What are the three structural modifications of the small intestine that increase its surface area for absorption?

The small intestine wall has three structural modifications that enlarge its absorptive surface: circular folds, villi, and microvilli. Circular folds are deep permanent folds of the mucosa and submucosa that slow chyme movement. Villi are fingerlike projections of the mucosa containing capillaries and a lacteal. Microvilli are cytoplasmic extensions on absorptive cells that form the brush border.

The intestinal wall is modified to greatly amplify surface area for absorption. Circular folds are deep, permanent folds of the mucosa and submucosa, about 1 cm tall, that force chyme to spiral and slow its passage so nutrients can be fully absorbed. Villi are fingerlike mucosal projections over 1 mm high that give the lining a velvety texture; each villus contains a capillary bed and a lymphatic lacteal for absorbing digested nutrients. Microvilli are long, tightly packed cytoplasmic extensions of the absorptive enterocytes, producing the brush border appearance; their plasma membranes contain brush border enzymes that complete digestion of carbohydrates and proteins. Together these three features increase the absorptive surface by more than 600 times.