Systems Biology
Digestive absorption involves getting food into our bodies by moving it from the intestinal lumen into blood or lymphatic vessels. Most nutrient absorption happens in the jejunum and ileum of the small intestines, and to a lesser extent, the duodenum. The lining of the small intestine contains finger-like projections called villi that protrude into the intestinal lumen. Villi are composed of multiple epithelial cells, also known as brush border cells. Each villus contains a network of capillaries and a lacteal, which is a lymphatic capillary. Nutrients absorbed by brush border cells travel either to a capillary to enter the bloodstream or into a lacteal that feeds into the lymphatic system.
Intestinal capillaries absorb water-soluble substances like carbohydrates, proteins, short-chain fatty acids, and water-soluble vitamins. On the other hand, lacteals absorb fat-soluble substances, including dietary fats and the fat-soluble vitamins A, D, E, and K. The duodenum absorbs calcium, iron, folate, and some water, while the ileum absorbs bile salts and vitamin B12 bound to intrinsic factor. Lastly, the colon brings the final round of absorption, removing water and salts from the lumen to concentrate fecal matter.
Lesson Outline
<ul> <li>Digestive absorption: getting nutrients into our bodies</li> <ul> <li>Locations: mostly jejunum and ileum, to some extent duodenum</li> </ul> <li>Stomach absorption</li> <ul> <li>Aspirin and alcohol begin their absorption in the stomach, but most absorption occurs in the small intestine</li> </ul> <li>Small intestine</li> <ul> <li>Three sections: duodenum, jejunum, ileum</li> <li>Anatomy for absorption</li> <ul> <li>Villi: finger-like projections into the intestinal lumen (made of brush border cells (epithelial cells))</li> <li>Microvilli: tiny projections on outer surface of brush border cells</li> <li>Villi and microvilli increase surface area for absorption</li> <li>Capillaries and lacteals in villi for nutrient absorption</li> </ul> </ul> <li>Capillaries</li> <ul> <li>Absorb water-soluble nutrients (protein, carbohydrates, vitamins, minerals, short-chain fatty acids)</li> </ul> <li>Lacteals</li> <ul> <li>Absorb fat-soluble nutrients (vitamins A, D, E, K)</li> </ul> <li>Small intestine absorption regions</li> <ul> <li>Duodenum: calcium, iron, folate, some water</li> <li>Jejunum: most nutrient absorption occurs here</li> <li>Ileum: bile salts, vitamin B12 bound to intrinsic factor</li> </ul> <li>Colon</li> <ul> <li>Water and salt absorption</li> </ul> <li>Rectum</li> <ul> <li>Excretion of unabsorbed nutrients and indigestible materials</li> </ul> </ul>
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FAQs
The small intestines are the primary site for digestive absorption. Almost all nutrients from the food we consume, such as proteins, fats, carbohydrates, vitamins, and minerals, are absorbed here. The small intestines have a large surface area and are lined with specialized structures called villi and microvilli, which help increase absorption efficiency. These structures contain brush border cells, capillaries, and lacteals that facilitate the uptake and transport of nutrients into the bloodstream and lymphatic system.
Brush border cells, along with the presence of villi and microvilli, contribute to the efficient absorption of nutrients in the small intestines. Villi are finger-like projections that line the inner surface of the small intestines, while microvilli are microscopic extensions on the surface of brush border cells. The function of villi and microvilli is to greatly increase the surface area available for nutrient absorption. The presence of these structures enhances the efficiency of nutrient uptake by maximizing the contact between the intestinal contents and the absorptive cells. Villi contain blood vessels and lacteals, which are specialized lymphatic vessels, allowing for the efficient transport of absorbed nutrients into the bloodstream and lymphatic system. The brush border cells, with their enzymes and membrane transport proteins, work in conjunction with the villi and microvilli to facilitate the absorption of breakdown products of carbohydrates, proteins, and fats. This coordinated action ensures that nutrients are efficiently absorbed into the body to support essential functions and provide energy.
Fat-soluble substances, such as lipids and fat-soluble vitamins, are absorbed through a different mechanism than water-soluble nutrients. After being broken down by bile salts and pancreatic lipases in the small intestine, fat-soluble substances form micelles that are absorbed by the brush border cells of the intestinal villi. Once inside the cells, they are reassembled into triglycerides and packaged into chylomicrons, which are then transported into lacteals. The lacteals are part of the lymphatic system, enabling the distribution of fat-soluble substances throughout the body.
The lymphatic system plays a vital role in the absorption of fat-soluble substances from the small intestine. Lacteals, which are small lymphatic vessels located within the villi, absorb the chylomicrons that contain fat-soluble nutrients. The lymphatic system then transports these chylomicrons to the thoracic duct, which eventually empties into the systemic circulation. By this mechanism, the lymphatic system enables the distribution of fat-soluble nutrients to the body's cells, tissues, and organs for utilization.
