Small intestine
The portion of the digestive tract between the stomach and large intestine. Most enzymatic digestion and nutrient absorption occur there.
Villi: Fingerlike projections of the intestinal lining that increase the surface area available for absorption. These projections expand the small intestine’s absorptive surface.
Duodenum: The first section of the small intestine, receiving stomach contents, bile, and pancreatic secretions. It is where acidic chyme first mixes with major digestive secretions.
Celiac disease: An autoimmune disorder in which gluten exposure damages the small-intestinal lining. Its villous injury demonstrates how damage to the lining impairs nutrient absorption.
Stomach: A muscular digestive organ that stores food, mixes it with acid and enzymes, and releases it into the small intestine. The stomach delivers partially digested food to the small intestine as chyme.
Microvilli: Tiny projections on the surface of intestinal epithelial cells that form the brush border. Their enzymes complete digestion at the surface where nutrients are absorbed.
Jejunum: The middle section of the small intestine, specialized for extensive nutrient absorption. Its lining absorbs much of the nutrients released during digestion.
Crohn's disease: A chronic inflammatory bowel disease that can affect any part of the digestive tract, often including the ileum. Small-intestinal inflammation can cause narrowing, pain, and malabsorption.
Pancreas: A gland that releases digestive enzymes and bicarbonate into the small intestine and hormones into the blood. Its enzymes digest food, while bicarbonate helps neutralize stomach acid in the duodenum.
Peristalsis: Wave-like muscular contractions that move contents through the digestive tract. These contractions propel intestinal contents while mixing them with digestive secretions.
Ileum: The final section of the small intestine, which connects to the large intestine. It absorbs vitamin B12 and bile salts before contents enter the colon.