Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 26

Overview of Gastrointestinal Function & Regulation 441

Overall water balance in the gastrointestinal tract is sum-
marized in Table 26–5. The intestines are presented each day
with about 2000 mL of ingested fluid plus 7000 mL of secre-
tions from the mucosa of the gastrointestinal tract and associ-
ated 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). Con-
  versely, the presence of glucose in the intestinal lumen facili-
  tates 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 electro-
genic mechanism for sodium absorption is expressed, particu-
larly 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.

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 gastrointesti-

nal 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 conduc-
  tance regulator (CFTR) channel that is defective in the disease
  of cystic fibrosis is quantitatively most important, and is acti-
  vated 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

  
  

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.

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/bi-

carbonate exchanger.

FIGURE 26–20 Electrogenic sodium absorption in the

colon. Sodium enters the epithelial cell via epithelial sodium channels

(ENaC).

Na+,K+-

ATPase

2K+ 3Na+

H+ NHE-3?NHE-2? Na+

KCC1

?

K+

Cl−

CLD Cl−

HCO 3 –

2K+

Na+

Cl−

ENaC

K+

Na+,K+-

ATPase

3Na+