**Core Concept:** Hemoglobin is a protein found in red blood cells that binds to oxygen and facilitates its transport from the lungs to the body's tissues. The oxygen saturation of arterial blood depends on the partial pressure of oxygen (PO2) in the alveoli, which in turn is influenced by the oxygen affinity of hemoglobin.

**Why the Correct Answer is Right:** Hemoglobin exhibits a phenomenon known as Bohr effect, where the oxygen affinity of hemoglobin increases as the pH of the blood decreases (i.e., when there is more carbon dioxide). This increase in oxygen affinity helps maintain adequate oxygen delivery to tissues despite the decreased partial pressure of oxygen in the alveoli during conditions like hypercapnia (increased CO2 levels).

**Why Each Wrong Option is Incorrect:**

A. This answer is incorrect because the correct explanation involves the Bohr effect, which is not directly proportional to the number of times oxygen concentration increases.
B. This answer is incorrect because it does not address the concept of the Bohr effect and how it increases oxygen affinity in response to decreased pH (increased CO2).
C. This answer is incorrect because it does not explain the Bohr effect and its influence on hemoglobin oxygen affinity.
D. This answer is incorrect because it does not address the concept of the Bohr effect and how it affects oxygen affinity in response to pH changes.

**Clinical Pearl:** The Bohr effect demonstrates how the body adapts physiological mechanisms to maintain tissue oxygenation despite changes in alveolar oxygen tension. This highlights the importance of considering both the partial pressure of oxygen and the pH when assessing arterial blood gas values.

**Correct Answer:** D. Hemoglobin increases its oxygen affinity by about 10 times in response to increased carbon dioxide levels, which is the correct answer. This is due to the Bohr effect, where an increase in CO2 levels causes a decrease in pH, which in turn increases oxygen affinity.