Oxyhemoglobin is of a bright scarlet color; hemoglobin itself, not combined with oxygen, is a very dark purplish color; partial combinations are brighter and brighter as they contain more oxygen, so that an expert can judge of the degree to which any specimen of hemoglobin is combined with oxygen by noting its color in comparison with fully saturated hemoglobin. The combination of hemoglobin with oxygen takes place as the blood is passing through the capillaries of the lungs; therefore the blood which leaves the lungs has the bright scarlet color characteristic of oxyhemoglobin. This blood is called arterial blood, the reason is that it is the kind that is found in the arteries of the body in general. It happens that it makes its first appearance in the pulmonary vein, by which it is conveyed from the lungs to the left side of the heart, so that the expression arterial blood does not mean anything in particular except to describe blood in which the hemoglobin is saturated with oxygen. This blood is pumped out by the left side of the heart to all the parts of the body; in its passage through the capillaries it is in a region where there is active utilization of oxygen by the living cells. These are steadily taking up oxygen from the tissue fluids about them, so the blood in the capillaries, which is carrying an abundant supply of oxygen, is brought in contact with tissue fluids containing little or none, with only the delicate wall of the capillaries between. Under these circumstances rapid diffusion of oxygen from the blood into the tissue fluids takes place and accompanying this there is a breakdown of oxyhemoglobin, so that not only most of the oxygen which was dissolved in the blood passes out, but also a considerable part of that which was formerly in combination with hemoglobin. Under ordinary circumstances only from one-fourth to one-third of the oxyhemoglobin decomposes during the rapid passage of the blood through the capillaries; thus the blood that goes on into the veins will still be carrying two-thirds or more of the total oxygen cargo. The color of venous blood will be darker than that of arterial blood, because it contains a good deal less oxyhemoglobin, but it is nowhere near so dark as is blood in which all the oxyhemoglobin has been decomposed. This is well recognized in melodramatic fiction where the wounds of persons who have met death by strangulation are described as oozing black blood. It is an actual fact that blood from which all the oxygen has been withdrawn is so much darker than ordinary venous blood that it gives the impression of being black, although it is really a dark purple.