If blood entering the normal arterial circulation of the heart is 100% saturated with oxygen, oxygen saturation of blood in the coronary sinus can be expected to be approximately:

D. 35%.

DISCUSSION: The heart has an unusually high rate of oxygen utilization and consumes approximately two thirds of the oxygen in the arterial blood. The oxygen saturation of the blood in the coronary sinus is usually about 30% to 35% and varies with the magnitude of cardiac disease. The body as a whole extracts approximately 25% of the oxygen it receives, thus emphasizing the great need of the heart for oxygen at rest as well as at exercise.