Is the increase of pressure due to the decrease in volume? Wouldn't the decrease in volume push the components of blood closer to one another resulting in an increase of repulsive forces, thus increasing pressure? Or is the increase in pressure due to the heart pumping the blood with more force in order to maintain blood flow?
Because pressure is inversely proportional to area. If you consider a pipe, if you squeeze it's tip to decrease it's area you'll observe that the velocity of water flow becomes high due to high pressure. Same is with blood in veins.