Ganong's Review of Medical Physiology, 23rd Edition

438 SECTION V Gastrointestinal Physiology
Gallbladder
FIGURE 26–11 Structure of the pancreas. (Reproduced with permission
from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The Mecha-
nisms of Body Function, 11th ed. McGraw-Hill, 2008.)
BILE
Duodenum
Exocrine cells
(secrete
enzymes)
Common bile
duct from gallbladder
Pancreas
Bile is made up of the bile acids, bile pigments, and other substances
dissolved in an alkaline electrolyte solution that resembles
pancreatic juice (Table 26–4). About 500 mL is
secreted per day. Some of the components of the bile are reab-
Right hepatic duct Left hepatic duct
Cystic
duct
Gallbladder
Common
hepatic
duct
Bile duct
Accessory
pancreatic
duct
Ampulla of bile duct Duodenum
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.)
Endocrine cells
of pancreas
Duct cells
(secrete
bicarbonate)
Pancreatic duct
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 – , SO4 2– , HPO4 2–
Digestive enzymes (see Table 26–1; 95% of protein in juice)
Other proteins
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 discussed
below.
The bile acids secreted into the bile are conjugated to glycine
or taurine, a derivative of cysteine. The bile acids are synthesized
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 chenodeoxycholic
acid. Ursodeoxycholic acid is a tautomer of chenodeoxycholic
acid at the 7-position. Because they are formed
Concentration of electrolytes
(meq/L) and amylase (U/mL)
150
120
90
60
30
Secretin 12.5 units/kg IV
(HCO 3 − )
(CI − )
(Amylase)
(K + )
0
−20 −10 0 +10 +20 +30 +40
Time (min)
Volume of
secretion (mL) 0.3 0.2 17.7 15.2 5.1 0.6
FIGURE 26–13 Effect of a single dose of secretin on the
composition and volume of the pancreatic juice in humans.