A red blood cell, also known as an erythrocyte, is a type of cell that is responsible for carrying oxygen and other nutrients throughout the body. It is the most common type of cell in the human body and is essential for life. Red blood cells are biconcave discs that contain hemoglobin, which is responsible for carrying oxygen to the body's tissues. They are produced in the bone marrow and circulate in the bloodstream for about 120 days before being recycled.