Chemistry
The active ingredients in senna are a group of substances known as the sennosides (A, B, C, and D).  These substances are part of a larger group of chemicals known as the anthraquinones, all of which have a similar chemical structure.

During digestion, the sennosides in the senna leaf acts on the walls of the bowel, causing:

• increased rate of movement, and
• increased levels of secretions.

The increased rate of movement means there is less time for water to be absorbed from the faeces.  This combines with the increased level of secretions to ensure that the faeces stay soft and bowel movement is not impeded.