"Aldosterone release: This steroid hormone is released from the adrenal gland in response to either high serum potassium levels or if angiotensin II is present. This hormone increases the excretion of potassium by the kidneys, while increasing sodium retention. Since sodium is the main ion which determines the amount of fluid in the blood vessels by osmosis, aldosterone will increase fluid retention, and indirectly, blood pressure."

from:
http://en.wikipedia.org/wiki/Blood_pressure

Aldosterone is produced by the adrenal glands. Because of mercury's effect on the adrenals, the production of Aldosterone can be altered and blood pressure can lower as a result.