The red color of red blood cells (RBCs) is primarily due to a molecule called "hemoglobin." Hemoglobin is a complex protein found inside RBCs, and it plays a crucial role in transporting oxygen from the lungs to the body's tissues and organs.
Hemoglobin contains iron atoms, and it has the ability to bind to oxygen molecules. When oxygen binds to hemoglobin, it forms a compound known as "oxyhemoglobin." This oxyhemoglobin complex has a bright red color. When RBCs are oxygenated in the lungs, they turn this characteristic red color, and they transport oxygen throughout the body.
Conversely, when RBCs release oxygen to the body's tissues and return to the lungs with carbon dioxide, they become "deoxyhemoglobin," which has a darker, bluish-red color. This is why venous blood, returning to the heart and lungs, appears darker than oxygenated arterial blood.