Several things affect your blood pressure. Two involve the heart.

If the heart beats faster or harder (pumping more blood per beat), this will increase your blood pressure. Coffee usually contains caffeine. A study that appeared in the American Journal of Hypertension in 2000 tested healthy college students and found that their blood pressure went up significantly when they drank juice containing caffeine. Students who drank juice without caffeine didn't have an increase in blood pressure. Neither the students nor the people measuring blood pressure knew which students were getting the caffeine in their juice. Why do you think the experiment was set up this way?

Another experiment showed that people who drank decaffeinated triple espressos had an increase in blood pressure, just like those people who drank regular triple espressos. Maybe other chemicals in coffee raised their BP, or maybe it was the "placebo effect," which is what happens when people have a physical response because they think they're getting a drug.

Another study showed that caffeine made arteries less flexible, which would also lead to high blood pressure. High blood pressure can lead to strokes, kidney disease, and heart attacks. That's why people worry about their blood pressure.

Some things that reduce blood pressure are:
not using caffeine or tobacco,
exercising regularly (during exercise your BP will go up, but your resting BP will be lower),
reducing fat, salt, and cholesterol in what you eat,
and avoiding and reducing stress.