The cardiovascular system circulates blood throughout the body using the heart as a pump. The heart has four chambers - two upper atria and two lower ventricles. It is surrounded by membranes and tissues. Blood enters the right atrium from the body, then passes to the right ventricle which pumps it to the lungs. Oxygenated blood returns to the left atrium and passes to the left ventricle which pumps it out to the body via the aorta. The heart contracts over 100,000 times per day to circulate blood through the pulmonary and systemic circuits. Valves ensure blood only flows in one direction through the heart.