I read these thin, kid encyclopedias in the early 2000s, but I think they were purchased in the 80s or 90s en masse from a catalog. These books were color-coded on the spines by subject (sciences, cultures/history, technology) and had that older picture book low-ink color scheme of pinks, teal, yellow and green. I remember there being a kind of cartoon mascot of sorts of a young boy in a yellow sweater with a bowl cut. These encyclopedias were pretty thorough overviews of anatomy (I remember distinctly reading a page about how the diaphragm fills the lungs that had a guide to an at-home experiment involving balloons and a cut 2-litre plastic bottle), biology/ecology, physics, and earth sciences. The pages were mostly white, kind of resembling a textbook, and they were hardcover. We had like 50 of them in the complete set.
Lost Classics of Teen Lit: Name That Book! 
This might be “A Child’s First Library of Learning” from Time-Life Books. The volume “Simple Experiments” does have a diaphragm model which sound like the description:
https://archive.org/details/simpleexperiment00alex
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I had “The Golden Book Children’s Encyclopedia” when I was little. Lots of cool illustrations (what a tornado could do, an iron lung for a polio patient – the things little kids are curious about). Had the A-Z set, as I recall…great times.
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