The Digestion and Absorption of Food
551
the blood to lower the blood volume and produce circulatory
complications. The large distension of the intestine by the
entering fl uid can also trigger vomiting in these patients. All
of these symptoms produced by the rapid entry of large quan-
tities of ingested material into the small intestine are known as
the
dumping syndrome
.
Once the contents of the stomach have emptied over
a period of several hours, the peristaltic waves cease and the
empty stomach is mostly quiescent. During this time, how-
ever, there are brief intervals of peristaltic activity that we will
describe along with the events controlling intestinal motility.
Pancreatic Secretions
The exocrine portion of the pancreas secretes bicarbonate ions
and a number of digestive enzymes into ducts that converge
into the pancreatic duct, which joins the common bile duct
from the liver just before it enters the duodenum (see Figure
15–4). The enzymes are secreted by gland cells at the pancreatic
end of the duct system, whereas bicarbonate ions are secreted
by the epithelial cells lining the ducts (
Figure 15–25
).
The mechanism of bicarbonate secretion is analogous to
that of hydrochloric acid secretion by the stomach (see Figure
15–18), except that the directions of hydrogen ion and bicar-
bonate ion movement are reversed. Hydrogen ions, derived
from a carbonic anhydrase-catalyzed reaction between carbon
dioxide and water, are actively transported out of the duct cells
by an H
+
/K
+
-ATPase pump and released into the blood. The
bicarbonate ions are secreted into the duct lumen.
The enzymes the pancreas secretes digest fat, polysaccha-
rides, proteins, and nucleic acids to fatty acids, sugars, amino
acids, and nucleotides, respectively. A partial list of these
Begin
Stomach
Gastric emptying
Duodenum
Acidity
Fat
Amino
acids
Hypertonicity
Distension
Plasma
enterogastrones
CNS
Parasympathetic
efferents
Sympathetic
efferents
Secretion of
enterogastrones
Stimulate
neural receptors
Short neural
reflexes via
enteric neurons
Long neural
reflexes
Figure 15–25
Structure of the pancreas.
Figure 15–24
Intestinal-phase pathways inhibiting gastric emptying.
Common bile
duct from gallbladder
Duodenum
Gallbladder
Pancreatic duct
Exocrine cells
(secrete
enzymes)
Endocrine cells
of pancreas
Duct cells
(secrete
bicarbonate)
Pancreas
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