Complex sentence: Secretin is released into circulation and/or intestinal lumen in response to low duodenal pH that ranges between 2 and 4.5 depending on species; the acidity is due to hydrochloric acid in the chyme that enters the duodenum from the stomach via the pyloric sphincter.
Split into simpler sentences: Secretin is released into circulation and/or intestinal lumen in response to low duodenal pH that ranges between 2 and 4.5 depending on species. The acidity is due to hydrochloric acid in the chyme that enters the duodenum from the stomach via the pyloric sphincter.