In terms of O2 saturation, which site typically has the lowest saturation?

The site with the lowest oxygen saturation in the human circulatory system is the right atrium. The right atrium receives deoxygenated blood from the body through the superior vena cava (SVC) and inferior vena cava (IVC). This blood has already delivered its oxygen to the tissues and has a lower saturation level.

While the SVC and IVC both carry deoxygenated blood, the right atrium collects blood from both these sources, resulting in it having the lowest oxygen saturation. The SVC typically carries blood from the upper body, while the IVC carries blood from the lower body, but both are lower in saturation compared to oxygenated blood returning from the lungs through the pulmonary veins.

The pulmonary veins, on the other hand, bring oxygen-rich blood back from the lungs to the left atrium, and are characterized by high oxygen saturation levels, making them the highest in terms of saturation when compared to the right atrium, SVC, and IVC.