Page 275 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

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Water and Mineral Absorption ! Na diffusion: Na in the colon is mainly ab-
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sorbed through luminal Na channels (! D5).
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The average intake of water (in beverages and This type of Na transport is electrogenic and
foodstuffs) is roughly 1.5 L per day. An addi- aldosterone-dependent (! p. 182). The related
tional 7 L of fluid are secreted into the gastro- lumen-negative transepithelial potential
intestinal (GI) tract (saliva, gastric juices, bile, (LNTP, see above) either leads to K secretion or
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pancreatic juice and intestinal secretions), drives Cl out of the lumen (! D2).
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whereas only about 0.1 L/day is eliminated in –
the feces. The digestive tract must therefore The Cl secretion mechanism of epithelial cells
(mainly Lieberkühn’s crypts, ! p. 245, A16) is similar
absorb a net volume of at least 8.4 L of water to that of the acini of salivary glands (! p. 236). The
per day. GI absorption of water occurs mainly efflux of Cl into the lumen and the associated efflux
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in the jejunum and ileum, with smaller quanti- of Na and water are stimulated by cAMP and regu-
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Nutrition and Digestion osmosis. When solutes (Na , Cl , etc.) are ab- iological function of this form of H 2O secretion could
lated by neurons and hormones such as VIP (vasoac-
ties being absorbed by the colon (! A). Water
tive intestinal peptide) and prostaglandins. The phys-
is driven through the intestinal epithelium by
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be to dilute viscous chyme or to ensure the recircu-
sorbed in the intestine, water follows (! B).
lation of water (from the crypts ! lumen ! villi
(The stool contains only small quantities of
! crypts) to promote the absorption of poorly
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Na , Cl and water.) Conversely, the secretion of
soluble substances. Cholera toxin inhibits the
substances into the lumen or the ingestion of
GTPase of the G s proteins (! p. 274), thereby main-
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a marked increase in Cl secretion. In response to it,
fluxes into the lumen. Poorly absorbable sub-
large quantities of water and Na are secreted into
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stances therefore act as laxatives (e.g. sulfate,
10 non-absorbable substances leads to water taining a maximal cAMP concentration and therefore
the lumen, which can lead to severe diarrhea (up to
sorbitol, polyethylene glycol).
Water absorption is mainly driven by the 1 L/hour!).
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absorption of Na , Cl and organic compounds In addition to HCO 3 from pancreatic juice,
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(! B). The luminal concentration of Na and Cl – HCO 3 is also secreted into the intestinal lumen
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steadily decreases from the duodenum to the by the small and large intestine (! A). K is
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colon. That of Na , for example, is approxi- secreted (aldosterone-dependent) by crypt
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mately 145 mmol/L in the duodenum, cells of the colon (luminal K concentration
125 mmol/L in the ileum and only 40 mmol/L ! 90 mmol/l!) and reabsorbed via the H -K +
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in the colon (! C). Na is absorbed by various pump of the surface epithelium (similar to the
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mechanisms, and the Na -K -ATPase on the ba- mechanism in the stomach). The aldosterone-
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solateral cell membrane is the primary driving dependent K secretion/absorption ratio de-
mechanism (! p. 26) for all of them (! B, D). termines the net amount of K excreted (! A
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! Symport of Na + and organic substances and p. 180). Diarrhea results in losses of K and
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(! see pp. 26ff. and 258): Na passively in- HCO 3 (hypokalemia and metabolic acidosis;
fluxes into cells of the duodenum and jejunum ! p. 142).
2+
via symporter carriers, which actively cotrans- Ca . The stool contains one-third of the di-
port glucose, amino acids, phosphates and etary Ca intake. Ca is absorbed in the upper
2+
2+
other compounds (secondary active transport; part of the small intestine (! A) with the aid of
! D1). Since this is an electrogenic transport intracellular calcium-binding protein (CaBP).
mechanism (! p. 28), a lumen-negative trans- Calcitriol increases CaBP synthesis, thereby in-
epithelial potential (LNTP; ! p. 162) that drives creasing Ca 2+ absorption (! p. 292). Deficien-
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Cl out of the lumen forms (! D2). cies of vitamin D or substances that form
2+
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! Parallel transport of Na and Cl : Na ions in water-insoluble compounds with Ca (phytin,
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the lumen of the ileum are exchanged for H + oxalate, fatty acids) decrease Ca 2+ absorption.
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ions (! D3) while Cl is exchanged for HCO 3 at Mg 2+ is absorbed by similar mechanisms, but
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the same time (! D4). The H ions combine iron (Fe) is absorbed by a different mechanism
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with HCO 3 to yield H 2O + CO 2, which diffuse (! p. 90).
out of the lumen. Most Na , Cl and, by sub-
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262 sequent osmosis, H 2O is absorbed by this elec-
troneutral transport mechanism.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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