Final answer:
The hemoglobin in blood leaving the lung capillaries to return to the heart is approximately 95-96% saturated with oxygen, indicating efficient oxygenation for bodily transport.
Step-by-step explanation:
By the time blood leaves the lung capillaries and returns to the heart, the hemoglobin in red blood cells is generally about 95-96% saturated with oxygen. This is due to the binding of O₂ to hemoglobin to form oxyhemoglobin (HbO₂) in the lungs. At this level of saturation, each hemoglobin molecule has, on average, four oxygen molecules bound to it, making the blood optimally oxygenated for transport to the rest of the body.