Red blood cells (also known as RBCs, red blood corpuscles or erythrocytes) are cells in the blood which transport oxygen. In women, there are about 4.8 million red blood cells per microliter of blood. In men, there are 5.4 million red blood cells per microliter of blood. Red blood cells are red because they have hemoglobin in them. Quantity of Red Blood Cells in the Human Body. The average male adult has about 5 million red blood cells per cubic millimeter of blood, while the average female adult has about 4.5 million red blood cells per cubic milliliter of blood. This may vary by about 300,000 to 500,000 red blood cells.