While the majority of the symptoms experienced by people infected with the SARS-CoV-2 (COVID-19) virus are mild, some develop the very serious condition of ARDS (acute respiratory distress syndrome), which results in a rapid and widespread inflammation of the lungs. The major function of the lungs-the process of breathing-facilitates the vital exchange of gases between the environment and the body. The rate of exchange of these gases is largely governed by diffusion, which takes place in the extensive network of tiny sacs in the lungs called alveoli-hundreds of millions of them! In fact, the surface area of the alveoli in the human lungs is so large that if every alveoli were laid flat, they would cover a surface area as large as 70 to 80 m². The diffusion of oxygen and carbon dioxide takes place over this large surface. Conditions like ARDS, and other diseases in the lungs, can reduce this surface area, resulting in an insufficient amount of oxygen being transferred to the blood during respiration, a condition known as hypoxemia (see the photo), which is why patients are given pure oxygen to breathe. ARDS results in considerable fluid accumulation in the lungs, which floods the alveoli and inhibits respiration. ARDS patients often have rapid breathing and are gasping for breath. This requires considerable physical exertion, and many patients quickly become exhausted. At this point, patients may be moved into an intensive care unit and placed on a mechanical ventilator to help them breathe. This is a very serious condition, and the overall prognosis for someone with ARDS is poor, with a mortality rate near 40%. For those who survive, they often experience a lower quality of life, due to irreversible damage. Consider a small section (5.20 m²) of the respiratory membrane that covers each alveolus. It is composed of a layer of squamous epithelial cells with a thickness of 1.00 pm. If the concentration of oxygen on one side of the membrane is 4.50 × 10-17 mol/μm³ and 7.80 x 10-18 mol/μm³ on the other side, what mass of oxygen (in micrograms) is transported across the membrane each second? The diffusion constant for oxygen across the membrane is 1.70 × 10-⁹ m²/s.