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.
I guess this happens because when you hold your breath after inhaling, you have inhaled enough oxygen to sustain yourself for a minute or two (which depends on the capacity of your lungs). Now this single minute is pretty long given the fact that we inhale and exhale almost every second.