The Small Intestine: Where Most Digestion Happens

Overview

The small intestine is the longest part of the digestive tract, measuring several meters in length. It is where most chemical digestion is completed and where the vast majority of nutrients are absorbed into the blood.

Three Parts

• Duodenum: The short, C-shaped first part that receives stomach contents along with bile from the liver and enzymes from the pancreas.
• Jejunum: The middle section, specialized for active nutrient absorption.
• Ileum: The final and longest section, which absorbs remaining nutrients, vitamin B12, and bile salts before passing contents into the large intestine.

A Surface Built for Absorption

The small intestine maximizes its absorptive surface through three nested features:

1. Circular folds in the wall.
2. Villi — tiny finger-like projections covering those folds.
3. Microvilli — even smaller projections on each cell, forming the "brush border."

Together these increase the surface area enormously, turning a tube into an absorption powerhouse. Each villus contains capillaries and a lymphatic vessel to carry absorbed nutrients away.

Clinical Significance

Conditions that damage the villi, such as celiac disease, reduce the absorptive surface and lead to malnutrition despite adequate food intake. The duodenum is also a common site for peptic ulcers.

Summary

The small intestine completes digestion and absorbs nutrients, relying on a brilliantly folded surface of folds, villi, and microvilli. Its three regions each play a distinct role in extracting nourishment from food.

For educational purposes only; not a substitute for medical advice.