When the gases Sulphur dioxide and hydrogen sulphide mix in the presence of water the reaction is s02 + 2H2S 2H2O + 35 Here
hydrogen sulphide is acting as :-

You exhale co2 which is produced during cellular respiration.  co2 combines with the water in your blood's plasma to make up one half of the body's most important buffer pair, carbonic acid.  the more physical activity you engage in, the more co2 your body is producing.  you can see this by putting some of the cabbage indicator in a glass and then blowing bubbles into it through a straw.  can you see a change in the color of the indicator?