I have a question that seems to be a paradox (only to me, probably).
Lavoisier showed that in every chemical reaction, the mass of the reactants and the mass of the products are the same. Life, and specifically the maintenance and sustenance of the human body, is a sum of chemical reactions. Since each such chemical reaction in the human body obeys the law of conservation of mass, so does the complete set of these chemical reactions that constitutes what we call metabolism.
So how is it that we can “lose weight” or “gain weight”?
And also, if we enclosed a human being inside a chamber isolated from the rest of the surrounding environment, allowing him a supply of food and water to keep him alive indefinitely, would the sum of the weight of the gases inhaled and exhaled and the person himself remain constant through time?
When you lose weight, a lot of it is in the carbon in the carbon dioxide that you exhale. That is, you breathe in oxygen, some of which combines with carbon in the body, giving CO2 which you breathe out.
And yes, conservation of mass applies to human beings, so that the total mass of a person plus all the inputs and outputs will be constant.
If you were to weigh all the things going into your body (solid food, drinks, inhaled air[sup]*[/sup]) and all the things coming out (urine, feces, exhaled gases, sweat (water vapor even when not visibly sweating), discarded skin and hair), the difference would be the amount of weight you gained or lost over the course of the experiment.
Do we take in anything else? Is water absorbed through the skin during bathing, for example?
Obligatory SDMB nitpick: There is a tiny, immeasurably small loss of mass, representing the energy content of the chemical bonds in the ingested food, etc.