Ganong's Review of Medical Physiology, 23rd Edition

TABLE 26–5 Daily water turnover (mL)
in the gastrointestinal tract.
Ingested 2000
Endogenous secretions 7000
Salivary glands 1500
Stomach 2500
Bile 500
Pancreas 1500
Intestine +1000
7000
Total input 9000
Reabsorbed 8800
Jejunum 5500
Ileum 2000
Colon +1300
8800
Balance in stool 200
Data from Moore EW: Physiology of Intestinal Water and Electrolyte Absorption.
American Gastroenterological Society, 1976.
Overall water balance in the gastrointestinal tract is summarized
in Table 26–5. The intestines are presented each day
with about 2000 mL of ingested fluid plus 7000 mL of secretions
from the mucosa of the gastrointestinal tract and associated
glands. Ninety-eight percent of this fluid is reabsorbed,
with a daily fluid loss of only 200 mL in the stools.
In the small intestine, secondary active transport of Na + is
important in bringing about absorption of glucose, some amino
acids, and other substances such as bile acids (see above). Conversely,
the presence of glucose in the intestinal lumen facilitates
the reabsorption of Na + . In the period between meals,
when nutrients are not present, sodium and chloride are
absorbed together from the lumen by the coupled activity of a
sodium/hydrogen exchanger (NHE) and chloride/bicarbonate
exchanger in the apical membrane, in a so-called electroneutral
mechanism (Figure 26–19). Water then follows to maintain an
osmotic balance. In the colon, moreover, an additional electrogenic
mechanism for sodium absorption is expressed, particularly
in the distal colon. In this mechanism, sodium enters
across the apical membrane via an ENaC (epithelial sodium)
channel that is identical to that expressed in the distal tubule of
the kidney (Figure 26–20). This underpins the ability of the
colon to desiccate the stool and ensure that only a small portion
of the fluid load used daily in the digestion and absorption of
meals is lost from the body. Following a low-salt diet, increased
expression of ENaC in response to aldosterone increases the
ability to reclaim sodium from the stool.
CHAPTER 26 Overview of Gastrointestinal Function & Regulation 441
H +
HCO 3 –
Na +
2K +
Na + ,K + -
ATPase
3Na +
FIGURE 26–19 Electroneutral NaCl absorption in the small
intestine and colon. NaCl enters across the apical membrane via the
coupled activity of a sodium/hydrogen exchanger and a chloride/bicarbonate
exchanger.
ENaC
NHE-3?
NHE-2?
CLD
Na +
Cl −
2K +
KCC1
?
Na + ,K + -
ATPase
FIGURE 26–20 Electrogenic sodium absorption in the
colon. Sodium enters the epithelial cell via epithelial sodium channels
(ENaC).
Despite the predominance of absorptive mechanisms,
secretion also takes place continuously throughout the small
intestine and colon to adjust the local fluidity of the intestinal
contents as needed for mixing, diffusion, and movement of
the meal and its residues along the length of the gastrointestinal
tract. Cl – normally enters enterocytes from the interstitial
fluid via Na + –K + –2Cl – cotransporters in their basolateral
membranes (Figure 26–21), and the Cl – is then secreted into
the intestinal lumen via channels that are regulated by various
protein kinases. The cystic fibrosis transmembrane conductance
regulator (CFTR) channel that is defective in the disease
of cystic fibrosis is quantitatively most important, and is activated
by protein kinase A and hence by cAMP (see Clinical
Box 26–2).
Water moves into or out of the intestine until the osmotic
pressure of the intestinal contents equals that of the plasma.
The osmolality of the duodenal contents may be hypertonic
or hypotonic, depending on the meal ingested, but by the time
the meal enters the jejunum, its osmolality is close to that of
Cl −
Cl −
3Na +
K +
K +