Diffusion is the main mechanism for absorption within the small intestine, so effective diffusion is the key to sufficient nutrient absorption. The small intestine has a very large surface area, created by folds and the presence of villi and microvilli. These are finger-like projections from the inner cells of the intestine which are only one-cell thick, allowing for a short diffusion distance between the intestinal lumen and surrounding capillaries. The vast network of capillaries surrounding the small intestine (located within the villi) ensures a constant concentration gradient is maintained, which also increases the rate of diffusion.