Now first let me state that any amount of exercise will be beneficial to your cardiovascular health, there are many causes for hypertension (high BP) and the effects of exercise will vary based on that. For example, you can be very fit but still have hypertension that is caused by a hormonal thyroid problem.

That said, there are actually a number of reasons why (that we know of through research; quite possibly other pathways as well)! Here are the major ones:

1.

Exercise makes your blood more efficient at delivering oxygen. Because your tissues need more oxygen when you exercise, your body produces more RBCs through bone marrow, which promotes vasodilation (=Less BP).
2.

Exercise brings more oxygen into your body. Because your lungs are more efficient and more powerful at bringing in oxygen, you have more oxygen dissolved in your blood. Higher partial pressures of Oxygen in blood causes vasodilation.
3.

Exercise improves the strength of skeletal muscles. Skeletal muscles act as a pump on veins to return blood to the heart, thus reducing the need for high pressure on your blood vessels.
4.

Exercise reduces stress hormones. These hormones act on your nervous system to increase heart rate and promote vasoconstriction (higher BP). Exercise reduces this.
you also can have a say on this, To be continued...