Explain how the cardiac cycle pumps blood around the body

The heart is made up of four chambers; the left atria, left ventricle, right atria and the right ventricle. The cardiac cycle is made up of repeating phases of systole (contraction) and diastole (relaxation). Both of these change the volume and thus the pressure inside the heart chambers. These changes pump the blood around the body. Starting at atrial systole, the contraction of the atria decreases their volume which forces the atrioventricular valves to open, pushing the blood into the ventricles. As the ventricles fill up with blood, their volume starts to rise. The atria then relax (atrial diastole) and the ventricles begin to contact (ventricular diastole). The atrioventricular valves close as the pressure in the ventricles exceeds that in the atria. This prevents blood from flowing back into the atria. Meanwhile, the rising pressure in the ventricles forces the semilunar valves to open and blood is forced through them and out through the pulmonary artery and the aorta. Next comes ventricular and atrial diastole; the ventricles and the atria both relax which increases the volume and decreases the pressure inside the chambers. The pressure in the aorta and pulmonary artery is greater than that of the ventricles which causes the semilunar valves to close, preventing back flow. The pressure is the vena cava and pulmonary vein is higher than the atrial pressure so blood flows into the atria and the cycle starts again.