The small intestine is a convoluted muscular tube where nutrients are further digested and nutrient absorption occurs. The small intestine is divided into three main regions: the duodenum, jejunum, and ileum. Stomach chyme enters the small intestine at the duodenum. Ducts of the gallbladder and pancreas empty into the duodenum to aid in further chemical digestion, the molecular breakdown of food molecules, and to neutralize the chyme coming from the highly acidic environment of the stomach. The jejunum makes up most of the small intestine and is heavily vascularized so that nutrients extracted from foods can be absorbed into the bloodstream. The ileum is the final portion of the small intestine that empties into the large intestine.
The accessory organs associated with the small intestine are the liver, gallbladder, and pancreas. The liver produces bile which is transported to the gallbladder where it is stored and concentrated. Bile is a fluid mixture containing cholesterol-derived bile salts that emulsify lipids. This emulsification is a form of mechanical digestion where large fat droplets, unable to dissolve in water, are broken down into smaller fat droplets. This process increases the surface area of the fat, exposing more surface for the digestive enzymes to act upon. The pancreas is an organ located horizontally across the abdomen near the duodenum of the small intestine. This organ has an endocrine function and an exocrine function. The endocrine cells of the pancreas produce the hormones insulin and glucagon into the bloodstream to regulate blood glucose levels. The exocrine cells produce an alkaline fluid containing digestive enzymes and release it into the lumen of the duodenum. Sodium bicarbonate in the pancreatic secretions neutralizes the acidic pH from the stomach chyme. Digestive enzymes include enzymes that break down carbohydrates (amylase, lactase, sucrase, etc.), proteins (proteases), lipids (lipases), and nucleic acids (nucleosidases and phosphatases) into their smaller components.
The small intestine has a high surface area to maximize nutrient absorption. Not only is the small intestine long (about six meters in adults), but the surface area of the intestinal wall is further increased by circular folds of the mucosa and submucosa, villi and microvilli. Villi are projections in the mucosal surface by the folding of this layer. Each villus is associated with an underlying lacteal and a blood capillary bed to collect absorbed nutrients. A lacteal is a lymphatic vessel in the small intestine that absorbs digested fat, transports it with the lymphatic fluid through the lymphatic vessels, and empties it into the bloodstream at the subclavian vein in the neck region. On a cellular level, additional projections on cellular membranes are present as microvilli. A microvillus (plural, microvilli) is one of several small projections on the apical side, or the surface, of the epithelial tissue. Each epithelial cell can have as many as 1,000 microvilli. These structures further increase the surface area of the mucosal surface, facilitating absorption in the small intestine. A series of cells with microvilli, resembling a brush on the mucosal surface, is referred to as a brush border. The columnar epithelial cells that make up the mucosa absorb nutrients and electrolytes directly into the cell and subsequently transport these substances out of the cell on the basal surface. Tight junctions hold these cells together, preventing materials from passing between the cells. A goblet cell is a simple columnar epithelial cell found in the small intestine that secretes mucus to protect the mucous membrane.