Here’s a structure and function question for your students to think about: how can a sponge soak up so much water? The anatomy and physiology answer: the same way your lungs can exchange blood gases so quickly. Structurally, a sponge has a large surface–every pore increases the amount of contact between the porous cellulose fibers and water molecules. Similarly, alveoli in the lungs increase the surface area for the uptake of oxygen from the environment. Then capillaries next to the alveoli allow gas exchange to occur between the internal and external environment.

Another question to pose to your students: what happens to a sponge when the liquid absorbed is spilled coffee instead of water? What happens to the larger molecules that get stuck in the pores? They stain the sponge, and the sponge must be rinsed out. What’s an analogous process in your lungs?

One more thing to consider: what if that spilled coffee is the last cup in the pot and has solid coffee grounds in it? The solid particles on the counter can be removed by the sponge, but now the sponge must be washed thoroughly, maybe even bleached, and some grounds may have to be picked out manually.

Our lungs perform a similar function with every breath we take. Rarely is the air we breathe perfectly proportioned atmospheric gases. Our respiratory system adapts to ranges in oxygen saturation with changes in altitude, changes in humidity (desert to rain forest or winter to summer), and changes in particulate matter (from dust to pollen to pathogens). It also adapts to the occasional guzzle of hot coffee that goes down the wrong way. It’s an amazing system!