Corpus Curare Spiritumque
The heart is a tough operating mechanism which moves blood around the body through a very advanced system called arteries and capillaries; the blood is then carried back to the heart by means of veins. Blood pressure is the thrust of this blood in the body pushing up against the inside walls of the arteries as the heart is pumping.
As the heart compresses it will drive this blood into the arteries which makes an increase in pressure. This increase in pressure is noted as systolic pressure.