We want to determine how fast the alveolar CO2 concentration can change in response to changes in blood CO2 concentration. Assume a spherically shaped alveolus (of radius 0.015 cm) with a spatially uniform internal gaseous composition at time t = 0. Calculate the time necessary to achieve 95% equilibration when the CO2 concentration in the alveolar wall is suddenly changed at t = 0. You may neglect any mass-transfer effects of the wall tissue and the liquid film in the alveolus. The diffusion coefficient of CO2 in air is 0.14 cm2/s.

Considering your answer, how important is the diffusion resistance to mass transport through the gas contained within the alveolus?

Fluorescein is a small fluorescent tracer molecule that is used in a wide variety of physiologic studies. In the eye, it is used in a technique known as fluorophotometry to characterize the transport characteristics of aqueous humor, the fluid that fills the anterior chamber behind the cornea.

A drop of fluorescein (50 μl of 0.005%, by mass, fluorescein in saline) is placed onto the cornea and spreads evenly over the corneal surface to create a thin film. It then diffuses through the cornea (D = 1 × 10–6 cm2/s) and enters the aqueous humor on the back side of the cornea. The thickness of the cornea is about 0.05 cm and its radius is 0.5 cm.