Digestion is a vital body process that is carried out by the organs in the digestive system. It involves the breaking down of food into small molecules that the body can absorb. The small intestine is a key organ in this body system where most of the chemical digestion and absorption of food is accomplished. It is the part of the gastrointestinal tract that comes after the stomach and before the large intestine.
The small intestine and its function
The small intestine is approximately 6.7 to 7.6 meters long. It is divided into three sections: the duodenum, the jejunum, and the ileum. The duodenum is the first section of the small intestine that comes right after the stomach; the jejunum is the second section while the ileum is the third section which directly connects to the large intestine.
The chief function of the small intestine is to digest food chemically and absorb the nutrients for the body to use. To optimize absorption, the small intestine is convoluted and has a vast surface area because of the presence of folds, projections (villi) and micro projections (microvilli). It is said that its total surface area is approximately 600 sq. meters (about the size of a baseball field).
The three sections of the small intestine vary in terms of function. The duodenum is the secretory portion of the small intestine. It is also where iron is absorbed. The jejunum is the main absorptive section where most nutrients are absorbed. Playing a minimal role in absorption, the ileum is where vitamin B12 and bile are absorbed.
Enzymes in the small intestine
There are enzymes, hormones and other substances in the small intestine to enable it to carry out its function in chemical digestion and absorption of nutrients. Digestive enzymes are substances that break down polymeric macromolecules into their smaller building blocks so that they can be easily absorbed.
Majority of the enzymes that act in the small intestine are produced by the pancreas and released into the small intestine via the pancreatic duct. Digestive enzymes are classified according to their targeted substrates or substances that they degrade. Those that break down proteins and peptides into amino acids are known as proteases and peptidases, respectively. Lipases are enzymes that split lipids and fats into fatty acids and glycerol. Maltase, sucrase, lactase, amylase, etc. are enzymes that break down carbohydrates like starch and sugars into their basic forms. And nucleases break down nucleic acids into nucleotides.
Pancreatic proteases released into the small intestine include trypsin and chymotrypsin which chop proteins into smaller peptides. A pancreatic brush border enzyme called carboxypeptidase breaks down peptides into amino acids one at a time. Aminopeptidase and dipeptidase act to free the resulting amino acid products.
Hormones produced by the small intestine
The duodenum) secretes a number of hormones, including secretin, cholecystokinin (CCK), gastric inhibitory peptide (GIP), motilin and somatostatin.
Secretin stimulates the pancreas to release bicarbonate into the small intestine to neutralize the potentially harmful acidity of chime coming from the stomach. CCK increases secretion of pancreatic juice, stimulates release of digestive enzymes into the small intestine, increases gallbladder contraction and release of bile into the small intestine and decreases gastric activity among others. GIP slows down gastric activity. Motilin acts on specialized receptors in the gastrointestinal (GI) tract causing increased GI motility. And somatostatin – which is produced by both the mucosal lining of the duodenum and the delta cells of the pancreas – acts to inhibit various secretory mechanisms.
Other secretions in the small intestine
Aside from the pancreas, the gallbladder also secretes a substance into the small intestine. This substance is the bile, a digestive fluid which is produced in the liver and stored in the gallbladder for release into the small intestine. Bile does not contain enzymes but it helps degrade fats into fatty acids to be absorbed by the digestive tract. It is mostly made up of cholesterol, bile acids (also known as bile salts), and bilirubin (produced when red blood are broken down). Bile also contains water and salts (potassium and sodium), and minimal amounts of copper and metals.
Absorption takes place once nutrients are broken down into their respective end products small enough for the body to absorb. Most nutrients are absorbed in the small intestine via the epithelial cells lining the intestinal mucosa. The absorbed substances are then transported into the capillaries then to the larger blood vessels for distribution to the various organs of the body.