What is Red blood cells job and how are they adapted?

They are the cells that transport oxygen everywhere. They are made small and have a biconcave disc shape for a larger surface area : volume which helps with absorbing/ releasing oxygen. They have haemoglobin which combines with oxygen to become oxyhaemoglobin. They can do the opposite. They don't have a nucleus, to carry more haemoglobin [oxygen] They are tiny, so they can fit through the capillaries.