The small intestine is about 6 meters long and fills the middle of the abdomen. It finishes most of digestion and absorbs almost all nutrients.
Three parts
The duodenum is short and C-shaped and receives bile and pancreatic juice. The jejunum absorbs most nutrients, and the ileum absorbs vitamin B12 and bile salts before joining the large intestine at the ileocecal valve.
A huge surface
Circular folds, finger-like villi and microvilli on each cell create a very large absorbing surface. Each villus holds capillaries for sugars and amino acids and a lymph vessel called a lacteal for fats.
Movement
Segmentation mixes chyme with enzymes, and peristalsis moves it forward. Food usually takes 3 to 5 hours to pass through.
Brush border enzymes
Enzymes on the microvilli finish digestion, such as lactase for milk sugar. Low lactase causes bloating and diarrhea after dairy.
At the bedside
Bowel sounds come mostly from gas and fluid moving through the small intestine. High-pitched tinkling sounds can suggest obstruction, and absent sounds can occur with ileus.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).