Why the blood is red

Blood is a liquid tissue composed of blood plasma and blood elements. Its function is to distribute nutrients throughout the body and to remove waste products. It is full of various substances fulfilling various functions from defense against foreign substances, through the transfer of information and nutrients to the cessation of bleeding in case of injury. But what is behind its red color?

Blood is an important component of the body and occupies approximately 7% of a person's total weight. Blood plasma, which occupies approximately 54.3% of the blood volume, is yellow. Red blood cells are in second place with 45% and white blood cells occupy approximately 0.7%. The other elements are negligible in volume. So it's no surprise that the blood is red due to the high concentration of red blood cells. The red blood cells are red thanks to a blood pigment called hemoglobin, which is found on the surface of the red blood cells. The purpose of hemoglobin is to transport oxygen from the lungs to the tissues, as well as a small amount of carbon dioxide from the tissues to the lungs.^[1]

It is also worth mentioning that the blood is not always the same red. Oxygenated blood that travels from the lungs to the tissues is light red because the oxygen bound to hemoglobin forms so-called oxyhemoglobin, which is light red. In contrast, non-oxygenated blood contains red blood cells with so-called deoxyhemoglobin, which is dark red. This phenomenon is used, for example, in pulse oximetry, in which the oxygenation value of the blood is measured by means of the translucency of the tissue by light.^[2]