Envision a crime scene.
If you’re anything like most people, your imagination has gone straight to the aftermath of a homicide. The details, however, are up to you. Is the corpse still lying around, or is there a chalk outline of where it used to be? Is there any evidence of what sort of weapon was used? Empty bullet casings littering the ground, perhaps? A knife hidden away in the shadows of the room? Does fluorescent yellow tape stretch across your view?
No matter the specifics of your mind’s eye, one thing’s for sure—there’s blood. It’s the staple of any publicized murder, picturesque in the most deplorable of ways.
There’s more to the role blood plays in a kill than just its visual flair, though. Blood can actually be a remarkable tool in reconstructing the events of a crime, especially a murder. This discipline, known as bloodstain pattern analysis, is respected among forensic specialists and has even lent itself to the creation of an entirely new career within the field of criminal investigation.
If you’ve already heard of bloodstain pattern analysis through a crime drama like CSI: Crime Scene Investigation or Dexter, be warned that Hollywood tends to oversimplify these things.
Still, that doesn’t necessarily mean that only experts in forensic science have the capacity to interpret the significance of consistency, spatter, and all else that makes bloodstains distinctive. In fact, you yourself will gain a rudimentary understanding of bloodstain pattern analysis by the end of this article.
Let’s begin with drip stains. Drip stains are blood droplets that have fallen from either an open wound or a surface covered in blood. Surface tension forces the blood drop to pull in on itself, so that a drip stain will take on a roughly circular shape. The size of these stains can reveal a fair amount of information about the height at which their blood fell from. As the diameter of the drip stain increases, the height of its blood source increases, as well.
Suppose there are two drip stains on the floor, the first of which has the dimensions of a dime, and the second the dimensions of a quarter; it would be safe to assume that the first drip stain fell from a blood source closer to the ground than that of the second drip stain.
If you’re seeking to learn more from blood than the height from which it fell, however, it’s important to understand the sort of blood spatter you’re looking at in the first place. Gunshot spatter, expirated spatter, cast-off, and arterial spray are all different forms of blood spatter that can tell you about the way a person was injured or killed.
As you may have guessed by now, gunshot spatter is blood spatter that originates from gunshot wounds. This accounts for both the back spatter from the entrance wound and the forward spatter from the exit wound. The consistency of back spatter and forward spatter varies, with forward spatter being finer than back spatter.
When blood from an internal injury mixes with air from the lungs and expels from the body through expiration, or the exhalation of breath, it's known as expirated spatter. Similar to forward gunshot spatter, expirated spatter forms a fine mist. Small air bubbles may be found in this spatter.
Cast-off is the grisly byproduct of when an object swings in an arc and flings blood onto a nearby surface. To illustrate, if an aggressor were to strike a victim with an already-bloodstained object, their swing would likely result in cast-off blood spatter. What’s more is that the number of arcs in a cast-off can indicate how many blows were delivered by an assailant and their weapon of choice.
The final kind of blood spatter is arterial spray. Arterial spray attributes to the blood spatter from when a major artery is severed and a sudden gush of blood propells from the blood vessel. Arterial spray will form patterns in arcs and large stains; a new one forms each time the heart pumps.
Once you can identify one of these particular blood spatters at a crime scene, determining how a victim was attacked becomes infinitely less challenging. When considering blood spatters in general, it’s worth noting that the smaller a blood spatter is, the stronger the force applied to its blood source was.
It should go without saying that there’s so much more to bloodstain pattern analysis than what you’ve just read, but it makes for a fine introduction. If you’ve taken an interest in this subject, by all means do some research! The International Association of Bloodstain Pattern Analysts is an invaluable resource for your pursuit of knowledge.
This was written by Valentina Rutledge.
Photo credits to Markus Gjengaar on Unsplash