Blood Stain Patterns

Crimes often involve blood, and when blood is involved it tends to leave trails or patterns which can be analyzed for forensic significance. By using concepts from biology and physics, investigators are able to discern quite a bit about the crime scene just by observing any blood that is present. While the patterns of drops, pools or smears of blood found at a crime scene may not seem significant at first glance, they often provide crucial information about the sequence of the crime as well as the weapon used and possible identifying information about the perpetrator. For example, blood spatter patterns can reveal the origin or where the attack causing the blood spatter occurred, distance from the target, direction of the force, position of persons involved, movement of persons involved, and number of blows or shots. Blood spatter can also be used to reconstruct a crime scene or to verify statements from witnesses, victims, or suspects.

Essentially there are three types of blood stains found in crime scenes: passive stains, transfer stains and projected stains.

Passive stains encompass patterns that result from gravity such as drops and pools of blood from a wound or injury.

Transfer stains are the result of some object coming into contact with a pool of blood and being transferred to another area or surface. Examples of transfer stains include wipes, swipes and even patterns of the object transferring the blood such as a shoe tread. A wipe occurs when a bloodstain is removed or altered because a non-bloody object has moved through a wet blood stain. A clean finger moving through a blood drop would be an example of a wipe. A swipe, conversely, occurs when blood is transferred from a bloody object to a previously unstained surface. Wiping a bloody finger across a clean wall would create a swipe pattern. These two types of stains can be difficult to tell apart in a crime scene, but are both significant in Bloodstain Pattern Analysis. In addition, blood can be transferred from various areas on or near the body such as from the hands, clothes, or hair to other surfaces like walls, ceilings, or bedding. When observing a crime scene for bloodstain patterns, it is important to understand that some of the patterns and actions may overlap such as a wipe and a swipe in the same vicinity. In this case, a puzzling pattern may be explained by the idea of double actions like wipe and swipe. Finally, blood will flow and pool as any liquid would depending on the location and contours present. A bleeding body that is moved will often leave swipes or smear marks as it is being dragged to a different location due to a pool of blood located under or near the body.

Projected stains are those that occur with impact and cause blood spatter. Projected stains include various patterns resulting from the gushing, splashing or arterial spurting of blood. In general, a faster moving force such as a bullet shot from a gun, will cause smaller droplets of blood spatter such as in a spray of blood. In particular, gunshots produce two types of blood spatter; forward spatter and back spatter. Forward spatter is a mist of blood that comes from the exit wound as the bullet exits. Back spatter usually produces larger drops originating from the entrance wound. A slower moving force, such as a baseball bat, will cause blood spatter with larger drops of blood.

Some other common patterns of blood spatter found in crime scenes include cast-off, arterial spray and expirated blood.

Cast-off is a blood pattern in which a weapon or object swings in an arc and deposits blood on a surface in the path of that arc. This typically occurs when the perpetrator is repeatedly hitting a victim with a hand or object in succession. The pattern and number of arcs can help investigators to determine the number of hits and the type of weapon used. Investigators can also tell the direction in which the perpetrator hit the victim by observing the shape of the blood droplets.

Arterial spray is a pattern of blood drops caused by an artery being cut and exposed as the heart continues to pump. The blood spurts and leaves a discernible pattern of blood on the surfaces near it. Often the pattern is that of an arc with large drops each time that the heart pumps.

Expirated blood is blood that exits the body after an internal injury through the nose or mouth in the process of breathing or coughing. When the blood exits the body with a breath, it usually has small air bubbles in the blood droplet and makes a very fine misting pattern. Investigators can see the air bubbles in the blood drops and know that it most likely originated during respiration or coughing.

Point of Convergence and Point of Origin

When more than one blood drop is present, investigators can determine the starting point of the bloodshed event known as the "point of convergence". This is determined by creating intersecting lines through blood drops to identify where all of the lines intersect. This intersection is the point of convergence. The significance of the point of convergence is that it shows the origin and direction from which the blood was traveling. Once the point of convergence is established, further analysis can be done to obtain the point of origin which describes where the suspect and victim were and possibly how they were positioned during the time that blood was shed. Point of origin differs from point of convergence in that it adds a third dimension to the scene recreation with the addition of the height at which the crime sequence occurred. While point of convergence conveys the crime sequence in 2D, point of origin can be thought of as conveying crime details in 3D. This 3D rendering of the crime sequence is accomplished by the use of protractors and elastic strings to show the angle and direction in which the blood traveled across the crime scene space. The intersection of the elastic strings provide information on the point of convergence and point of origin, as well as insight to the location of the person and the injury that was the source of the blood that produced the spatter. Furthermore, it provides all of the information necessary to determine the angle of impact when the victim was attacked.

The Scientific Method

By connecting a line of string from the point of origin to the blood spatter, an investigator can use a protractor to determine the angle of impact. When that is not possible, the angle of impact can be estimated reproducing spatter patterns at various angles and observing the outcome. Typically, when a drop of blood is falling it forms a spherical shape in flight. When it hits a surface, the drop of blood leaves a bloodstain of varying elliptical degrees that can indicate the angle of impact based on the shape. One calculation used to find the angle of impact is: $LaTeX: \sin (\alpha)=w\div l$ . In this equation, $LaTeX: \sin\left(\alpha\right)$ is the angle of impact whereas, w is the width of the bloodstain and l is the length of the bloodstain. In other words, the ratio between the length and the width of a blood drop is the function of the sine of the angle of impact. In this way, accurate measurement of the stain can give an accurate estimation of the angle of impact even if the point of origin is unknown. The example below shows the moving blood impact (Fig 1), the blood movement upon impact (Fig 2), and the impact from a top view (Fig 3).

What BPA Can Determine

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