Red blood cells, also known as erythrocytes, are the most common type of cells in our blood. Their main job is to carry oxygen from the lungs to all parts of the body and return carbon dioxide back to the lungs for exhalation. These cells are shaped like a doughnut without a hole in the middle, which helps them move easily through blood vessels. Each red blood cell contains a protein called hemoglobin, which binds to oxygen and gives blood its red color. They are produced in the bone marrow and have a lifespan of about 120 days. After that, they are broken down and recycled by the body, ensuring a constant supply of fresh cells.