Sodium Is Actively Transported Through the Intestinal Membrane. Twenty to 30 grams of sodium are secreted in the intestinal secretions each day. In addition, the average person eats 5 to 8 grams of sodium each day. Therefore, to prevent net loss of sodium into the feces, the intestines must absorb 25 to 35 grams of sodium each day, which is equal to about one seventh of all the sodium present in the body.
Whenever significant amounts of intestinal secretions are lost to the exterior, as in extreme diarrhea, the sodium reserves of the body can sometimes be depleted to lethal levels within hours. Normally, however, less than 0.5 percent of the intestinal sodium is lost in the feces each day because it is rapidly absorbed through the intestinal mucosa. Sodium also plays an important role in helping to absorb sugars and amino acids, as subsequent discussions reveal.
The basic mechanism of sodium absorption from the intestine is shown in Figure1. The principles of this mechanism, discussed in Chapter 4, are also essentially the same as for absorption of sodium from the gallbladder and renal tubules, as discussed in Chapter 28.
Fig1. Absorption of sodium, chloride, glucose, and amino acids through the intestinal epithelium. Note also osmotic absorption of water (i.e., water “follows” sodium through the epithelial membrane).
Sodium absorption is powered by active transport of sodium from inside the epithelial cells through the basal and lateral walls of these cells into paracellular spaces. This active transport obeys the usual laws of active trans port: It requires energy, and the energy process is catalyzed by appropriate adenosine triphosphatase (ATPase) enzymes in the cell membrane. Part of the sodium is absorbed along with chloride ions; in fact, the negatively charged chloride ions are mainly passively “dragged” by the positive electrical charges of the sodium ions.
Active transport of sodium through the basolateral membranes of the cell reduces the sodium concentration inside the cell to a low value (≈50 mEq/L). Because the sodium concentration in the chyme is normally about 142 mEq/L (i.e., about equal to that in plasma), sodium moves down this steep electrochemical gradient from the chyme through the brush border of the epithelial cell into the epithelial cell cytoplasm. Sodium is also co-transported through the brush border membrane by several specific carrier proteins, including (1) the sodium-glucose co-transporter, (2) sodium-amino acid co-transporters, and (3) the sodium-hydrogen exchanger. These transporters function similarly as in the renal tubules, described in Chapter 28, and provide still more sodium ions to be transported by the epithelial cells into the interstitial fluid and paracellular spaces. At the same time, they also provide secondary active absorption of glucose and amino acids, powered by the active sodium-potassium (Na+-K+) ATPase pump on the basolateral membrane.
Osmosis of the Water. The next step in the transport process is osmosis of water by transcellular and paracellular pathways. This osmosis occurs because a large osmotic gradient has been created by the elevated concentration of ions in the paracellular space. Much of this osmosis occurs through the tight junctions between the apical borders of the epithelial cells (the paracellular pathway), but much also occurs through the cells themselves (the transcellular pathway). Osmotic movement of water creates flow of fluid into and through the para cellular spaces and, finally, into the circulating blood of the villus.
Aldosterone Greatly Enhances Sodium Absorption. When a person becomes dehydrated, large amounts of aldosterone are secreted by the cortices of the adrenal glands. Within 1 to 3 hours this aldosterone causes increased activation of the enzyme and transport mechanisms for all aspects of sodium absorption by the intestinal epithelium. The increased sodium absorption in turn causes secondary increases in absorption of chloride ions, water, and some other substances.
This effect of aldosterone is especially important in the colon because it allows virtually no loss of sodium chloride in the feces and also little water loss. Thus, the function of aldosterone in the intestinal tract is the same as that achieved by aldosterone in the renal tubules, which also serves to conserve sodium chloride and water in the body when a person becomes depleted of sodium chloride and dehydrated.
Absorption of Chloride Ions in the Small Intestine. In the upper part of the small intestine, chloride ion absorption is rapid and occurs mainly by diffusion (i.e., absorption of sodium ions through the epithelium creates electronegativity in the chyme and electropositivity in the paracellular spaces between the epithelial cells). Chloride ions then move along this electrical gradient to “follow” the sodium ions. Chloride is also absorbed across the brush border membrane of parts of the ileum and large intestine by a brush border membrane chloride bicarbonate exchanger. Chloride exits the cell on the basolateral membrane through chloride channels.
