Blood is confined to blood vessels in a closed circulatory system.This allows the pressure to be regulated.During a typical cardiac cycle, pressure changes are associated with the contraction of the atria and ventricles and the opening and closing of valves.A. Left ventricular pressure is low at first but gradually increases as the ventricles fill and the atria contract.B. The AV valves close and the pressure in the ventricles rises dramatically when the ventricles contract.C.The pressure rises above that of the arteries and blood is forced through the semilunar valves into the arteries. The pressure falls as the ventricles empty and the walls relax.D.Atrial pressure is always relatively low because of the thin walls.E. Atrial pressure drops when the AV valve closes and the atrial walls relax. F. Aortic pressure rises when the ventricles contract. The pressure never falls below about 10kPa because the elastic wall recoils.G. Left ventricular volume rises as the atria contract then drops suddenly as blood leaves the left ventricle and enters the aorta.