Red blood cells don't have nuclei?

There's nothing they need to 'know' to do. We know that RBC don't have a nucleus to have more haemoglobin to carry as much oxygen as possible and also so that RBCs have a bi-concave structure. Haemoglobin have a high affinity for Oxygen in the lungs so oxygen moves in by diffusion into the blood to bind with the haemoglobin. They then have low oxygen affinity in the respiring cells so oxygen moves out by diffusion from the RBCs.

Red blood cells are not like other cells, they don't need to do things like respiration so a nucleus is not really necessary to them. Everything happens by diffusion which is a passive process.

As the person above said, RBCs don't undergo mitosis to form adult RBCs, which is why they are created in the red bone marrow of bones and are initially stem cells.

Ahhh, okay! Thank you!