An adequate level of pressure is needed in the arteries to keep the blood flowing through the cardiovascular system. The chief determinant of arterial pressure is the volume of blood in the arteries and the important determinants of this are the cardiac output and the peripheral vascular resistance. The resistance offered by the arterioles accounts for almost half the total resistance in the systemic circulation. The muscle in the arterial wall allows them to constrict or dilate. When they constrict, the resistance increases and the blood pressure increases. The vasomotor centre in the medulla when stimulated sends impulses via the sympathetic to the smooth muscle in the arteries, arterioles and veins which results in their constriction.