a) The gas ______ is carried within the red blood cells.  It is carried on a molecule known as ______ found in RBC’s.  There are around 250 million ______ molecules in each RBC.   Each may carry ______ oxygen molecules.   The gas ______ is primarily transported in the ______ of blood rather than in the RBCs.   It combines with the water in blood to generate the carbonic acid that rapidly changes chemically to ______ , a safe way to transport the gas in the blood.   When the ______ reaches lungs, it is transformed back to ______ gas, which then diffuses out of blood and is exhaled.

b) Key factors which influence respiratory rate and depth are ______  such as talking or coughing, ______ (such as, sometimes we try to breath at a several pace to relax), ______ like being scared, and ______  like the amount of ______  or ______  in the blood.