Leech saliva has relaxing, vasodiating effect which helps the blood flow so heart needs to use less pressure to push the blood through narrowed vessels. Hirudin in leech saliva thins the blood which also eases the blood flow. Enzymes such as apyrase, destabilase and others help to clean the blood vessels from plaques, clots and unhealthy atherosclerotic deposits.
As a result cleaner blood circulates easier and pressure reduces.
Leech therapy can help regulate both high and low blood pressure (except when too low) and gradually bring it to healthy norm.