Commonly found at the scenes of violent crime, the forensic analysis of blood can be divided into the biological approach, serology, and the physical approach, blood spatter analysis. Information on the biological study of blood may be found on the forensic biology page.
Blood spatter analysis is based on the idea that blood will follow a certain set of physics, meaning that any blood spatter found at the scene can potentially be used to reconstruct the sequence of events that occurred. The size, quantity, shape, distribution, location, and angle of impact of a bloodstain are all studied. Such an analysis can determine the movement and direction of a blood source, the position of individuals or objects during the bloodshed and after it, the object used to create a specific pattern, and the direction the stain was travelling in when deposited.
A bloodstain can be placed in one of three categories in terms of its velocity. Low velocity impact spatter (LVIS) is caused by blood travelling at less than 1.5 m/s. Medium velocity impact spatter (MVIS) is the cause of blood travelling between 1.5 m/s and 7.5 m/s. And finally, high velocity impact spatter (HVIS) is caused by blood travelling at over 30 m/s.
Blood dropping vertically onto a flat surface will create a circular stain, suggesting its source was stationary at the time. Blood falling directly into a pre-existing blood stain will cause further drip patterns around the stain. A moving source dropping blood onto a flat surface will create elongated stains in the shape of an exclamation mark. The shape of these droplets can often yield information regarding the direction and speed of the source. Larger droplets suggest the blood was travelling at a low velocity, whereas small droplets indicate the opposite. Measuring the degree of circular distortion of a stain can determine the impact angle of the blood. A line of blood spots on the ceiling might suggest a bloodstained object was swung through the air, for example a bloodied knife swung above the perpetrator’s head. This is known as cast-off. Blood exiting the body under great pressure from a breached artery will cause arterial spurting. Expiratory blood is blown out of the nose, mouth, or a wound as a result of air pressure. The resulting spatter will often appear as a fine misting of blood. Large stains might suggest a bloodied object or individual was in contact with the surface for a certain amount of time before being moved. A void in an otherwise uniform spatter is indicative of an object blocking the spray of blood and later being removed. Smeared blood stains can be indicative of either a bloodied object being moved across the surface, or an object or person smearing a pre-existing bloodstain. The texture of the surface is particularly important in determining the type of blood spatter. Blood landing on a smooth surface will often produce a spherical blood stain, whereas a porous surface may cause additional spatter.