Nearly all of the dietary nutrients and approximately 95 to 98% of the water and electrolytes that enter the upper small intestine are absorbed. The absorption of electrolytes and minerals involves both passive and active processes, resulting in the movement of electrolytes, water, and metabolic substrates into the blood for distribution and use throughout the body.
Sodium. The GI system is well equipped to handle the large amount of Na+ entering the GI lumen daily—on average, about 25 to 35 g of Na+ every day. Around 5 to 8 g are derived from the diet, and the rest from salivary, gastric, biliary, pancreatic, and small intestinal secretions. The GI tract is extremely efficient in conserving Na+: only 0.5% of intestinal Na+ is lost in the feces. The jejunum absorbs more than half of the total Na+, and the ileum and colon absorb the remainder. The small intestine absorbs the bulk of the Na+ presented to it, but the colon is most efficient in conserving Na+.
Sodium is absorbed by several different mechanisms operating at varying degrees in different parts of the GI tract. When a meal that is hypotonic to plasma is ingested, considerable absorption of water from the lumen to the blood takes place, predominantly through tight junctions and intercellular spaces between the enterocytes, resulting in the absorption of small solutes such as Na+ and Cl" ions. This mode of absorption, called solvent drag, is responsible for a significant amount of the Na+ absorption by the duode-
num and jejunum, but it probably plays a minor role in Na+ absorption by the ileum and colon because more distal regions of the intestine are lined by a "tight" epithelium (see Chapter 2).
In the jejunum, Na+ is actively pumped out of the baso-lateral surface of enterocytes by a Na+/K+-ATPase (Fig. 27.29A). The result is low intracellular Na+ concentration, and the luminal Na+ enters enterocytes down the electrochemical gradient, providing energy for the extrusion of H+ into the lumen (via a Na+/H+ exchanger). The H + then reacts with HCO3~ in bile and pancreatic secretions in the intestinal lumen to form H2CO3. Carbonic acid dissociates to form CO2 and H2O. The CO2 readily diffuses across the small intestine into the blood. Another mode of Na+ uptake is via a carrier located in the enterocyte brush border membrane, which transports Na+ together with a monosaccharide (e.g., glucose) or an amino acid molecule (a symport type of transport).
In the ileum, the presence of a Na+/K+-ATPase at the basolateral membrane also creates a low intracellular Na+ concentration, and luminal Na+ enters enterocytes down the electrochemical gradient. Sodium absorption by Na+-coupled symporters is not as great as in the jejunum because most of the monosaccharides and amino acids have already been absorbed by the small intestine (Fig. 27.29B). Sodium chloride is transported via two exchangers located at the brush border membrane. One is a Cl~/HCO3~ exchanger, and the other is a Na+/H+ exchanger. The downhill movement of Na+ into the cell provides the energy required for the uphill movement of the H+ from the cell to the lumen. Similarly, the downhill movement of HCO3~ out of the cell provides the energy for the uphill entry of Cl" into the enterocytes. The Cl" then leaves the cell through facilitated transport. This mode of Na+ uptake is called Na+/H + -Cl~/HCO3" countertransport.
In the colon, the mechanisms for Na+ absorption are mostly similar to those described for the ileum. There is no sugar- or amino acid-coupled Na+ transport because most sugars and amino acids have already been absorbed. Sodium is also absorbed here via Na+-selective ion channels in the apical cell membrane (electrogenic Na+ absorption).
Potassium. The average daily intake of K+ is about 4 g. Absorption takes place throughout the intestine by passive
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