| A | B |
|---|
| Step 1 | O2-poor blood from the body enters the right atrium through the upper and lower venae cavae |
| Step 2 | The right atrium contracts, pushing O2-poor blood through a valve and into the right ventricle |
| Step 3 | The right ventricle contracts, pushing O2-poor blood through a valve and to the lungs through the pulmonary artery |
| Step 4 | O2-rich blood returns from the lungs through the pulmonary veins and enters the left atrium |
| Step 5 | The left atrium contracts, pushing O2-rich blood through a valve and into the left ventricle |
| Step 6 | The left ventricle contracts, pushing O2-rich blood through a valve and into the aorta |
| Step 7 | The aorta branches into smaller and smaller arteries, distributing O2-rich blood throughout the body |
| Step 8 | Smaller veins drain O2-poor blood from the body into larger and larger veins on its way back to the heart |
