This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
The influence of the circulatory system upon digestion appears in the composition of the blood and in its rate of supply to the digestive glands. Vigorous active circulation accompanies good digestion and maintains a normal local reaction and temperature, and feeble, sluggish circulation produces local congestion of the viscera and interferes with gland secretion and absorption. When the nerves of the salivary glands are experimentally stimulated in animals by an electric current the blood vessels are altered in calibre. If the chorda-tympani nerve is stimulated the vessels are dilated and the rapidity of the blood flow is accelerated so that the venous blood issuing from the gland is of a red arterial hue, it not having lingered long enough to undergo the ordinary changes in regard to its gases. The salivary secretion becomes watery and contains a smaller percentage of solids.
On the other hand, when the sympathetic nerves are stimulated exactly the reverse occurs. Hence the blood supply is shown to alter the digestive power of a secretion by modifying its composition.
Blood which is impoverished in composition, watery, anaemic, or deficient in albuminous ingredients, will furnish poor materials for the manufacture of the digestive secretions, and, further, the muscular walls of the alimentary canal will suffer from malnutrition and peristaltic action will be diminished.