Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which contains both of those minerals) and is the main component of pearls and the shells of marine organisms, snails, and eggs.
An ordinary breath for a 70-kg adult is about 500ml of air. We call this the tidal volume. Since oxygen is 21% by volume of air, then the volume of oxygen inhaled is about 105ml. The density of pure oxygen at sea level is 1.43kg per cubic metre.
But the crux of the matter is the oxygen-carbon dioxide tradeoff. Your lungs just aren't as good as you think they are at oxygen exchange. Every time you take a breath, which is 21 percent oxygen, your lungs are only able to extract about a fifth of that oxygen. The rest gets breathed right back out again. There wasn't much carbon dioxide in the air you breathed in: unless you are in a failing submarine or spaceship, it's around 0.04 percent. But when you breathe out, it's still pretty low, about 4 percent. There's not a huge difference between inhaled and exhaled air, chemically speaking.