Question

1. In pathologic conditions where the alveolar membrane is thickened there is little effect on the exchange of carbon dioxide, but oxygen diffusion is impaired. Based on your understanding of the respiratory membrane function, propose an explanation for this phenomenon.

2. Would breathing pure oxygen cause a large increase in the partial pressure of oxygen within the blood stream? Why or why not?

Answer 1

1. Both Carbon dioxide and oxygen diffuse through the alveolar membrane for gas exchange in lungs. Normally, thinner the alveolar membrane, faster is the rate of diffusion.

Solubility of CO2 is higher than the solubility of oxygen in blood. CO2 solubility is 24 times higher than that of oxygen. Although large differences in partial pressure of the gases compensate for this solubility differences in normal conditions, oxygen transport will be slower in diseased states.

Relative rates of diffusion for Oxygen and CO2 is 0.85. Hence, total diffusion rate of CO2= 0.85 X 24= 20 times higher than that for oxygen at the same partial pressure. Further, rate of diffusion of CO2 through a liquid such as blood is 1.18 times lower than that seen for oxygen. As a result, total diffusion rate is affected. This will allow the CO2 equilibrium to be reached faster between capillary and the alveolus. Oxygen takes a longer time to equilibrate with alveolar gas. Hence, CO2 will still diffuse through the much thicker alveolar membrane in diseased state such as pulmonary fibrosis, but diffusion of oxygen will be impaired.

2. Pure oxygen is 100% oxygen. It results in a higher PaO2 than 20% oxygen that we normally breathe. This pure oxygen can damage the pulmonary epithelium. It can also cause surfactant inactivation and result in formation of edema in the alveoli. There may be fibrosis in the lung and interstitial thickening. There is increased greater stiffness and hysteresis in the lungs after exposure to pure oxygen. All these changes may lead to development of pulmonary atelectasis. Thus, pure oxygen if administered for longer duration damages the lungs due to oxygen toxicity. This toxicity is mostly due to generation of oxygen free radicals due to increase concentration of oxygen. These radicals will inactivate enzymes, affect protein transport, create oxidative stress Pure oxygen can only be tolerated by lungs for 24-48 hr at sea level.

The partial pressure of oxygen in arteries will increase by 5 times at sea level when inhaling pure oxygen. Pure oxygen is 100% oxygen while normal oxygen that is taken in is 21% oxygen. In case of 21% oxygen, the rest of the volume of air has water, CO2 and nitrogen. When the 21% oxygen is replaced by 100% oxygen, the air is denitrified, although CO2 and water content remain the same. Partial pressure of water is 47 mm Hg.

Partial pressure of oxygen in arteries PAO2= FiO2 X (Pb-47)-1.25 X partial pressure of CO2

At 21% oxygen, FiO2 is 0.21, while at 100% oxygen, it is 1. This will make the PAO2 at 21% oxygen to be 104 mm. However, as FiO2 increases at 100% oxygen, PAO2 will become 673 mm Hg. Thus, inhaling pure oxygen will cause large increase in partial pressures in blood stream.