Food from the stomach is allowed into the duodenum through the
pylorus by a muscle called the
pyloric sphincter.
Digestion The small intestine is where most chemical digestion takes place. Many of the
digestive enzymes that act in the small intestine are secreted by the
pancreas and
liver and enter the small intestine via the
pancreatic duct. Pancreatic enzymes and
bile from the gallbladder enter the small intestine in response to the hormone
cholecystokinin, which is produced in the response to the presence of nutrients.
Secretin, another hormone produced in the small intestine, causes additional effects on the pancreas, where it promotes the release of
bicarbonate into the duodenum in order to neutralize the potentially harmful acid coming from the stomach. The three major classes of nutrients that undergo digestion are
proteins,
lipids (fats) and
carbohydrates: • Proteins are degraded into small
peptides and
amino acids before absorption. Chemical breakdown begins in the stomach and continues in the small intestine. Proteolytic enzymes, including
trypsin and
chymotrypsin, are secreted by the
pancreas and cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic
brush border enzyme, splits one amino acid at a time.
Aminopeptidase and
dipeptidase free the end amino acid products. • Lipids (fats) are degraded into
fatty acids and
glycerol. Pancreatic lipase breaks down
triglycerides into free fatty acids and
monoglycerides. Pancreatic lipase works with the help of the salts from the
bile secreted by the
liver and stored in the
gall bladder. Bile salts attach to triglycerides to help
emulsify them, which aids access by pancreatic lipase. This occurs because the lipase is water-soluble but the fatty triglycerides are hydrophobic and tend to orient towards each other and away from the watery intestinal surroundings. The bile salts emulsify the triglycerides in the watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption. • Some carbohydrates are degraded into simple sugars, or
monosaccharides (e.g.,
glucose). Pancreatic amylase breaks down some carbohydrates (notably
starch) into oligosaccharides. Other carbohydrates pass undigested into the large intestine for further handling by
intestinal bacteria. Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase, which further break down oligosaccharides. Other brush border enzymes are maltase, sucrase and lactase. Lactase is absent in some adult humans and, for them, lactose (a disaccharide), as well as most polysaccharides, is not digested in the small intestine. Some carbohydrates, such as
cellulose, are not digested at all, despite being made of multiple
glucose units. This is because the cellulose is made out of beta-glucose, making the inter-monosaccharidal bindings different from the ones present in starch, which consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds, something reserved for herbivores and bacteria from the large intestine.
Absorption Digested food is now able to pass into the blood vessels in the wall of the intestine through either
diffusion or
active transport. The small intestine is the site where most of the nutrients from ingested food are absorbed. The inner wall, or mucosa, of the small intestine, is lined with
intestinal epithelium, a
simple columnar epithelium. Structurally, the mucosa is covered in wrinkles or flaps called
circular folds, which are considered permanent features in the mucosa. They are distinct from
rugae which are considered non-permanent or temporary allowing for distention and contraction. From the circular folds project microscopic finger-like pieces of tissue called
villi (
Latin for "shaggy hair"). The individual epithelial cells also have finger-like projections known as
microvilli. The functions of the circular folds, the villi, and the microvilli are to increase the amount of surface area available for the absorption of
nutrients, and to limit the loss of said nutrients to intestinal fauna. Each villus has a network of
capillaries and fine lymphatic vessels called
lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries (amino acids and carbohydrates) and lacteals (lipids). The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by our body. The material that remains undigested and unabsorbed passes into the large intestine. Absorption of the majority of nutrients takes place in the
jejunum, with the following notable exceptions: •
Iron is absorbed in the duodenum. •
Folate (Vitamin B9) is absorbed in the duodenum and jejunum. •
Vitamin B12 and
bile salts are absorbed in the
terminal ileum. Vitamin B12 will only be absorbed by the ileum after binding to a protein known as
intrinsic factor. • Water is absorbed by
osmosis and
lipids by passive diffusion throughout the small intestine. •
Sodium bicarbonate is absorbed by active transport and
glucose and
amino acid co-transport •
Fructose is absorbed by
facilitated diffusion.
Water Immunological The small intestine supports the body's
immune system. The presence of
gut flora appears to contribute positively to the host's immune system.
Peyer's patches, located within the ileum of the small intestine, are an important part of the digestive tract's local immune system. They are part of the lymphatic system, and provide a site for antigens from potentially harmful bacteria or other microorganisms in the digestive tract to be sampled, and subsequently presented to the immune system. ==Clinical significance==