Humans can have doubt but there is so much information out there, so many videos, articles, videos, diagrams, humans can look at the details if they want to. Those with more doubt should be the ones looking at what doctor Robert Young said, look at his videos, he shows photos of the red blood cells, specifically with the hemoglobin which contains the oxygen. Think of the hemoglobin as air balloons. Imagine what would happen if those air balloons begin to pop. Try to visualize your cells getting less and less oxygen due to the air balloons getting depleted more and more over time. Try to imagine what that might do. You can have doubt about that all you want and yet you can also simply look at those balloons popping. Pop pop pop. But you have the right to doubt the sound of balloons popping.