Explain why the tidal volume and breathing rate increase over the course of a spirometer experiment?

Since the spirometer is a closed system, over time the oxygen concentration will decrease and therefore the oxygen concentration in the blood per inhalation will decrease. When the blood reaches the medulla oblongata, receptors will recognize the low oxygen levels and through the autonomic nervous system, sends impulses to the diaphragm which causes stronger and more frequent contractions to bring more of the air more frequently into the alveoli to compensate for the oxygen deficient air.