The reason why blood is red is that it contains a large number of red blood cells, which in turn contain hemoglobin, the color of which is red. Therefore, red blood cells take on a red color, resulting in bright red blood. There is also a clear distinction between the red color of human blood, which can be divided into bright red and dark red. This is closely related to the level of oxygen concentration contained in the blood, for example, when the oxygen concentration in the blood is high, oxyhemoglobin will be formed, and this is when the blood will appear bright red. If the blood is venous, the concentration of carbon dioxide in venous blood increases and the concentration of carboxyhemoglobin increases, and this is when the blood appears dark red. So human blood is red, but there is bright red and dark red. In patients with the same anemia, the blood color will be light red because of the decrease in the number of red blood cells and the decrease in the density of red blood cells.