Final answer:
The normal PaCO2-etCO2 gradient is 2-5 mmHg, reflecting the amount of CO2 not exhaled by the lungs. A gradient notably outside this range could signify an issue with gas exchange.
Step-by-step explanation:
The normal PaCO2-etCO2 gradient, which is the difference in partial pressure of carbon dioxide between arterial blood (PaCO2) and end-tidal carbon dioxide (etCO2), is typically 2-5 mmHg.
This gradient represents the amount of carbon dioxide that is not exhaled by the lungs and, although it can vary slightly among individuals, a gradient significantly higher than this may indicate some form of physiological or anatomical impairment in gas exchange. Thus, the correct answer to the question is b) 2-5 mmHg. In terms of carbon dioxide exchange in the respiratory system, when the partial pressure of carbon dioxide is higher in the blood (45 mm Hg) than in the alveoli (40 mm Hg), carbon dioxide will diffuse from the blood into the alveoli to be expelled from the body.