After chemical digestion is complete, virtually all nutrients such as vitamins, water, and carbohydrates from food are absorbed into the bloodstream in the small intestine. Nutrients that are unable to cross the membrane of the intestinal lumen of the small intestine and enter the bloodstream are not absorbed.
During absorption, nutrients are taken up by the intestinal walls and then enter the bloodstream. The small intestine is anatomically divided into three regions. The first part is the duodenum, which functions to digest food that first enters the small intestine. The jejunum is the second part where nutrients from the digested food are absorbed back into the intestinal walls. The last part of the small intestine is the ileum, where more nutrients like Vitamin B12 are absorbed back into circulation. The small intestine is covered by a brush border, the surface of the intestinal epithelial cells that is covered with microvilli. These microvilli increase the surface area of the intestine to facilitate more efficient nutrient absorption. Digested proteins and carbohydrates are taken up directly by the bloodstream and lipids are absorbed by a lacteal, or a lymphatic vessel in the small intestine that absorbs digested fat before it enters the bloodstream.
Proteins are chemically digested in the stomach using pepsin and further in the small intestine using many pancreatic proteases. An enterocyte or absorptive cell, is a type of epithelial cell predominantly found in the lumen of the small intestine or internal lining. The enterocyte cells contain sodium-dependent amino acid transporter proteins embedded on their plasma membranes. These transporters are unique because they have four distinct receptors on their surface which allow either neutral acids, basic amino acids, acidic amino acids, or imino acid to bind for transport. When proteins are digested, amino acids are cotransported with the help of sodium ions, across the intestinal lumen. That is, sodium first binds to the cell surface receptor on the amino acid transporter followed by the binding of an amino acid to its specific cell surface receptor on the same transporter. After the amino acid binds, the transporter experiences a conformational change, which initiates the process of moving the amino acid into the lumen of the small intestine. Sodium is also removed from the surface of the transporter during this process. It is important that the sodium ion is removed because this ion plays a role in establishing a concentration gradient that generates energy for the amino acid transportation process. The sodium-dependent amino acid transporter actively pumps sodium ions outside the enterocyte while potassium ions are pumped in, establishing a gradient that creates cellular energy to drive this cotransport process. Once the amino acids are inside the intestinal lumen, additional protein transporters that do not depend on sodium gradients drive a facilitated diffusion process by helping to transport the amino acids from the intestinal lumen into blood without the need of cellular energy.
Anatomy of the Small Intestine
Lipids are not water soluble, so bile is used to mechanically digest the lipids into smaller droplets. Lipases are involved in chemical digestion (emulsification) of lipids in the small intestine. When fats are digested, fatty acids and monoglycerides associate with bile salts and phospholipids to produce micelles. A micelle is small collection of lipids and bile salts that interact with intestinal epithelial cells. Fats diffuse into intestinal epithelial cells and are assembled into triglycerides and then combined with proteins to form chylomicrons. A chylomicron is a lipoprotein particle formed in order to transport lipids from the intestines to the lymphatic system.
Amino Acid Absorption
Chemical digestion of carbohydrates begins in the mouth using salivary amylase and continues in the small intestine using pancreatic amylase, lactase, maltase, sucrase, and other enzymes. Before these dietary carbohydrates are absorbed into the intestinal lumen, they must be further broken down or digested with the help of digestive enzymes found on the brush border. For example, when sucrase comes into contact with these enzymes it breaks down into glucose and fructose. These smaller molecules are taken inside the intestinal lumen by cotransport with sodium ions. Specifically for glucose (and another molecule called galactose) it must bind to a transporter that also binds to sodium. When sodium binds to the transporter, the transporter undergoes a conformational change that creates a small opening for glucose to bind to. Following this binding of glucose, the transporter encases around glucose and begins to transport it inside the cell. Sodium is dissociated from the transporter before glucose enters. This type of cotransport or secondary active transport process with glucose requires cellular energy in the form of adenosine triphosphate (ATP), which is created from the use of Na+/K+ pumps that establish a sodium-potassium concentration gradient. Other molecules like fructose do not need to use this cotransport process because they can enter the lumen without sodium binding to a transporter. After glucose is inside the lumen it is exported into blood by way of a facilitated diffusion process, which involves the use of transporters that are found on the basolateral membrane and help diffuse glucose molecules into blood.
Water, ions, vitamins, and minerals are also absorbed in the intestines. Large amounts of water are absorbed by the small intestine in a passive diffusion process. This net movement of water happens because of osmosis where water moves across a selectively permeable membrane by way of an osmotic gradient that is established when there is a difference in solute concentration on either side of the membrane. The main solute involved with water absorption in the intestine is sodium. Food products that cannot be absorbed are mixed with other waste to become feces. Defecation is the release of feces from the body.