The Small Intestine
The small intestine is the part of the gastrointestinal tract between the stomach and the large intestine where much of the digestion of food takes place. The primary function of the small intestine is the absorption of nutrients and minerals found in food.
Digested nutrients pass into the blood vessels in the wall of the intestine through a process of diffusion. The inner wall, or mucosa, of the small intestine is lined with simple columnar epithelial tissue.
Structurally, the mucosa is covered in wrinkles or folds called plicae circulares—these are permanent features in the wall of the organ. They are distinct from the rugae, which are non-permanent features that allow for distention and contraction.
From the plicae circulares project microscopic finger-like pieces of tissue called villi (Latin for shaggy hair). The individual epithelial cells also have finger-like projections known as microvilli. The function of the plicae circulares, the villi, and the microvilli is to increase the amount of surface area available for the absorption of nutrients.
Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries ( amino acids and carbohydrates) and lacteals (lipids).
The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances, such as the proteins required by our body. The food that remains undigested and unabsorbed passes into the large intestine.
Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions:
Iron is absorbed in the duodenum.
Vitamin B12 and bile salts are absorbed in the terminal ileum.
Water and lipids are absorbed by passive diffusion throughout the small intestine.
Sodium bicarbonate is absorbed by active transport and glucose and amino acid co-transport.
Fructose is absorbed by facilitated diffusion.