Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 26

Overview of Gastrointestinal Function & Regulation 435

Gastric secretion adds about 2.5 L per day to the intestinal
contents. However, despite their substantial volume and fine
control, gastric secretions are dispensable for the full digestion
and absorption of a meal, with the exception of cobalamin
absorption. This illustrates an important facet of gastrointesti-
nal physiology, that digestive and absorptive capacity are mark-
edly in excess of normal requirements. On the other hand, if
gastric secretion is chronically reduced, individuals may display
increased susceptibility to infections acquired via the oral route.

PANCREATIC SECRETION

The pancreatic juice contains enzymes that are of major im-

portance in digestion (see Table 26–2). Its secretion is con-

trolled in part by a reflex mechanism and in part by the

gastrointestinal hormones secretin and cholecystokinin

(CCK).

ANATOMIC CONSIDERATIONS

The portion of the pancreas that secretes pancreatic juice is a

compound alveolar gland resembling the salivary glands.

Granules containing the digestive enzymes

(zymogen gran-

ules)

are formed in the cell and discharged by exocytosis (see

Chapter 2) from the apexes of the cells into the lumens of the

pancreatic ducts (Figure 26–11). The small duct radicles coa-

lesce into a single duct (pancreatic duct of Wirsung), which

usually joins the common bile duct to form the ampulla of

Vater (Figure 26–12). The ampulla opens through the duode-

nal papilla, and its orifice is encircled by the sphincter of Oddi.

Some individuals have an accessory pancreatic duct (duct of

Santorini) that enters the duodenum more proximally.

COMPOSITION OF PANCREATIC JUICE

The pancreatic juice is alkaline (Table 26–3) and has a high

HCO

3

• content (approximately 113 mEq/L vs. 24 mEq/L in
  plasma). About 1500 mL of pancreatic juice is secreted per
  day. Bile and intestinal juices are also neutral or alkaline, and
  these three secretions neutralize the gastric acid, raising the
  pH of the duodenal contents to 6.0 to 7.0. By the time the
  chyme reaches the jejunum, its pH is nearly neutral, but the
  intestinal contents are rarely alkaline.

FIGURE 26–8
Composite diagram of a parietal cell, showing
the resting state (lower left) and the active state (upper right).
The resting cell has intracellular canaliculi (IC), which open on the api-
cal membrane of the cell, and many tubulovesicular structures (TV) in
the cytoplasm. When the cell is activated, the TVs fuse with the cell
membrane and microvilli (MV) project into the canaliculi, so the area of
cell membrane in contact with gastric lumen is greatly increased. M,
mitochondrion; G, Golgi apparatus.
(Adapted from Junqueira LC, Carneiro J:
Basic Histology: Text & Atlas,
10th ed. McGraw-Hill, 2003.)

IC

IC

IC

G

TV

M

IC

MV M

M

FIGURE 26–9
Parietal cell receptors and schematic representation of the morphological changes depicted in Figure 26–7.
Amplifi-
cation of the apical surface area is accompanied by an increased density of H

• , K


• 
  

  –ATPase molecules at this site. Note that acetylcholine (ACh) and
  gastrin signal via calcium, whereas histamine signals via cAMP.
  (Adapted from Barrett KE:
  Gastrointestinal Physiology

  
  

. McGraw-Hill, 2006.)

Resting

Canaliculus

Tubulo- H+,K+ATPase

vesicle

M 3

H 2

M 3

H 2

CCK−B

CCK−B

Secreting

ACh

Histamine

cAMP

Gastrin

Ca Ca++

++