The part of the red blood cell that carries oxygen is called hemoglobin, which is a protein found inside the red blood cells. Hemoglobin contains iron, which allows it to pick up oxygen from the air we breathe and deliver it to all the tissues and organs in the body. Red blood cells are adapted for the transport of oxygen, and they are small and flexible so they can fit through narrow vessels, have a bi-concave shape which maximizes their surface area to absorb oxygen, have a thin membrane so gases easily diffuse through, and contain hemoglobin which binds to oxygen. When the red blood cells reach tissues that need oxygen, the oxygen is released from the hemoglobin and diffuses into the cells where it is used to make energy.