Blood Spatter

The understanding of blood spatter patterns is not a widely recognized forensic practice. Bloodstain pattern interpretation (BPI) is commonly used in murder investigations, but could be utilized in everything from simple assault to mass murders if the number of trained professionals increased. BPI can reveal critical information into reinventing a given crime scene. Everything from the number of blows, stabs or shots a victim was given, the movement that was undergone by the victim and assailant after bloodshed began, position of objects at the crime scene and the type of weapon, if any, that was used can be uncovered.

Blood differs from water in that it is a less viscous liquid. The teardrop that is formed by rain is not formed by blood falling at an equal pace. As a result blood tends to hold the shape of a sphere throughout its flight path. Sobriety of the victim will have no significant effect on how bloodstain patterns are produced(wwwbrazoriacounty.com). By blood holding a generally geometric form the angle of impact (AOI), and eventually the point of origin (POO) can de determined. To determine the angle of impact, assuming the drop is contacting a flat surface, simply measure the width of the stain on the ground. This width will equal the diameter of the droplet in the air. The width will be equal to the short side of a right triangle. This triangle can be placed over the spatter and the angle at which the droplet traveled will be inline with the angle of the triangle(James & Eckhart, 1999). This can be recreated on a scientific calculator with the TAN option.

Now that the AOI has been determined the point from which the droplet originated from needs to be determined; in other words where was the victim at the time they began to bleed. This can be determined by drawing a line or placing a string through the long axis of 3 or more droplets. This triangulation technique will cause the lines to cross at the POO. By following the AOI to the point of origin, the area from which the blood was produced can be pinpointed within a foot.

Blood spatters are classified into three velocities: low, medium, and high velocity. The velocity of the blood spatter is not the speed at which the droplet is traveling, rather the force put to work onto the blood causing it to generate speed. Low velocity stains are produced by normal gravity, roughly 5 fps. They tend to form a circle upon impact of a flat surface because they are dripping off either a person, weapon, or other object. Low velocity blood stains are the largest of the three velocities measuring 3mm and larger. Medium velocity stains are the result of an external force higher than normal gravity (5 fps) but less than 25 fps. These stains are caused by fists and hand held weapons, such as a baseball bat or hammer.

Medium velocity stains typically measure between 1 and 3 mm. Arterial spurts also fall into this range. While arterial spurts are more difficult to interpret as they are not geometrical shapes rather arcs, they are often coupled with bloody handprints, swipe or wipe. Swipe is blood that is already present on a surface and is contacted or swiped through with an object or portion of a person. Wipe is simply blood present on an individual that is obscurely transferred to a surface or object.

High velocity blood spatter is the smallest and travels the shortest distance from the point of origin because its momentum is minimal. Often referred to as mist, high velocity spatter is caused by an object traveling over 100 fps. This mist is typically caused by a projectile but can also be caused by coughing or wheezing up blood. Blood that is directly taken from the lungs, as exhibited in, for example, a sucking chest wound, is typically coupled with bubbles of air, a pinkish hue, and coagulation. Expirated blood can be easily identified by these characteristics. Bubble rings form when the blood containing air bubbles dries and retains the bubble's circular configuration as a dried outline.

Another element of BPI is cast off. Cast off is most commonly associated with a weapon. It is flung off the weapon or tool during and after the act as a result of centrifugal force. Cast-off spatter is often elliptical in shape which is why it is easily confused with arterial spurting (Sutton, 1998). However, it can also resemble a typical medium velocity droplet any time the blood hits a surface. For example, a blow to the top of the head with an over head strike or simply cast off forming from blood dripping off a weapon after use will result in a similar pattern to a medium or low velocity droplet. Smooth metallic weaponry used will generate a more profound pattern than a textured weapon. In other words an aluminum baseball bat will yield a greater amount of cast off blood than a wooden baseball bat.

All these elements are important in uncovering the truth behind a crime scene, and in a controlled environment seem simple. Implementing these techniques in an actual situation is more difficult as crime scenes are all unique. A multitude of elements need to be considered before forming an opinion from blood spatter. Spot size, quantity, shape, distribution, location, AOI, and target surface are only a few of these elements. As with other types of forensic science negative evidence can be used in BPI.

Void is the absence of blood spatter in an otherwise continuous pattern. This leads evidence leads us to believe that an object