intestine is a complex tube which lies in a spiral, allowing it to fit into a small space, and serves as the primary site for digestion and absorption. The small intestine consists of the duodenum, ileum and jejunum.
Gastrointestinal tract of the pig.
The wall of the small intestine has many villi (tiny finger-like projections), which increase the absorptive area of the small intestine. The cells lining the small intestine produce enzymes that aid digestion and absorb digested feed.
Chyme enters the small intestine through the pylorus. The chime is mixed by rhythmic segmentation of the small intestine. This is a type of intestinal movement that does not move the contents along the intestines but merely mixes them. At intervals, this rhythmic segmentation is interrupted by peristalsis which propels the ingesta along the intestine, towards the anus.
Functions of the intestinal movements:
- To mix ingesta with digestive enzymes.
- To bring ingesta into close contact with the mucous membrane for absorption.
- To move ingesta through the gut.
- To assist the flow of lymph and blood through the intestinal wall.
- To expel faeces from the rectum.
In the first section of the small intestine (the duodenum), secretions from the liver and pancreas are added. Secretions from the liver are stored in the gall bladder and pass into the intestine through the bile duct. These bile secretions aid in the digestion of fats.
The wall of the duodenum together with its associated glandular structures (pancreas, liver and intestinal glands) are responsible for producing and secreting enzymes and hormones necessary for the conversion of the partly digested products of the stomach into materials that can be absorbed by the body.
Protein-digesting enzymes (e.g., trypsin) from the pancreas and intestinal mucosa complete protein digestion in the small intestine. Fat-digesting enzymes from the pancreas (lipase) and intestinal mucosa complete fat digestion in the small intestine. To assist in fat digestion, bile from the liver emulsifies fat into smaller droplets in the small intestine.
After intestinal digestion chyme becomes chyle, which is a milky material that passes through the epithelium (lining) of the gut into the lymphatic vessels in the villi and from there into the bloodstream.
Most food nutrients are absorbed in the second and third parts of the small intestine, called the jejunum and the ileum, respectively. Undigested nutrients and secretions pass on to the large intestine through the ileocecal valve.
The intestinal glands
An intestinal gland is a gland found between villi in the intestinal epithelium lining of the small intestine and large intestine (or colon). Intestinal juice is derived from intestinal glands in the wall of the small intestine. These include:
Intestinal crypts (or crypts of Lieberkühn)
These intestinal glands are scattered throughout the entire small intestine. They secrete digestive enzymes, provide outlet ports for Brunner glands, and produce cells that replace surface-membrane cells shed from the tips of villi.
Duodenal glands (or Brunner’s glands)
These intestinal glands contribute mucous and are found only in the duodenum – more specifically in the submucosa of the duodenum. They secrete an alkaline fluid or mucous (called succus entericus) containing mucin (mucoproteins), water, and hydrogen carbonate ions. This succus entericus fluid helps to counteract the highly acidic and proteolytic chyme entering the small intestine from the stomach, and thus protects the duodenum from damage.
A hormone, enterocrinin, is secreted from the duodenum in response to the arrival of chyme from the stomach. This hormone affects the duodenum by promoting the production of large amounts of alkaline mucus by the submucosal glands of the small intestine (i.e., the duodenal glands).
The secretion of intestinal juices takes place as a result of food intake and the mechanical stimulation of the intestinal lining of the small intestine.
The functions of enzymes found in the intestinal juice:
- Enterokinase – converts trypsinogen into its active form trypsin, which aids in protein digestion.
- Maltase – converts maltose to two molecules of glucose.
- Sucrase – converts sucrose to glucose and fructose.
- Lactase – converts lactose to glucose and galactose.
- Lipase – digests fats by changing them into glycerol and fatty acids.
Peptidase – breaks peptones down to amino acids