Think of your circulatory/vascular system as a liquid “plumbing” network consisting of 1) pump 2) pipes, and 3) fluid.
Now imagine that this system is sealed off and circulating. The pressure in this system depends upon the force of the pump, the size, number, and elasticity of the pipes, and the volume of the fluid.
We develop high blood pressure, in general, because 1) our pump is working too hard, 2) the pipes are too stiff or constricted, or; 3) there is too much fluid in the circuit. This is affected by our heredity, our diet (obesity, too much salt), our stress level, aging, atherosclerosis (hardening of the arteries), kidney disease, diabetes, and a host of other factors.
So, you can see that to reduce blood pressure with medications we can give drugs (or natural supplements) that: 1) turn down the pump (e.g., beta blockers); 2) dilate the pipes, or open up new sets of pipes (calcium channel blockers, ACE inhibitors, and many others), or; 3) drain off some fluid (diuretics/water pills).
Some prescription pills come in combinations two or more drugs and some medications do double duty all by themselves. Blood pressure medications accomplish one or more of these three tasks through a variety of mechanisms. Some work directly on the heart or blood vessel muscle layers. Some work through the central nervous system. Some work through direct or “chemical signal” methods via the kidneys.
But the principle is simple: “PUMP, PIPES, FLUID”.