(2) Red blood cells (RBCs),also called erythrocytes, are the most common type of blood cell and the vertebrate’s principal means of delivering oxygen (O2) to the body tissues—via blood flow through the circulatory system. The cytoplasm of erythrocytes is rich in hemoglobin, an iron containing biomolecule that can bind oxygen and is responsible for the red color of the cells. The cell membrane is composed of proteins and lipids, and this structure provides properties essential for physiological cell function such as deformability and stability while traversing the circulatory system and specifically the capillary network.