Small Intestine

The small intestine is the longest part of the digestive tract, averaging 3 to 5 meters in life. Despite its name, it is where the majority of chemical digestion and nearly all nutrient absorption take place, between the stomach's pyloric outlet and the ileocecal junction with the large intestine.

Structure and Location

It has three continuous segments. The duodenum is the short first part (about 20 to 25 cm) that curves around the head of the pancreas and receives bile and pancreatic juice. The jejunum, roughly 2.5 meters, is the main site of nutrient absorption. The ileum, around 3 meters, is the final and longest segment, absorbing vitamin B12 and bile salts before connecting to the cecum. The coiled jejunum and ileum fill much of the central and lower abdomen and are suspended by the mesentery, which carries their blood supply from the superior mesenteric artery.

Function

The small intestine is built to maximize surface area for absorption. Its lining is folded into circular ridges called plicae circulares, which are carpeted with finger-like villi, and each villus cell carries microscopic microvilli (the "brush border"). This layering multiplies the absorptive surface enormously. Here, pancreatic enzymes and bile (delivered into the duodenum) break carbohydrates, proteins, and fats into small molecules that pass into the blood and lymph through the villi. The intestine also has endocrine and immune roles, releasing hormones that regulate digestion and housing immune tissue that monitors gut contents.

Clinical Relevance

Disorders of the small intestine often impair nutrition. Celiac disease damages the villi in response to gluten, reducing absorption. Crohn disease, a form of inflammatory bowel disease, frequently affects the ileum. Other conditions include duodenal ulcers (often H. pylori-related), bowel obstruction, and Meckel diverticulum, a common congenital remnant. This content is educational, not medical advice.