Red blood cells ( also known as RBCs , red blood corpuscles or erythrocytes ) are cells in the blood which transport oxygen . Red blood cells are very large in number ; in women , there are 4.8 million red blood cells per microliter of blood . Several blood tests involve red blood cells , including the RBC count ( the number of red blood cells per volume of blood ) , the hematocrit ( percentage of blood volume occupied by red blood cells ) , and the erythrocyte sedimentation rate .