Except for some lower animals (especially insects), the blood color of most animals is red. Blood is usually composed of plasma and blood cells. These blood cells are again of three types—red blood cells, white blood cells, and platelets. Blood is red mainly because of these red blood cells.
Red blood cells perform important functions in the body. Its job is to deliver the oxygen we take in to the cells of the body when we breathe. Again, these particles carry carbon dioxide or other waste materials from each cell and deliver them to the lungs. Otherwise it would not be possible for any animal to survive on earth.
Red blood cells contain a protein called hemoglobin. Its color is bright red. Because iron is associated with hemoglobin. Hemoglobin is mainly responsible for the exchange of oxygen or carbon dioxide between the lungs and every cell in the body.
Hemoglobin is purified by taking oxygen from the lungs. At this time the blood is bright red. Then, under the pressure of the heart's pump, pure blood moves from cell to cell with pure oxygen. As a result, the cells can do their own work. When cells finish their work, carbon dioxide is produced as waste. As the blood delivers oxygen to the cells, it becomes polluted with carbon dioxide instead. This contaminated blood looks a bit dark in color. It again reaches the lungs through the blood. Lungs expel carbon dioxide with respiration.
Normally, pure blood flows through arteries and contaminated blood flows through veins. This is why arterial blood looks more red. Venous blood is slightly bluish red.