**Core Concept:**
Inhalation of pure oxygen (O2) increases the partial pressure of O2 in the alveoli and blood, which may lead to a decrease in the partial pressure of carbon dioxide (CO2) and subsequent increase in the partial pressure of CO2 in the blood.

**Why the Correct Answer is Right:**
When the concentration of inhaled O2 is doubled, the partial pressure of O2 in the alveoli increases. This leads to a decrease in the partial pressure of CO2 in the alveoli and blood, resulting in a decrease in CO2 levels in the blood. This is essential for the normal functioning of the respiratory and cardiovascular systems, as it facilitates the gas exchange process in the lungs and helps maintain the acid-base balance in the blood.

**Why Each Wrong Option is Incorrect:**
A. Increasing the O2 concentration would be expected to increase the partial pressure of O2 in the alveoli and blood, but the correct answer is D (decrease in CO2 levels).
B. Increasing the O2 concentration does not directly affect the partial pressure of HCO3- ions or H+ ions in the blood, which are primarily influenced by the acid-base balance.
C. Increasing the O2 concentration does not directly influence the pH of the blood; instead, it affects the partial pressure of CO2 and HCO3- ions.
D. Increasing the O2 concentration does not directly affect the oxygen saturation of the blood; rather, it influences the partial pressure of CO2 and HCO3- ions in the blood.

**Clinical Pearl:**
In clinical practice, understanding the relationship between inhaled O2 concentration and CO2 levels is crucial for managing patients with respiratory disorders, as it helps to tailor oxygen therapy according to the patient's specific needs to optimize gas exchange and maintain acid-base homeostasis.

**Correct Answer:**
D. Increase in partial pressure of CO2