The large artery, aorta branches into innumerable smaller arteries and capillaries. Left ventricle is pumping chamber and pumps out blood in to these big and small arteries. Network of smaller arteries tends to contract and raises the blood pressure in large arteries and when they get opened the pressure in large arteries is achieved by layer of muscles in the walls of the arteries, which is controlled by many complex factors, involving nerve stimulation and chemical stimulation. In systemic circulation blood flow has to withstand this peripheral resistance. As mentioned earlier left ventricle is a pumping chamber and expels out blood in to systemic circulation when it contracts.
Pressure reaches to peak point at that time and is called systolic pressure .
This is the highest reading that may be 110 mm of Hg. This expelles blood flows at a more rapid rate out the peripheral arterial network. The left ventricle stops ejecting blood. Pressure in the arteries drops steadily as the blood flows out the arteries. The lowest pressure level is called the diastolic pressure which is around 70mm of Hg.