The diagram below represents one of many microscopic air sacs in a human lung. The alveolus (air sac) is the place where oxygen (O2) and carbon dioxide (CO2) move into or out of the blood, as represented in the diagram. Which statement best explains why these gases are able to move in the directions shown in the diagram?
O The CO2 moves out of the capillary and into the alveolus to make more room for the blood to carry O2.
O The O2 is needed by the cells, so it is actively transported into the blood. The CO2, which is not needed, is actively transported out of the blood.
O The blood coming to the lungs is low in CO2 and high in O2, so the gases each diffuse from a lower to a higher concentration in this area.
O The blood coming to the lungs is high in CO2 and low in O2, so the gases each diffuse from a higher to a lower concentration in this area.
