The affinity of hemoglobin for oxygen changes with ph. what is affecting the ph of the blood?
a. the blood changes its ph based on the amount of carbon dioxide dissolved
b. the blood changes its ph based on the amount of oxygen dissolved
c. the ph of the blood is adjusted centrally in the hypothalamus
d. the ph of the blood is constant and never changes

a. The blood changes its pH based on the amount of carbon dioxide dissolved  

Explanation:

Carbon dioxide in the blood combines with water to form carbonic acid. The carbonic acid then freely dissociates into hydrogen carbonate and hydrogen ions.

CO₂(aq) + H₂O(ℓ) ⇌ H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq)

The added hydrogen ions affect the pH of the blood.

b. is wrong. The amount of dissolved oxygen does not affect the pH of the blood.

c. is wrong. One of the functions of the hypothalamus is to control blood volume, not pH.

d. is wrong. The "normal" pH of arterial blood is 7.4, but it can range from 7.35 to 7.45.

the answer is:

carbon dioxide