In humans cystic fibrosis is a recessive genetic disorder that causes mucous to build up in the lungs and digestive tract. A healthy couple has
their first child and later discover that the baby has cystic fibrosis. How is
it possible that two parents without the disorder can have a child with the
disorder? *

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?