Red blood cells, also called erythrocytes, are the most abundant cell type in the blood. Other major blood components include plasma, white blood cells, and platelets. The primary function of red blood cells is to transport oxygen to body cells and deliver carbon dioxide to the lungs. A red blood cell has what is known as a biconcave shape.

Red blood cell, also called erythrocyte, cellular component of blood, millions of which in the circulation of vertebrates give the blood its characteristic colour and carry oxygen from the lungs to the tissues. The mature human red blood cell is small, round, and biconcave; it appears dumbbell-shaped in profile.