Active bloodstains are those made to travel by a force other than gravity. These can occur from the force of impact with a weapon like a baseball bat, more likely blunt weapons than blades. These impact blood stains will result in numerous droplets of blood covering the target surface. Active bloodstains may also be caused by the blood pressure shooting out of breached arteries in gushes (large amounts) or spurts (small amounts). Since the blood assumes a spherical shape in the air, on collision with a surface like a wall it will create a tear shape. This is incredibly useful information. The first thing it letss you determine in the direction of travel.
As in this example, the tails clearly indicate the direction of blood splatter. As the angle at which the blood hits the surface becomes shallower the blood droplets' tail becomes shorter. by finding the completely circular blood drops you can find the trajectory and point of origin of the blood splatter. This is where maths, for the first time in my entire life is actually interesting. By using sin ratios and two blood droplets you can work out the location of their origin. Since we know blood travels through the air in a spherical shape, the width at the widest point of the stain equals the height of the sphere, this gives you the opposite in a triangle and the tail length is the adjacent so using trigonometry you can calculate the angle of the hypotenuse which will be the direction it flew,the equation for anybody finding the maths hard to follow is angle of impact = sin^-1(Width/Length). If you do this for two blood droplets and then use algebra to find the intersection point you know the distance they came from.
Passive bloodstains are those formed by gravity. This means blood flows pools and drops from the victim (no gushes or spurts). These are helpful as the flow patterns which look unusual are a clear indicator that the body has been moved after death. In addition passive blood stains may provide clues as to the length of time since bloodshed, by the analysis of drying times simply through observation of sample against a recreation (same amount of blood, same surface, same temperature etc.). Blood droplets form a circular crust within minutes of being exposed to air. This crust is very difficult to wipe away so the presence of these circles shows that some has attempted to clean up the blood.
Transfer Stains are those which occur as a result of direct contact with the objects covered in wet blood. These stains are extremely common at any type of bloody crime scene and will often give valuable information as to the movements of any individual involved. This is a relatively intuitive process. Repeated marks will give away the movements of the individual, for example footprints will be strongest at the start of the trail and fade with each step until they must be traced using a reagent caused luminol. Whose blood it is must also be taken into consideration but DNA analysis is a topic for another day.
Well there you go, you now know your way around a bloody crime scene. I feel these techniques prove why preservation of the crime scene is so important as smudging the blood, even slightly in some cases can make your analyses veer off. All that's required to piece together a violent crime is a well-preserved crime scene, some decent maths, a little common sense and possibly some luminol. Now you can get annoyed when TV detectives miss simple clues like these. Thanks for reading!
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