Red blood cells, also known as erythrocytes, are specialized cells in the blood responsible for transporting oxygen from the lungs to the body's tissues. They contain a protein called hemoglobin, which binds to oxygen and gives these cells their red color. Red blood cells are produced in the bone marrow and have a lifespan of about 120 days. These cells are unique in shape, resembling a doughnut without a hole in the center, which allows them to move easily through blood vessels. The body regulates the production of red blood cells based on its oxygen needs, ensuring that tissues receive adequate oxygen for energy and function.