Ganong's Review of Medical Physiology, 23rd Edition

M
IC
IC
TV
IC
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 apical
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.)
G
IC
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 gastrointestinal
physiology, that digestive and absorptive capacity are markedly
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.
Tubulovesicle
MV
M 3
M
Resting
H 2
CHAPTER 26 Overview of Gastrointestinal Function & Regulation 435
M
Canaliculus
CCK−B
H + ,K + ATPase
PANCREATIC SECRETION
The pancreatic juice contains enzymes that are of major importance
in digestion (see Table 26–2). Its secretion is controlled
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 granules)
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 coalesce
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 duodenal
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 26–9 Parietal cell receptors and schematic representation of the morphological changes depicted in Figure 26–7. Amplification
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.)
M 3
ACh
Secreting
cAMP
Ca++
H2 Histamine
Ca ++
CCK−B
Gastrin