Gas Exchange & Partial Pressures

Gas exchange in the lungs involves the diffusion of oxygen and carbon dioxide between the lungs and pulmonary capillaries. The partial pressure gradient is a key driver of diffusion. In healthy lungs, oxygen and carbon dioxide diffuse rapidly and achieve equilibrium. Representative portion of the tracheobronchial tree, with an alveolar sac ballooning from its terminal end. A pulmonary capillary, which is where gas exchange occurs in the lungs. Mixed systemic venous blood becomes oxygenated systemic arterial blood in this vessel. Mixed systemic venous blood is blood returning from the peripheral tissues, so it is poorly oxygenated. To understand gas exchange, we need to understand how the partial pressures of oxygen and carbon dioxide change as we move through the respiratory tract. First, recall that the air we breathe is a mixed gas – it comprises nitrogen, oxygen, carbon dioxide, and water vapor. Each of these gases exerts a different force on the surfaces it encounters (i.e., our respiratory tract); partial pressure is the pressure of a single gas in the mixture. For example, atmospheric pressure at sea level is 760 mmHg – this is the total pressure of all gases in the atmosphere; the partial pressure of oxygen in the atmosphere is only a fraction of this, 160 mmHg. Let's see how we use Dalton's and Henry's Laws to home in on specific gases in respiration. Dalton's Law indicates that the partial pressure of a gas is the pressure that gas would exert if it occupied the total volume of a mixture. In other words, the total pressure of a mixture of gas types is equal to the sum of the partial pressures of each gas present. We show this with a series of containers: First, draw a container of solution with two types of molecules; we'll call them "a" and "b." The total pressure of the gases in this mixture equals 7 mmHg. Then, show that this is equal to the partial pressure of "a," which happens to be 4 mmHg, plus the partial pressure of "b," which happens to be 3 mmHg. The partial pressure of a gas is not the same as its concentration, but the two are related: Henry's Law states that the concentration of a dissolved gas is equal to its partial pressure multiplied by a solubility coefficient. Now, let's return to our diagram and show how the partial pressures of oxygen and carbon dioxide change with inspiration and facilitate gas exchange. At the opening of the trachea, show the following values for dry inspired air: The partial pressure of oxygen is 160 mmHg. The partial pressure of carbon dioxide is negligible so we say it is 0 mmHg. As air moves through the moist environment of the trachea, the oxygen is "diluted" by water vapor. Thus, partial pressure of humidified tracheal (P HT) oxygen is ~150 mmHg; Partial pressure of carbon dioxide remains unchanged, at 0 mmHg. In the systemic mixed venous blood in the pulmonary capillary the partial pressures (PV) are as follows: The partial pressure of oxygen is only 40 mmHg because oxygen was taken up by the peripheral tissues; The partial pressure of carbon dioxide is 46 mmHg because CO2 was added in the peripheral tissues. Carbon dioxide moves from the capillary blood to the alveolar air, which leads to carbon dioxide equilibrium: Alveolar partial pressure (PA) of carbon dioxide raises to 40 mmHg. Systemic arterial blood partial pressure (Pa) drops to 40 mmHg. Similarly, oxygen moves from the alveolar air to the capillary blood to reach equilibrium: Alveolar air oxygen partial pressure drops to 100 mmHg. Systemic arterial blood partial pressure raises to 100 mmHg. This is an example of perfusion-limited transport, because the only way to increase gas exchange at this point would be to increase pulmonary blood perfusion and the rate at which mixed venous blood arrived at the alveoli. Finally, let's consider the rate of diffusion of oxygen and carbon dioxide. Fick's law states that the diffusion rate is determined by: the diffusion coefficient (D), the partial pressure gradient between two points (P1-P2), the surface area (A), and, the thickness of the barrier (T). Thus, we can use the following diffusion rate equation: The rate of gas diffusion = (D ((P1 - P2)) A)/T The diffusion coefficient multiplied by the partial pressure gradient multiplied by the surface area, all divided by the thickness of the barrier. With this understanding, we can predict that respiratory diseases will negatively affect diffusion rates. Emphysema is characterized by alveolar destruction, and, therefore, decreased surface area available for diffusion. At the other extreme, fibrotic diseases thicken the alveolar wall, and, therefore, increase the barrier to diffusion.