During relaxation of the heart (diastole) the left ventricle of the heart fills with blood returning from the lungs. The left ventricle then contracts and pumps blood into the arteries (systole).
The blood pressure during contraction of the ventricle (systolic pressure) when blood is being actively ejected into the arteries is higher than during relaxation of the ventricle (diastolic pressure).
The pulse that we can feel when we place our fingers over an artery is caused by the contraction of the left ventricle.
Blood pressure is determined by two factors: 1) The amount of blood pumped by the left ventricle of the heart into the arteries, and 2) the resistance to the flow of blood caused by the walls of the arterioles (smaller arteries).
Generally, blood pressure tends to be higher if more blood is pumped into the arteries or if the arterioles are narrow and stiff. (Narrow and stiff arterioles, by resisting the flow of blood, increase blood pressure.) This often happens when older patients develop atherosclerosis.
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