Ganong's Review of Medical Physiology, 23rd Edition

438
SECTION V
Gastrointestinal Physiology

BILE

Bile is made up of the bile acids, bile pigments, and other sub-
stances dissolved in an alkaline electrolyte solution that re-
sembles pancreatic juice (Table 26–4). About 500 mL is
secreted per day. Some of the components of the bile are reab-

sorbed in the intestine and then excreted again by the liver

(enterohepatic circulation).

The glucuronides of the

bile pigments,

bilirubin and

biliverdin, are responsible for the golden yellow color of bile.

The formation of these breakdown products of hemoglobin is

discussed in detail in Chapter 29, and their excretion is dis-

cussed below.

The

bile acids

secreted into the bile are conjugated to gly-

cine or taurine, a derivative of cysteine. The bile acids are syn-

thesized from cholesterol. The four major bile acids found in

humans are listed in Figure 26–15. In common with vitamin

D, cholesterol, a variety of steroid hormones, and the digitalis

glycosides, the bile acids contain the steroid nucleus (see

Chapter 22). The two principal (primary) bile acids formed in

the liver are cholic acid and chenodeoxycholic acid. In the

colon, bacteria convert cholic acid to deoxycholic acid and

chenodeoxycholic acid to lithocholic acid. In addition, small

quantities of ursodeoxycholic acid are formed from chenode-

oxycholic acid. Ursodeoxycholic acid is a tautomer of cheno-

deoxycholic acid at the 7-position. Because they are formed

FIGURE 26–11
Structure of the pancreas.
(Reproduced with per-
mission from Widmaier EP, Raff H, Strang KT:
Vander’s Human Physiology: The Mecha-
nisms of Body Function
, 11th ed. McGraw-Hill, 2008.)

FIGURE 26–12
Connections of the ducts of the gallbladder,
liver, and pancreas.
(Adapted from Bell GH, Emslie-Smith D, Paterson CR:
Textbook
of Physiology and Biochemistry,
9th ed. Churchill Livingstone, 1976.)

Common bile

Duodenum duct from gallbladder

Gallbladder

Pancreatic duct

Exocrine cells

(secrete

enzymes)

Endocrine cells

of pancreas

Duct cells

(secrete

bicarbonate)

Pancreas

Right hepatic duct Left hepatic duct

Common

hepatic

duct

Bile duct

Cystic

duct

Gall-

bladder

Accessory

pancreatic

duct

Ampulla of bile duct Duodenum

Pancreas

Pancreatic

duct

TABLE 26–3

Composition of normal human

pancreatic juice.

Cations: Na

+

, K

+

, Ca

2+

, Mg

2+

(pH approximately 8.0)

Anions: HCO

3

• , Cl
  
  
  • , SO
    4
    2–
    , HPO
    4
    2–
  
  
  
  

Digestive enzymes (see Table 26–1; 95% of protein in juice)

Other proteins

FIGURE 26–13

Effect of a single dose of secretin on the

composition and volume of the pancreatic juice in humans.

Secretin 12.5 units/kg IV

(K+)

(HCO 3 −)

(CI−)

(Amylase)

150

120

90

60

30

0

− 20 − 10 0 +10 +20 +30 +40

Time (min)

Concentration of electrolytes(meq/L) and amylase (U/mL)

Volume of

secretion (mL) 0.3 0.2 17.7 15.2 5.1 0.6