Absorption of Water and ions in alimentary canal

 Absorption of Water and ions in alimentary canal

Absorption of Water by Osmosis

Isosmotic Absorption

Water is transported through the intestinal membrane entirely by diffusion. Furthermore, this diffusion obeys the usual laws of osmosis. Therefore, when the chyme is dilute enough, water is absorbed through the intestinal mucosa into the blood of the villi almost entirely by osmosis.

Conversely, water can also be transported in the opposite direction from plasma into the chyme. This occurs especially when hyperosmotic solutions are discharged from the stomach into the duodenum. Within minutes, sufficient water usually will be transferred by osmosis to make the chyme isosmotic with the plasma.

Absorption of Ions

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 principle of this mechanism is also essentially the same as for absorption of sodium from the gallbladder and renal tubules. The motive power for sodium absorption is provided 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 transport: It requires energy, and the energy process is catalyzed by appropriate adenosine triphosphatase (ATP) 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

·        Sodium-glucose co-transporters,

·        Sodium amino acid co-transporters,

·        Sodium-hydrogen exchanger.

These transporters function similarly as in the renal tubules, and provide still more sodium ions to be transported by the epithelial cells into the paracellular spaces. At the same time they also provide secondary active absorption of glucose and amino acids, powered by the active 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 Para cellular pathways. This occurs because a large osmotic gradient has been created by the elevated concentration of ions in the para cellular space. Much of this osmosis occurs through the tight junctions between the apical borders of the epithelial cells (para cellular pathway) but much also occurs through the cells themselves (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.

When a person becomes dehydrated, large amounts of aldosterone almost always 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. And 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 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 electro negativity in the chyme and electro positivity in the para cellular spaces between the epithelial cells). Then chloride ions 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, moderate 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. Thus, this is so-called active absorption of bicarbonate ions.

Secretion of Bicarbonate Ions in the Ileum and Large Intestine

Simultaneous Absorption of Chloride Ions

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. This is important because it provides alkaline bicarbonate ions that neutralize acid products formed by bacteria in the large intestine.

Deep in the spaces between the intestinal epithelial folds are immature epithelial cells that continually Na divide to form new epithelial cells. These in turn 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 loss of 5 to 10 liters of water and sodium chloride as diarrhea each day. Extreme diarrheal secretion is initiated by entry of a subunit of cholera toxin into the epithelial cells.

This 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 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 it can be lethal because of serious dehydration of the whole body that might ensue.