NORMAL INTESTINAL PHYSIOLOGY



The intestine normally receives 8 to 10 liters of fluid per day, 1500 to 2000 ml from injested food and liquids and the rest from salivary, gastric, pancreatic, biliary, and small intestinal secre­tions. The small bowel absorbs all but about one liter of this fluid, and the colon absorbs 90 per cent of the remaining fluid, resulting in a fecal output of 100 to 150 ml per day.

The mechanisms responsible for solute and fluid absorption differ in different regions of the gut. All obey the general principle, however, that solutes (salts and other substances) are absorbed by specific mechanisms, while water follows pas­sively according to osmotic gradients. Many of the cellular mechanisms responsible for small intes­tinal solute transport, shown in Figure 36-3, ap­pear to be sodium-coupled processes; i.e., they use the energy of the sodium gradient (established by the sodium pump) to drive entry or exit of a wide range of compounds, including protons, chloride, glucose, amino acids, and bile acids across cell membranes. Bicarbonate secretion, dri­ven by sodium/hydrogen exchange, accounts for the relatively alkaline nature of ileal and colonic contents. Thus, jejunal and ileal.fluid typically contains Na + , 140 mM; K + , 6.0 mM; CI”, 100 mM; and HC03~, 30 mM.

Colonic solute transport is limited to electro­lytes and occurs through slightly different mech­anisms. Sodium absorption, which occurs via a specific sodium channel, generates an electrical potential across the colon wall, which in turn drives both chloride absorption and potassium se­cretion, leading to the typically high potassium concentration of colonic contents. In “addition, or­ganic acids, produced by colonic bacteria from nonabsorbed carbohydrate or fat, react with bi­carbonate to produce organic anions and C02. Thus, colonic fluid typically contains Na + , 40 mM; K+, 90 mM; CI”, 15 mM; HC03″, 30 mM; and organic anions, $5 mM.

Both small bowel and colon also secrete elec­trolytes and water. In the small bowel, secretion probably originates in crypt cells and may be due to sodium-coupled entry of anions (CI”) across the serosal cell membrane, followed by secretion of chloride across the luminal cell membrane. So­dium and water are thought to follow chloride passively according to electrical and osmotic gra­dients (Fig. 36-4). Colonic chloride secretion maypossibly occur via a similar mechanism in which chloride uptake across the serosal membrane is coupled to both sodium and potassium entry.

Although intestinal water movement is passive, it is not inconsequential. Since water absorption is linked to solute absorption, the presence of poorly absorbed, osmotically active solutes such as Mg*2, S04~2, and P04″2 in the gut lumen will impair water absorption or cause water secretion.