Complex sentence: The concentration of carbon dioxide (CO 2) in healthy alveoli is known-it is equal to its concentration in blood since CO 2 rapidly equilibrates across the alveolar-capillary membrane.
Split into simpler sentences: The concentration of carbon dioxide (CO 2) in healthy alveoli is known. It is equal to its concentration in blood since CO 2 rapidly equilibrates across the alveolar--capillary membrane.