Completed biology worksheet featuring a labeled diagram of the human heart's chambers and major vessels.

- The muscle wall on the left side of the heart is thicker because it must generate much higher pressure to pump oxygenated blood through the aorta to the entire body, which is a long and high-resistance circuit. The right side only pumps deoxygenated blood to the nearby lungs, requiring less force.
- The heart has valves between the chambers to ensure unidirectional blood flow. They open to allow blood to move forward from atria to ventricles and then out to the arteries, and close tightly to prevent backflow into the chambers when the heart relaxes or contracts.