Venous blood, which is the blood flowing in the veins of the peripheral circulation and the blood flowing from the right ventricle to the pulmonary artery in the pulmonary circulation, has a dark red color because it contains a lot of carbon dioxide and various tissue metabolites, while arterial blood has a bright red color because it is rich in oxygen. As can be seen, the color of blood to the naked eye changes depending on the composition of the blood, with a purplish-black appearance if the blood contains high levels of methemoglobin or other types of hemoglobin derivatives. Normal serum looks clear and yellowish because it contains a small amount of bilirubin. If the blood is rich in fat particles or other types of fat derivatives, it may appear milky and cloudy, while once hemolysis occurs, it appears reddish blood-like. What flows in the veins is not necessarily venous blood, and what flows in the arteries is not necessarily arterial blood, because what flows in the pulmonary arteries is venous blood, while what flows in the pulmonary veins is arterial blood, and the exact color of the venous blood depends on the changes in the components of the blood.