What is the concentration of oxygen in blue side heart?

Deoxygenated or venous blood flows from the body into the blue side of the heart. The concentration of oxygen in blue side heart is usually ~5%, which is much lower than the concentration of oxygen in red side heart (~95%). This difference in oxygen concentration is due to the fact that oxygen is picked up by hemoglobin in the lungs and transported to the rest of the body, where it is used for cellular respiration. The deoxygenated blood then returns to the heart and is pumped to the lungs, where it is re-oxygenated.