The intrinsic factor is secreted by the stomach and thus exists in both the gastric juice and the gastric mucous membrane. The optimal pH for its action is around 7. Its concentration is unrelated to the amount of HCl or pepsin in the gastric juice; for example, intrinsic factor may be present even when pepsin is scarce. The intrinsic factor's formation site differs between species. In pigs, it is obtained from the pylorus and the beginning of the duodenum; in humans, it is found in the fundus and stomach body.