The pancreas consists of clusters if endocrine cells (the islets of Langerhans) and exocrine cells whose secretions drain into the duodenum.
Pancreatic fluid contains:
- sodium bicarbonate (NaHCO3). This neutralizes the acidity of the fluid arriving from the stomach raising its pH to about 8.
- pancreatic amylase. This enzyme hydrolyzes starch into a mixture of maltose and glucose.
- pancreatic lipase. The enzyme hydrolyzes ingested fats into a mixture of fatty acids and monoglycerides. Its action is enhanced by the detergent effect of bile.
In April 1999, the FDA approved orlistat as a treatment for obesity. Orlistat inactivates pancreatic lipase. About one-third of ingested fats fails to be broken down into absorbable fatty acids and monoglycerides and simply passes out in the feces.
- 4 "zymogens" — proteins that are precursors to active proteases. These are immediately converted into the active proteolytic enzymes:
- trypsin. Trypsin cleaves peptide bonds on the C-terminal side of arginines and lysines.
- chymotrypsin. Chymotrypsin cuts on the C-terminal side of tyrosine, phenylalanine, and tryptophan residues (the same bonds as pepsin, whose action ceases when the NaHCO3 raises the pH of the intestinal contents).
- elastase. Elastase cuts peptide bonds next to small, uncharged side chains such as those of alanine and serine.
Trypsin, chymotrypsin, and elastase are members of the family of serine proteases. Link to discussion.
- carboxypeptidase. This enzyme removes, one by one, the amino acids at the C-terminal of peptides.
- nucleases. These hydrolyze ingested nucleic acids (RNA and DNA) into their component nucleotides.
The secretion of pancreatic fluid is controlled by two hormones:
- secretin, which mainly affects the release of sodium bicarbonate, and
- cholecystokinin (CCK), which stimulates the release of the digestive enzymes.
The small intestine
Digestion within the small intestine produces a mixture of disaccharides, peptides, fatty acids, and monoglycerides. The final digestion and absorption of these substances occurs in the villi, which line the inner surface of the small intestine.
This scanning electron micrograph (courtesy of Keith R. Porter) shows the villi carpeting the inner surface of the small intestine.
The crypts at the base of the villi contain stem cells that continuously divide by mitosis producing
- more stem cells
- cells that migrate up the surface of the villus while differentiating into
- columnar epithelial cells (the majority). They are responsible for digestion and absorption.
- goblet cells, which secrete mucus;
- endocrine cells, which secrete a variety of hormones;
Link to Gut Hormones.
- Paneth cells, which secrete antimicrobial peptides [Link to discussion] that sterilize the contents of the intestine.
All of these cells replace older cells that continuously die by apoptosis.
The villi increase the surface area of the small intestine to many times what it would be if it were simply a tube with smooth walls. In addition, the apical (exposed) surface of the epithelial cells of each villus is covered with microvilli (also known as a "brush border"). Thanks largely to these, the total surface area of the intestine is almost 200 square meters, about the size of the singles area of a tennis court and some 100 times the surface area of the exterior of the body.
The electron micrograph (courtesy of Dr. Sam L. Clark) shows the microvilli of a mouse intestinal cell.
Incorporated in the plasma membrane of the microvilli are a number of enzymes that complete digestion:
- aminopeptidases attack the amino terminal (N-terminal) of peptides producing amino acids.
- disaccharidasesThese enzymes convert disaccharides into their monosaccharide subunits.
Fructose simply diffuses into the villi, but both glucose and galactose are absorbed by active transport.
- fatty acids and monoglycerides. These become resynthesized into fats as they enter the cells of the villus. The resulting small droplets of fat are then discharged by exocytosis into the lymph vessels, called lacteals, draining the villi.
Humans with a rare genetic inability to form microvilli die of starvation.