What is wrong with the way plants breathe? Leaves breathe by direct ventilation, after all, through their stomata.
After reptiles, animals wanted their skin dry. There was only a tiny place left for air, and that was in the lungs.
O2 is a gas, and gases move about quickly, "twisting and rocking", and there must be a great deal of space, relatively speaking, between one gas molecule and another. Every molecule needs a certain radius, and molecules in their gas form need the most. If O2 dissolved in water, (without changing the pressure), it would reach a limit of about 22%. No more Oxygen could get in unless the pressure increased. (Think of the way water bubbles from a faucet when you first open it up --the gases that were under pressure are escaping!)
But animals living on land needed more oxygen, and raising the pressure was not a choice. Cells cannot hold more free oxygen. So how could they get more?
RETURN TO Start
Hemoglobin -+- Index-+-
