What is the name of the protein that carries oxygen in blood?

Haemoglobin, built around four iron atoms, each able to hold one oxygen molecule - and its cleverness is that it does not hold them all equally. Binding the first oxygen changes the protein's shape so the next binds more easily, and releasing one makes the rest let go more easily too. That cooperativity is why it can load up almost completely in the lungs and unload sharply in tissue that needs it.

It is also responsive to conditions. Working muscle is warmer, more acidic and full of carbon dioxide, and all three of those make haemoglobin release oxygen more readily - so the tissue that needs it most gets it, automatically, with no signalling involved. It is a piece of chemistry that behaves like a control system.

The same molecule explains a whole family of diseases. Sickle cell is a single amino acid substitution that makes haemoglobin polymerise when it gives up its oxygen, distorting the cell into a shape that jams capillaries - and the reason a lethal mutation persists is that carrying one copy protects against malaria. Carbon monoxide is dangerous for a related reason: it binds the same iron sites over two hundred times more tightly than oxygen does, so it occupies them and will not leave.