Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 26Overview of Gastrointestinal Function & Regulation 449

muscle and sympathetic noradrenergic activity generally de-
creasing it while causing sphincters to contract. The pregangli-
onic parasympathetic fibers consist of about 2000 vagal
efferents and other efferents in the sacral nerves. They generally
end on cholinergic nerve cells of the myenteric and submucous
plexuses. The sympathetic fibers are postganglionic, but many
of them end on postganglionic cholinergic neurons, where the
norepinephrine they secrete inhibits acetylcholine secretion by
activating α 2 presynaptic receptors. Other sympathetic fibers
appear to end directly on intestinal smooth muscle cells. The
electrical properties of intestinal smooth muscle are discussed
in Chapter 5. Still other fibers innervate blood vessels, where
they produce vasoconstriction. It appears that the intestinal
blood vessels have a dual innervation: They have an extrinsic
noradrenergic innervation and an intrinsic innervation by fi-
bers of the enteric nervous system. VIP and NO are among the
mediators in the intrinsic innervation, which seems, among
other things, to be responsible for the hyperemia that accompa-
nies digestion of food. It is unsettled whether the blood vessels
have an additional cholinergic innervation.

GASTROINTESTINAL

(SPLANCHNIC) CIRCULATION

A final general point that should be made about the gas-
trointestinal tract relates to its unusual circulatory features.
The blood flow to the stomach, intestines, pancreas, and liver
is arranged in a series of parallel circuits, with all the blood
from the intestines and pancreas draining via the portal vein
to the liver (Figure 26–25). The blood from the intestines,
pancreas, and spleen drains via the hepatic portal vein to the
liver and from the liver via the hepatic veins to the inferior
vena cava. The viscera and the liver receive about 30% of the
cardiac output via the celiac, superior mesenteric, and inferi-
or mesenteric arteries. The liver receives about 1300 mL/min
from the portal vein and 500 mL/min from the hepatic artery
during fasting, and the portal supply increases still further af-
ter meals.

CHAPTER SUMMARY

■ The gastrointestinal system evolved as a portal to permit con-

trolled nutrient uptake in multicellular organisms. It is func-

tionally continuous with the outside environment and is

defended by a well-developed mucosal immune system. Never-

theless, the gut usually lives in harmony with an extensive com-

mensal microflora, particularly in the colon.

■ Digestive secretions serve to chemically alter the components of

meals (particularly macromolecules) such that their constitu-

ents can be absorbed across the epithelium. Meal components

are acted on sequentially by saliva, gastric juice, pancreatic juice,

and bile, which contain enzymes, ions, water, and other special-

ized components.

■ The intestine and the organs that drain into it secrete about 8 L

of fluid per day, which are added to water consumed in food and

beverages. Most of this fluid is reabsorbed, leaving only approx-

imately 200 mL to be lost to the stool. Fluid secretion and

absorption are both dependent on the active epithelial transport

of ions, nutrients, or both.

■ Gastrointestinal functions are regulated in an integrated fashion

by endocrine, paracrine, and neurocrine mechanisms. Hor-

mones and paracrine factors are released from enteroendocrine

cells in response to signals coincident with the intake of meals.

■ The enteric nervous system conveys information from the cen-

tral nervous system to the gastrointestinal tract, but also often

can activate programmed responses of secretion and motility in

an autonomous fashion.

■ The intestine has an unusual circulation, in that the majority of

its venous outflow does not return directly to the heart, but rath-

er is directed initially to the liver via the portal vein.

FIGURE 26–25 Schematic of the splanchnic circulation

under fasting conditions. Note that even during fasting, the liver

receives the majority of its blood supply via the portal vein.

Vena

cava Hepatic veins

500 mL/min 1300 mL/min

700 mL/min

700 mL/min

Aorta

Rest of

body

Inferior

mesenteric artery

Superior

mesenteric artery

*Branches of the hepatic artery also supply the stomach,

pancreas and small intestine

Spleen

Stomach

Pancreas

Colon

Small

intestine

400 mL/min

Liver

Celiac artery

Portal vein

Heart

He

pat

ica

rtery*