The secretion of the liver varies less in the amount formed at different times than that of other digestive glands. Although the changes in the rate of its secretion are not so marked, they follow the same general rule as those of other glands connected with digestion, i. e., after food is taken there is a sudden rise, then a gradual fall, followed by a second rise in the rate of secretion. This is well seen in the case of the pancreas. Want of food is said to check the secretion of bile, but only does so in a slight degree, for the more important work of the liver is continuous, as is the activity of all glands whose duty it is to eliminate noxious substances or otherwise influence the composition of the blood. At the end of a period of fasting, the gall bladder is always found greatly distended, because the secretion has continued to flow into that receptacle, and there has been no call for its discharge into the duodenum.
The amount of bile produced by dogs is much influenced by their diet. It is very great when meat alone is consumed, less with vegetable, and very small with a diet of pure fat. As a general rule, bile is more abundantly produced in herbivorous than in carnivorous animals.
The rate of secretion is much influenced by the amount of blood flowing through the organ, which probably explains the increase during digestion. Ligature of the portal vein causes arrest of the secretion, and death. After ligature of the hepatic artery the secretion continues, but soon diminishes from malnutrition of the tissue of the liver, which ultimately causes death if the entire vessel be tied.
These variations in the rate of secretion may depend on direct nervous influence, but no special secretory nerve mechanism has been discovered for the liver, and it is quite possible that the changes in the activity of the gland which accompany the different periods of digestion may be accounted for by changes in the intestinal blood supply, which give rise to corresponding differences in the amount of blood flowing through the portal vein.
The force with which the bile is secreted is very small. That is to say, the pressure in the ducts never exceeds that of the blood (as is the case in the salivary glands); but, on the contrary, when a pressure of about 16 mm. (.63 in.) mercury is attained, the evacuation of the bile ceases, and with a little increase of opposing force the fluid in the manometer retreats and finds its way into the blood. The low pressure which can be reached in the gall ducts does not imply any want of secretory power on the part of the liver cells, but merely that there exists a great facility of communication between the duct radicles and the blood vessels, probably through the medium of the lymphatics. This is made obvious by experiment, by which it can be shown that with a comparatively low pressure (200 mm. = nearly 8 in. of water for a guinea-pig) fluid can be forced into the circulation from the bile ducts.
This is seen also in stoppage of the bile ducts in the human subject, when some of the bile constituents continue to be formed, and pass into the blood, where their presence is demonstrated by the yellow color characteristic of jaundice. The ready evacuation of the bile is a matter of great importance for health, the least check to its free exit causing the secretion to be forced into the circulating blood instead of into the gall passages. Under normal circumstances, the large receptacle of the gall bladder being always ready to receive the bile, ensures its easy exit from the ducts, but the forces which cause its flow are extremely weak. The smooth muscle in the walls of the duct seem rather for the purpose of regulating than aiding the flow.
When food from the stomach begins to flow into the duodenum, the muscular coat of the gall bladder contracts and sends a flow of bile into the intestine, which action is doubtless brought about by a reflex nerve impulse, for it is only when this part is stimulated that the bile flows freely from the bladder. The acid gastric contents seem to be the most efficacious stimulus.
In the human subject the quantity of bile secreted has been found to be about 600 c.c. (21 oz.) per diem in cases where there were biliary fistulas. This would equal about 13 grms. per kilo of the body weight.
In the guinea-pig and rabbit, it has been estimated to be about 150 grms. per kilo of the body weight.