Final answer:
H+ ions from fixed and metabolic acids are primarily managed by the kidneys, which have a large capacity for handling acids and bases, though over a slower timeframe. The lungs assist in this process through rapid exhalation of CO2.
Step-by-step explanation:
Hydrogen ions (H+) generated by fixed and metabolic acids must be handled primarily by the kidneys and the lungs. The lungs play a crucial role in acid-base homeostasis by rapidly (in seconds to minutes) converting bicarbonate (HCO3-) into carbon dioxide (CO₂), which then gets exhaled. This process, however, has a limited capacity in terms of handling significant acid challenges. On the other hand, the kidneys provide a more substantial and long-term solution (minutes to hours) by not only neutralizing acids but also ridding the body of excess acids and bases through various mechanisms. This is done by secreting H+ into the urine in the proximal convoluted tubule (PCT) and the collecting ducts, where intercalated cells have proton pumps for active H+ secretion.
Considering the options given in the question, the correct answer would be the kidneys as they have a large capacity for handling both acids and bases over a slower time frame, which contributes to longer-term pH balance in the bloodstream.