Final answer:
A leftward shift on the hemoglobin oxygen saturation curve indicates increased affinity for oxygen, commonly due to higher pH, lower CO₂ levels, or reduced temperatures.
Step-by-step explanation:
A shift to the left on a % Hb saturation vs PO₂ graph indicates an increased affinity for O₂. This means that hemoglobin is more tightly binding oxygen, which generally occurs under conditions where the blood pH increases (becomes more alkaline). Affinity for oxygen can also increase with lower levels of carbon dioxide (CO₂) or lower temperatures. Conversely, a shift to the right on this graph would suggest a decreased affinity for oxygen, often due to conditions like acidosis (lower pH), hypercapnia (increased CO₂), or elevated temperatures, which facilitate oxygen release to the tissues.