CNS receives information from the GI tract (afferent input)
and has a vital inﬂ uence on GI function (efferent output).
This completes our overview of the gastrointestinal sys-
tem. Because its major tasks are digestion and absorption, we
begin our more detailed description with these processes.
Subsequent sections of the chapter will then describe, organ
by organ, regulation of the secretions and motility that pro-
duce the optimal conditions for digestion and absorption. A
prerequisite for this physiology, however, is an understanding
of the structure of the gastrointestinal tract wall.
Structure of the Gastrointestinal
From the mid-esophagus to the anus, the wall of the gastro-
intestinal tract has the general structure illustrated in
. Most of the luminal (inside) surface is highly convo-
luted, a feature that greatly increases the surface area available
for absorption. From the stomach on, this surface is covered
by a single layer of epithelial cells linked together along the
edges of their luminal surfaces by tight junctions.
secrete mucus into the lumen of the tract and endocrine cells that
release hormones into the blood. Invaginations of the epithelium
into the underlying tissue form exocrine glands that secrete acid,
enzymes, water, and ions, as well as mucus, into the lumen.
Common hepatic duct
Common bile duct
Bile ducts from the liver converge to form the
common hepatic duct, from which branches the duct
leading to the gallbladder. Beyond this branch, the
common hepatic duct becomes the common bile
duct. The common bile duct and the pancreatic duct
converge and empty their contents into the duodenum
at the sphincter of Oddi.
Average amounts of solids and ﬂ uid ingested, secreted, absorbed,
and excreted from the gastrointestinal tract daily.
Feces 100 ml water; 50 g solids excreted
1500 ml intestinal secretions
(primarily small intestine)
1500 ml pancreatic secretions
500 ml bile
2000 ml gastric secretions
1500 ml salivary secretions
1200 ml water/day; 500 800 g solids/day ingested
1400 ml absorbed