What are the three special structures on the luminal surface of the small intestine?

The lumen of the small intestine features three specialized structures: microvilli, intestinal villi, and circular folds.

1. Microvilli

Microvilli are microscopic projections formed by the cell membrane of the columnar epithelial cells lining the small intestine, extending into the intestinal lumen. They constitute a critically important site for absorption.

2. Intestinal Villi

Intestinal villi appear as leaf-like, finger-like, or cone-shaped structures. They primarily function to filter substances and also assist in nutrient absorption.

3. Circular Folds

Circular folds are mainly located at the distal end of the duodenum and the proximal portion of the jejunum. Their primary role is to increase the surface area of the small intestine, thereby enhancing effective absorption of nutrients.

These three specialized structures greatly expand the internal surface area of the small intestine, promoting its absorptive function. In daily life, it's important to maintain regular eating habits and reduce gastrointestinal strain.