15) Heart valves open when the heart pumps to allow blood to flow. They close quickly between heartbeats to make sure blood does not flow backward. Any trouble in this normal flow will make it hard for the heart to pump the blood where it needs to go.
18) Oxygen is picked up from your lungs and distributed throughout the body by red blood cells which travel through the blood-stream to the heart. The waste gas is carried to the lungs by the circulatory system are able to pass from the cappillaries to the alveoli so they can be removed from the body