The small intestine is where most chemical digestion occurs. Most of the digestive enzymes that act in the small intestine are secreted by the pancreas and enter the small intestine via the pancreatic duct. The enzymes enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients. The hormone secretin also causes bicarbonate to be released into the small intestine from the pancreas in order to neutralize the potentially harmful acid coming from the stomach.
Small Intestine
This image shows the position of the small intestine in the gastrointestinal tract.
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 large 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 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 are the "main man" that holds 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 most adult humans and for them lactose, like most polysaccharides, are 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.
The fat soluble vitamins A, D, and E are absorbed in the upper small intestine. The factors that cause malabsorption of fat can also affect absorption of these vitamins. Vitamin B12 is absorbed in the ilium and must be bound to intrinsic factor, a protein secreted in the stomach, in order to be absorbed. If intrinsic factor is missing, then Vitamin B12 is not absorbed and pernicious anemia results. Of the water soluble vitamins, transport of Folate and B12 across the apical membrane are Na+ independent, but the other water soluble vitamins are absorbed by Na+ co-transporters. In physiology, the primary ions of electrolytes are sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), chloride (Cl−), hydrogen phosphate (HPO42−), and hydrogen carbonate (HCO3−). Sodium is the main electrolyte found in extracellular fluid and is involved in fluid balance and blood pressure control. Electrolyte balance is regulated by hormones, generally with the kidneys flushing out excess levels. In humans, electrolyte homeostasis is regulated by hormones such as antidiuretic, aldosterone, and parathyroid hormone. Serious electrolyte disturbances, such as dehydration and overhydration, may lead to cardiac and neurological complications and, unless they are rapidly resolved, will result in a medical emergency. Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the pH is slightly acidic about 5.6 - 6.9.