The heart beats and pumps blood in to the aorta and throughout the arteries, it replaces blood which has escaped through the capillaries ensuring the volume is maintained in the arterial system. The artery walls contain muscle and elastic fibre and as the heart relaxes the fibres contract and therefore reduce the capacity of the circulatory system which ensures pressure is maintained. The arterioles which are small minute arterial vessels leading from the smallest arteries to the capillaries, have muscular walls. As the blood flow through the capillaries the muscles contract, which in turn controls the rate that blood escapes from the arterial system, through the capillaries and into the
