In a model, the rate of diffusion of oxygen across a layer is (A × Δ p)/t in arbitrary units…

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In a model, the rate of diffusion of oxygen across a layer is $\dfrac{A \times \Delta p}{t}$ in arbitrary units. Here $A$ is the area, $\Delta p$ is the pressure difference and $t$ is the thickness. A healthy air sac wall has $t = 0.5\ \mu\text{m}$ , $A = 70\ \text{m}^2$ and $\Delta p = 60\ \text{mm Hg}$ .

What is the rate for the healthy wall?

A disease thickens the wall to $2\ \mu\text{m}$ and the other numbers stay the same. What is the new rate?

In another disease, the thickness is $2\ \mu\text{m}$ and the area is also halved, to $35\ \text{m}^2$ . What is the rate now?

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Epithelial tissue forms sheets that cover the body and line its cavities and tubes. It has little matrix and no blood vessels, and it rests on a basement membrane. Simple epithelium has one layer, and compound epithelium has more. The cells are squamous (flat), cuboidal (cube shaped) or columnar (tall). Simple squamous epithelium lines blood vessels and air sacs and forms a thin boundary for diffusion. Cuboidal epithelium lines gland ducts and kidney tubules and helps in secretion and absorption. Columnar epithelium lines the stomach and intestine. Ciliated epithelium moves mucus or particles. Compound epithelium protects, as in the skin. The rate of diffusion across a layer is inversely proportional to its thickness.