Which of the following ranges of haemoglobin O2 saturation from systemic venous to systemic aerial blood represents a normal resting condition?: (A) 25 to 75%, (B) 40 to 75%, (C) 40 to 95%, (D) 75 to 98%
Answer is D. In a normal res ng condition, the blood leaving the lungs is 98% saturated with oxygen, and the blood returning to the lungs is 75% saturated with oxygen. With vigorous exercise, blood leaving the lungs is still 98% saturated, but blood returning is usually less than 75% saturated because more oxygen is unloaded from hemoglobin in exercising muscles