Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 26

Overview of Gastrointestinal Function & Regulation 439

by bacterial action, deoxycholic, lithocholic, and ursodeoxy-

cholic acids are called secondary bile acids.

The bile salts have a number of important actions: they

reduce surface tension and, in conjunction with phospholip-

ids and monoglycerides, are responsible for the emulsification

of fat preparatory to its digestion and absorption in the small

intestine (see Chapter 27). They are

amphipathic,

that is, they

have both hydrophilic and hydrophobic domains; one surface

of the molecule is hydrophilic because the polar peptide bond

and the carboxyl and hydroxyl groups are on that surface,

whereas the other surface is hydrophobic. Therefore, the bile

salts tend to form cylindrical disks called

micelles.

A top view

of micelles is shown in Figure 26–16 and a side view of one in

FIGURE 26–14
Ion transport pathways present in pancreatic duct cells.
CA, carbonic anhydrase; NHE-1, sodium/hydrogen exchanger-
1; NBC, sodium-bicarbonate cotransporter.
(Adapted from Barrett KE:
Gastrointestinal Physiology

. McGraw-Hill, 2006.)

CFTR cAMP

Duct lumen Basolateral

Cl−

NHE-1

Na+

Na+

3Na Na+, K+

+

2K+

K+

H+

2HCO 3

ATPase

CO 2 + H 2 O

HCO 3 + H+

C.A

NBC

+

−

Cl−/HCO 3

Exchanger

−

HCO 3 −

−

channel

TABLE 26–4
Composition of human hepatic duct bile.

Water 97.0%

Bile salts 0.7%

Bile pigments 0.2%

Cholesterol 0.06%

Inorganic salts 0.7%

Fatty acids 0.15%

Phosphatidylcholine 0.2%

Fat 0.1%

Alkaline phosphatase ...

FIGURE 26–15
Human bile acids.
The numbers in the formula
for cholic acid refer to the positions in the steroid ring.

HO OH

COOH

OH

37

CH 3

CH 3

Cholic acid

12

Cholic acid

Chenodeoxycholic acid

Deoxycholic acid

Lithocholic acid

3

OH

OH

OH

OH

Percent in

human bile

50

30

15

5

OH

H

OH

H

OH

OH

H

H

712

Group at position

FIGURE 26–16

Lipid digestion and passage to intestinal

mucosa.

Fatty acids (FA) are liberated by the action of pancreatic li-

pase on dietary triglycerides and, in the presence of bile salts (BS), form

micelles (the circular structures), which diffuse through the unstirred

layer to the mucosal surface.

(Adapted from Thomson ABR: Intestinal

absorption of lipids: Influence of the unstirred water layer and bile acid micelle. In:

Disturbances in Lipid and Lipoprotein Metabolism.

Dietschy JM, Gotto AM Jr, Ontko JA

[editors]: American Physiological Society, 1978.)

Dietary

triglyceride

Pancreatic

lipase

Mucosa

BULK SOLUTION

OF INTESTINAL

CONTENTS

UNSTIRRED

LAYER

FA absorption

FA absorption in presence of BS

in absence of BS