The duodenum absorbs the nutrients and it does it even more than the stomach. Because of that, in obese people, the duodenum is frequently bypassed in gastric bypass surgery to decrease the absorption of nutrients.
About 20,000 protein-coding genes are expressed in human cells and 70 % of these genes are expressed in the normal duodenum. Some 300 of these genes are more specifically expressed in the duodenum with very few genes expressed only in the duodenum. The corresponding specific proteins are expressed in the duodenal mucosa and many of these are also expressed in the small intestine, such as ANPEP, a digestive enzyme, ACE an enzyme involved in the control of blood pressure, and RBP2 a protein involved in the uptake of vitamin A. From here the foods moves into the Jejunum.
The Jejunum (subpage digestion content)
The lining of the jejunum is specialized for the absorption, by enterocytes, of small nutrient particles which have been previously digested by enzymes in the duodenum. Once absorbed, nutrients (with the exception of fat, which goes to the lymph) pass from the enterocytes into the enterohepatic circulation and enter the liver via the hepatic portal vein, where the blood is processed. The jejunum is involved in magnesium absorption. The pH in the jejunum is usually between 7 and 9 (neutral or slightly alkaline). The jejunum has less fat inside its mesentery than the ileum.
The jejunum is typical of larger diameter than the ileum.
The ileum is the third and final part of the small intestine. It follows the jejunum and ends at the ileocecal junction, where the terminal ileum communicates with the cecum of the large intestine through the ileocecal valve. The ileum has more fat inside the mesentery than the jejunum. The diameter of its lumen is smaller and has thinner walls than the jejunum.
Its circular folds are smaller and absent in the terminal part of the ileum. While the length of the intestinal tract contains lymphoid tissue, only the ileum has abundant Peyer’s patches, unencapsulated lymphoid nodules that contain large numbers of lymphocytes and other cells of the immune system.
The function of the ileum is mainly to absorb vitamin B12 and bile salts and whatever products of digestion were not absorbed by the jejunum. The wall itself is made up of folds, each of which has many tiny finger-like projections known as villi on its surface. In turn, the epithelial cells that line these villi possess even larger numbers of microvilli. Therefore, the ileum has an extremely large surface area both for the adsorption (attachment) of enzyme molecules and for the absorption of products of digestion. The DNES (diffuse neuroendocrine system) cells of the ileum secrete various hormones (gastrin, secretin, cholecystokinin) into the blood. Cells in the lining of the ileum secrete the protease and carbohydrase enzymes responsible for the final stages of protein and carbohydrate digestion into the lumen of the intestine. These enzymes are present in the cytoplasm of the epithelial cells.
The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the hepatic portal vein and the liver. Lacteals are small lymph vessels and are present in villi. They absorb fatty acid and glycerol, the products of fat digestion. Layers of circular and longitudinal smooth muscle enable the chyme (partly digested food and water) to be pushed along the ileum by waves of muscle contractions called peristalsis. The remaining chyme is passed to the colon.
Problems with digestion
In general, it is possible to say that the first problems with our health start with food digestion. Babies have problems with an empty stomach but also when food is not digested the right way. Our digestion can be considered one of the important parts of the aging process. It is extensively used and suffers from many attacks. The production of saliva, enzymes and other products varies during a lifetime. But surely and steadily there is decline. This decline cannot be stopped but delayed.
To see what can go wrong here are a few simple samples. The umbrella term for stomach disease is gastropathy. Some common stomach disorders include:
Dyspepsia: This is a condition characterized by a feeling of fullness; indigestion; and pain in the upper abdomen or lower chest. Other symptoms include nausea and vomiting. Dyspepsia can be a precursor to GERD and may also indicate angina.
GERD: Gastroesophageal reflux disease is a condition in which the stomach acids rise up the esophageal passage. Heartburn is a common symptom of the condition.
Peptic Ulcers – These can occur when the protective mucous lining of the stomach walls is damaged by the stomach acids. The bacterium, Helicobacter pylori is thought to be an important factor in the development of gastric and duodenal ulcers.