When the pH of the gastric contents is low, acid secretion stops but, if gastrin is injected, the hydrogen ion secretion will restart. The intestinal phase of gastric secretion and also the opposing action of inhibition of gastric secretion, e.g. by fats or by a low pH in the duodenum, is mediated by hormones but it is not absolutely certain which hormones are involved. BILE Bile is an isotonic secretion containing the usual ions plus bile salts and bile pigments. Bile salts are steroid-like derivatives of cholesterol with a polar, hydrophilic part, so that they act like detergents. They are essential for the absorption of fats which will be considered further in the lecture on absorption.
Bile pigments are the route by which the breakdown products of red cells are excreted.
When red cells are destroyed in the liver, spleen or bone marrow, the globin chains are broken down to amino acids which are added to the circulating pools in the blood. The iron is removed from the haem and stored as ferritin, the rest of the haem is turned into bilirubin, which is insoluble and carried to the liver bound to plasma protein. In the liver, the bilirubin is conjugated with glucuronic acid, which makes it soluble. The conjugated bilirubin is secreted into the bile and, after it has been modified by bacteria, it is responsible for the colour of the faeces. The formation and release of the bile and its control Bile is formed in the liver and passed down the hepatic and bile ducts.