Postmortem examinations, review of medical records, accident reports and photos taken at the scene are used to analyze and classify asphyxial deaths. There are non-specific physical signs used to attribute death to asphyxia. These include visceral congestion via dilation of the venous blood vessels and blood stasis, petechiae, cyanosis and fluidity of the blood. Petechiae are tiny hemorrhages. Blood vessels, usually small veins, are broken by high intravascular pressure. They can occur in various parts of the body, such as over the surface of the heart and organs, in the eye, the skin and the scalp. If a large area is affected, they may be termed ecchymoses and appear as bruising. Hemoglobin [Hb] in red blood cells turns from red to blue when it loses oxygen. This loss of oxygen is the reason veins are described as blue since they carry blood that has lost oxygen to the body's cells back to the lungs where it can be reoxygenated. As asphyxia progresses and more oxygen is depleted, a dark discoloration of the skin and tissues called cyanosis develops. Cyanotic tissue is described as blue, black or purplish in color. After death, changes in blood chemistry and the breakdown of clotting factors such as fibrin lower the viscosity of the blood; this is sometimes called 'fluidity'. The study of flow is called rheology, thus; those who specialize in the study of blood flow behavior are called rheologists or, more specifically, hemorheologists.