Absorption in the Small Intestine: General Mechanisms
Virtually all nutrients from the diet are absorbed into blood across the mucosa of the small intestine. In addition, the intestine absorbs water and electrolytes, thus playing a critical role in maintenance of body water and acid-base balance.
It's probably fair to say that the single most important process that takes place in the small gut to make such absorption possible is establishment of an electrochemical gradient of sodium across the epithelial cell boundary of the lumen. This is a critical concept and actually quite interesting. Also, as we will see, understanding this process has undeniably resulted in the saving of millions of lives.
To remain viable, all cells are required to maintain a low intracellular concentration of sodium. In polarized epithelial cells like enterocytes, low intracellular sodium is maintained by a large number of Na+/K+ ATPases - so-called sodium pumps - embedded in the basolateral membrane. These pumps export 3 sodium ions from the cell in exchange for 2 potassium ions, thus establishing a gradient of both charge and sodium concentration across the basolateral membrane.
In rats, as a model of all mammals, there are about 150,000 sodium pumps per small intestinal enterocyte which collectively allow each cell to transport about 4.5 billion sodium ions out of each cell per minute (J Membr Biol 53:119-128, 1980). Pretty impressive! This flow and accumulation of sodium is ultimately responsible for absorption of water, amino acids and carbohydrates.
Aside from the electrochemical gradient of sodium just discussed, several other concepts are required to understand absorption in the small intestine. Also, dietary sources of protein, carbohydrate and fat must all undergo the final stages of chemical digestion just prior to absorption of, for example, amino acids, glucose and fatty acids.
At this point, its easiest to talk separately about absorption of each of the major food groups, recognizing that all of these processes take place simultaneously.
- Water and electrolytes
- Carbohydrates, after digestion to monosaccharides
- Proteins, after digestion to small peptides and amino acids
- Neutral fat, after digestion to monoglyceride and free fatty acids
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