Angiotensin-converting enzyme (ACE) inhibitors are very important drugs for the control of heart failure. They relax blood vessels and lower blood pressure. This improves blood flow.

In the body there are systems that work together to keep blood pressure at a normal level. This level involves a voltage high enough to carry oxygen and nutrients to the body, but not so high as to damage blood vessels or organs. ACE inhibitors and ARBs reduce blood pressure by affecting these systems.
The kidneys have sensors that can detect if the blood pressure drops too much. When this happens, a hormone called angiotensin I is released into the bloodstream. By itself, angiotensin I does not produce a large effect, but when combined with the angiotensin-converting enzyme (ACE) in the blood, it is converted to angiotensin II.

Angiotensin II can raise blood pressure in two ways:

The first, by narrowing the blood vessels, which increases the pressure of the circulating blood (as if you were squeezing a hose).
The second, by causing the release of another hormone, called aldosterone. This gives the body the message to retain the liquid, instead of eliminating it in the urine. The extra fluid stays in the blood, which increases the body’s blood volume. The blood pressure increases, because there is a greater amount of blood driven by the same space.

ACE inhibitors block the action of said enzyme, so that angiotensin I does not become angiotensin II. Thus, ACE inhibitors relax the walls of blood vessels and reduce blood pressure.
ARBs interfere with the action of angiotensin II on blood vessels, and also reduce blood pressure.