Anatomically speaking, the small intestine is divided into three parts. These are the duodenum which is the first part of the small intestine. It is connected with the stomach and there is a valve between the stomach and the duodenum that is called gastro-duodenal valve that prevents food from back-flow of food from the duodenum to the stomach.
The second part of the small intestine is called the jejunum and the third part is called the ileum.
In contrast to the large intestine which has no observable role in absorption of nutrients except for absorbing water, the small intestine is very active in the process of nutrients absorption.
Enzymes in the form of lipases and amylases are secreted from the pancreas into the duodenum. These enzymes are not active and become functional and activated in the duodenum. These enzymes help in digesting carbohydrates and fats in the food. Lipase being specific for fats and amylase being specific for carbohydrates.
Secretions from the gall bladder that are called bile secretions also contribute to digesting the food in the intestine by emulsifying it.
The propulsion of food in the small intestine is controlled by both segmentation movement and peristalsis.
The secretion of hormones also occur in the small intestine in addition to the stomach. An example of such a secretion is the hormone cholecystokinin which is secreted by the jejunum. The secretion of cholecystokinin is stimulated by fatty food in the stomach. It increases the secretion of bile from the gallbladder . Thus contributing to the digestion of food. In addition to this role of cholecystokinin it also inhibits the movement of the stomach slightly. This helps in completing the digesting process of food in the stomach.
Another hormone that is secreted by the duodenum is called secretin. Its secretion is stimulated by excess acid in the gastric juice in the stomach. It functions by inhibiting the movement of the gastrointestinal tract slightly.
Another inhibitory hormone to the function of the stomach motility is called gastric inhibitory peptide or GIP. It is secreted by the small intestine in its earlier portion.
Some disorders of the small intestine include abnormal digestion of food due to failure of the pancreas.
The pancreas is responsible for secreting digestive enzymes from the pancreas to the duodenum such as lipases and amylases. These enzymes function by degrading carbohydrates and fats into its constituents.
In pancreartitis the pancreas is unable to secrete these enzymes, thus the digestion in the small intestine is impaired.
Another malabsorption condition is called Sprue. An example occurs as a result of the toxin gluten. Gluten destroys the epithelium of the small intestine probably due to autoimmune reaction. Thus impairing the absorption process of food in the intestinal canal.
The small intestine is connected with the large intestine through the ileo-cecal connection. The ileo-cecal valve prevents food from back-flowing from the cecum of the colon to the ileum of the small intestine.