So this region here is region number eight, the alveolar space. Now, within the alveolar space, we have a partial pressure of oxygen equaling to 105 mercury, while the partial pressure due to our carbon dioxide molecules, that is 40 mercury, now, inside the lumen of our pulmonary arterial, these are the concentrations, these are the partial pressures of these same gas molecules. Notice the oxygen is 40 mercury, which is less than inside the alveolar space, while the carbon dioxide has a higher concentration, 45 mercury, inside the lumen of the arterial compared to our alveolar space. So we have a difference in pressure. And whenever we have a difference in pressure, we know we have a pressure gradient. And these gas molecules will begin to move down their gradient from a high pressure to a low pressure.