Final answer:
Blood oxygen levels are most likely low when blood fills the right atrium, as this is where deoxygenated blood from the body collects before being sent to the lungs for oxygenation.
Step-by-step explanation:
Blood oxygen levels are most likely low when blood fills the right atrium.
The circulatory system transports blood throughout the body, and it has different levels of oxygen saturation at various points in the cycle. Oxygen-poor blood returns to the right atrium through the vessels known as vena cavae. The right atrium receives this deoxygenated blood from the body tissues after oxygen has been delivered and carbon dioxide has been picked up. This deoxygenated blood is then sent to the right ventricle, which pumps it toward the lungs where it gets re-oxygenated.
On the contrary, blood that leaves the aorta, flows through arteries, or reaches body tissues generally still contains relatively high levels of oxygen, as these are points in the circulatory system either delivering freshly oxygenated blood from the lungs or in the process of delivering oxygen to the tissues.