Similar cells form tissues; tissues work together in organs; organs interact in organ systems. Lungs and the respiratory system exchange gases, while the heart and circulatory system transport blood. In alveoli, oxygen passes into blood and carbon dioxide passes out. Many alveoli provide a large total area with thin walls near capillaries.
The heart pumps blood. Arteries carry it away from the heart; veins carry it toward the heart. These names describe direction, not guaranteed oxygen content. Blood transports oxygen to cells for aerobic respiration. Keep the processes separate: breathing moves air, circulation moves blood, and respiration transfers energy through reactions in cells.
Example 1
Why do many small alveoli support gas exchange better than one small smooth sac?
Together they provide a much larger exchange surface, with thin walls close to capillaries.
Example 2
A vessel carries blood from the heart to the lungs. Is it an artery or vein?
An artery, because it carries blood away from the heart, even though this blood has relatively little oxygen.
Watch out: Arteries carry blood away from the heart. The pulmonary artery carries oxygen-poor blood to the lungs.