Absorption of Bicarbonate Ions in the Duodenum and Jejunum. Often large quantities of bicarbonate ions must be reabsorbed from the upper small intestine because large amounts of bicarbonate ions have been secreted into the duodenum in both pancreatic secretion and bile. The bicarbonate ion is absorbed in an indirect way as follows: When sodium ions are absorbed, moder ate amounts of hydrogen ions are secreted into the lumen of the gut in exchange for some of the sodium. These hydrogen ions, in turn, combine with the bicarbonate ions to form carbonic acid (H2CO3), which then dissociates to form water and carbon dioxide. The water remains as part of the chyme in the intestines, but the carbon dioxide is readily absorbed into the blood and subsequently expired through the lungs. This process is the so-called “active absorption of bicarbonate ions.” It is the same mechanism that occurs in the tubules of the kidneys.
Secretion of Bicarbonate and Absorption of Chloride Ions in the Ileum and Large Intestine. The epithelial cells on the surfaces of the villi in the ileum, as well as on all surfaces of the large intestine, have a special capability of secreting bicarbonate ions in exchange for absorption of chloride ions (see Figure 1). This capability is important because it provides alkaline bicarbonate ions that neutralize acid products formed by bacteria in the large intestine.
Extreme Secretion of Chloride Ions, Sodium Ions, and Water from the Large Intestine Epithelium in Some Types of Diarrhea. Immature epithelial cells that continually divide to form new epithelial cells are found deep in the spaces between the intestinal epithelial folds. These new epithelial cells spread outward over the luminal surfaces of the intestines. While still in the deep folds, the epithelial cells secrete sodium chloride and water into the intestinal lumen. This secretion, in turn, is reabsorbed by the older epithelial cells outside the folds, thus providing flow of water for absorbing intestinal digestates.
The toxins of cholera and of some other types of diarrheal bacteria can stimulate the epithelial fold secretion so greatly that this secretion often becomes much greater than can be reabsorbed, thus sometimes causing a loss of 5 to 10 liters of water and sodium chloride as diarrhea each day. Within 1 to 5 days, many severely affected patients die of this loss of fluid alone.
Extreme diarrheal secretion is initiated by entry of a subunit of cholera toxin into the epithelial cells. This subunit stimulates formation of excess cyclic adenosine monophosphate, which opens tremendous numbers of chloride channels, allowing chloride ions to flow rapidly from inside the cell into the intestinal crypts. In turn, this action is believed to activate a sodium pump that pumps sodium ions into the crypts to go along with the chloride ions. Finally, all this extra sodium chloride causes extreme osmosis of water from the blood, thus providing rapid flow of fluid along with the salt. All this excess fluid washes away most of the bacteria and is of value in combating the disease, but too much of a good thing can be lethal because of serious dehydration of the whole body that might ensue. In most instances, the life of a person with cholera can be saved by the administration of tremendous amounts of sodium chloride solution to make up for the loss.
Active Absorption of Calcium, Iron, Potassium, Magnesium, and Phosphate. Calcium ions are actively absorbed into the blood, especially from the duodenum, and the amount of calcium ion absorption is exactly controlled to supply the daily need of the body for calcium. One important factor controlling calcium absorption is parathyroid hormone secreted by the parathyroid glands, and another is vitamin D. Parathyroid hormone activates vitamin D, and the activated vitamin D in turn greatly enhances calcium absorption. These effects are discussed in Chapter 80.
Iron ions are also actively absorbed from the small intestine. The principles of iron absorption and regulation of its absorption in proportion to the body’s need for iron, especially for the formation of hemoglobin, are discussed in Chapter 33.
Potassium, magnesium, phosphate, and probably still other ions can also be actively absorbed through the intestinal mucosa. In general, the monovalent ions are absorbed with ease and in great quantities. Bivalent ions are normally absorbed in only small amounts; for example, maximum absorption of calcium ions is only 1/50 as great as the normal absorption of sodium ions. Fortunately, only small quantities of the bivalent ions are normally required daily by the body.
