Letâs first look at the loading of oxygen into the blood. Each alveolus is surrounded by a network of capillaries. Here for simplicity, we are looking at a portion of one alveolus and one capillary. The PO2 of the alveolar air is 104 mm of mercury. At rest, the oxygen poor blood entering the pulmonary capillaries has a PO2 of 40 mm of mercury. Now watch the PO2 increase as blood flows past the alveolus. Notice that there is a net diffusion of oxygen along its partial pressure gradient from the alveolus into the blood until equilibrium is reached. The PO2 of the oxygen rich blood has increased to 104 mm of mercury. Letâs look at a graph of these changes in the blood. As indicated in the graph, equilibrium is reached rapidly within the first third of a pulmonary capillary. Now letâs look at the unloading of carbon dioxide from the blood into the alveolus. The PCO2 of the alveolar air is 40 mm of mercury. At rest, the PCO2 of the blood entering the pulmonary capillaries is 45 mm of mercury. Letâs watch the PCO2 decrease as blood flows past the alveolus. Carbon dioxide diffuses along its partial pressure gradient from the blood into the alveolus until equilibrium is reached. The PCO2 of the blood has decreased to 40 mm of mercury. Now letâs see a graph of these changes in the blood. As indicated in the graph, equilibrium is reached rapidly within the first four tenths of the pulmonary capillary. Loading oxygen and unloading carbon dioxide actually occur simultaneously. As you inhale, you replenish oxygen and as you exhale, you eliminate carbon dioxide. Letâs watch the simultaneous exchange of oxygen and carbon dioxide. Notice how much smaller carbon dioxideâs partial pressure gradient is than oxygenâs. As Henryâs law states, the number of molecules, which dissolve in a liquid is proportional to both the partial pressure and the gas solubility. Since carbon dioxide is very soluble in blood, a large number of molecules diffuse along this small partial pressure gradient. Oxygen, which is less soluble, requires a much larger concentration gradient to provide adequate oxygen to the body. Now letâs turn our attention to internal respiration: During internal respiration, oxygen diffuses from the systemic capillaries into the cells and carbon dioxide diffuses from cells into the systemic capillaries Just as in external respiration, the exchange of oxygen and carbon dioxide depends on several factors: The first factor is the available surface area, which varies in different tissues throughout the body. Second, as we saw in the lungs, gases diffuse along their partial pressure gradients. The third factor is the rate of blood flow in a specific tissue. Blood flow in a tissue varies for many reasons including the tissues metabolic rate. Recall that during metabolism cells use oxygen and produce carbon dioxide. Letâs look at the exchange of oxygen and carbon dioxide during internal respiration. Remember that capillaries typically branch profusely within tissues, but here we are looking at only one capillary and two layers of cells. In relatively inactive organs, the tissue cells have a PO2 of 40 mm of mercury and a PCO2.of 45 mm of mercury. As the blood enters the systemic capillaries it has a PO2 of 100 mm of mercury and a PCO2 of 40 mm of mercury. Notice that the PO2 entering the systemic capillaries is lower than the alveolar PO2 of 104 mm of mercury. This small decrease is due primarily to imperfect ventilation-perfusion coupling in the lungs. Now letâs watch the simultaneous exchange of both oxygen and carbon dioxide. Gas exchange continues until equilibrium is reached. At equilibrium, the blood in the systemic capillaries has a PO2 of 40 mm of mercury and the PCO2 of 45 mm of mercury. The oxygen poor blood now returns through the systemic veins to the right side of the heart.