Lungs–A structure that solves the problem via diffusion

All organisms of the animal kingdom have a continual need to get oxygen into the body and carbon dioxide out of the body. Have kids ponder how a structure might be designed so that this can be accomplished by diffusion. The requirement is a large surface area where blood flows in close proximity to fresh air. After kids have made suggestions show them diagrams of lungs (below)–one of nature’s solutions to this problem. Please post questions that arise.

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Note that other organs and parts of the body likewise have fine structures that enable transfer of materials via diffusion.