Bloodstain Pattern Analysis Lab

Bloodstain pattern analysis (BPA) is the interpretation of bloodstains at a crime scene in order to recreate the actions that caused the bloodshed. Analysts examine the size, shape, distribution, and location of the bloodstains to decide what happened. BPA uses biology (behavior of blood), physics (cohesion, surface tension, and velocity) and mathematics (geometry, distance, and angle) to assist investigators in answering questions like:

• From where did the blood originate from?

• What was the cause of the bloodshed? (weapons used)

• From what direction was the victim wounded?

• What were the positions of the victim/ attacker during the bloodshed?

• Did anyone/anything move around after the bloodshed?

• How many possible people were present

during the bloodshed?
• Does the bloodstain evidence support or go against the witness statements recorded?
Blood behaves according to certain scientific principles. This allows analysts to examine the blood evidence left behind conclude how the blood may have been shed. A random distribution of bloodstains at a crime scene can easily be categorized by analysts by gathering information from spatter patterns, transfers, and other marks. This helps investigators recreate the sequence of events that occurred at the crime scene.
BPA provides information not only about what happened, but just as importantly, what could not have happened. This information can assist the investigator in reconstructing the crime, supporting statements from witnesses, and including or excluding potential criminals from the investigation.
Facts About Blood
When blood is impacted, droplets are spread throughout the air. When these droplets strike a surface, the shape of the stain changes depends on the angle of impact, velocity, and distance travelled. Generally, the stain shape will vary from circular to elliptical, with tails or spines extending in the direction of travel. Smaller satellite stains may also break away from the initial drop.
Blood is considered to be a fluid. A fluid is a substance with no fixed shape and is subject to external pressure. Surface tension is the force that pulls the surface molecules towards the interior of a fluid, decreasing the surface area of the fluid and causing the fluid to resist splitting up. Surface tension causes the surface of a fluid to shrink into the smallest area possible (i.e. - it causes blood form the shape of a sphere when it is moving). (See Figure 1) The fluid is able to do this because the cohesive forces are stronger on the surface of fluids as there are no neighboring molecules above. As a result there are stronger attractive forces between molecules and their nearest neighbors on the surface.
The formula to calculate surface tension is: the surface tension (Y) is the extent of the force (F) exerted divided by the length (L) of the line over which the force acts:
Y= F/L
Surface tension is measured in force per unit length (newtons per meter). (See Figure 2)
Due to its surface tension, when a blood drop hits a surface, the lower portion of it will stop while the upper half will continue to move. (Newton’s Law of Inertia) Therefore, the diameter of the drop while it was in motion and the length of the bloodstain on the surface will create a right triangle, with the hypotenuse being the length of the triangle. (See Figure 3)
Blood acts as a non-Newtonian fluid, which means that its viscosity changes due to the shear rate. The shear rate can be classified as the velocity. When blood moves quickly, it becomes thinner. And when it moves slowly, it is thicker and stickier. This is known as shear-thinning, non-Newtonian behavior of whole blood.(See Figure 4)
Some other things to keep in mind about blood are that it:
• On average accounts for 8% of total body weight
• There are about 5 to 6 liters of blood in males
• And there are about 4 to 5 liters of blood in females
• A blood loss of 1.5 liters will cause a person to faint
• A 40% blood volume loss in the human body will most likely cause the victim’s death.

Path of Blood Droplets
Blood moves according to basic projectile movement, where the only force affecting it is gravity(assuming that there is no air resistance) Newton’s law state that the net force F on an object is equal to the mass m and acceleration a:
F = ma
Where F is the sum of all forces on an object.

(See figure 5)

In figure WHATEVER, the only force is gravity (Fg.) This force can be described by the equation:

Fg = mg,

Gravity is a descending path. This causes a projectile to move in the downward direction. Since both of them are perpendicular, the force of gravity could never alter the horizontal velocity of an object because they are free of each other. A vertical force does not affect a horizontal motion. The result of vertical force acting upon a horizontally moving object is to cause the object to stray from its otherwise line path. (See Figure 6)

Before, I mentioned that we are assuming there is no air resistance and thus, the only force acting on the blood drop is gravity. However, this is not the case. Air resistance plays a huge role on the path blood drops take. ( See Figure 7) The path it takes will start off parabolic, but will turn linear as it reaches terminal velocity. The drag force (and terminal velocity) differ for different particles in the scene depending on their size and shape. Stokes’ Law applies for spherical objects in a fluid. This law states the drag force

as:

Where μ is the viscosity of the fluid and v is the particle velocity.
If we combine all the constants into one term k, we can write:
Fd = −kv,
Where k = 6πμr
Now that we know how both forces Fd and Fg can be described as, from
Newton’s law (F=ma)
Fg+ Fd=ma
Mg-kv=ma

Angle of Impact
The angle of impact tells us the angle at which the blood hit the surface. For instance, if I held out my arm and it was bleeding, the angle of impact of the blood would be 90 degrees, since my arm is directly perpendicular to the floor. To calculate the angle of impact, take the width measurement and the length measurement of the blood stain, keeping in mind not to measure the tail. The tail is caused by the pull of gravity on blood, which is why is disregarded. The larger the tail, the smaller the angle of impact. (See Figure 8) One must imagine that as the blood droplet is hitting the surface it is making a right triangle. To calculate, do the following:
1. Measure the length and width of the blood droplet. (Excluding the tail)
2. Use i to represent the angle of impact, the hypotenuse represents the length, and the side opposite the hypotenuse is the width. (See figure 9)
3. Divide the length by the width.
4. Find the inverse sine.
* Using this method you will get an estimate. As long as your answer is within 4º-6° it will not affect the area of convergence.
For example, say the length (or the hypotenuse) is 5 and the width (or the opposite side) is 3. You would first divide 3 by 5, which is .6. Lastly, you take the inverse sine of .6, which is about 37 degrees. This is your angle of impact.
When calculating the angle of impact it is important to keep in mind that (sin-1) does not mean (1/sin). Instead, it means the inverse sin function. When dividing, make sure is the width is the smaller number, as the angle of impact cannot be greater than 90 degrees.
As the angle of impact changes, so does the appearance of the resulting stain. A blood drop striking at a 90° angle will end in an almost circular stain. There is little elongation, and the spines and satellites are fairly evenly distributed around the outside of the drop. Below 75°, spines begin to become more noticeable. As the angle of impact decreases, the spatter stain becomes more elliptical, and the spines become larger .At very low angles, a satellite may break off to form a second stain. This forms a distinctive “exclamation point” stain. (See Figure 10)

Area of convergence
The area of convergence tells us where the spatter may have originated. To do this, strings are taken and are attached to each blood stain down its axis (See figure 11), and then drawn in the opposite direction of which they were travelling. (See figure 12). It is important to look at the direction of the blood stains, because it will show you which spatter is due to swinging the object backward and which is due to forward motion. If we know where the perpetrator was, we can get a much clearer understanding on which hand of the assailant is their dominant hand. There are times when the attacker will attempt to make a killing appear as a suicide. Area of convergence plays a very big role in this. It shows if there are any inconsistencies at the scene.
Area of Origin
Although the area of convergence plays a huge role in blood spatter analysis, it is only a two-dimensional measurement of a three-dimensional crime scene. Area of origin shows the height at which blood fell. It is important to determine the height at which the blood fell, because it tells us the height that the blood drop originated from. (See Figure 13) It is possible that the victim and the assailant both leave spatter evidence. Due to this, it is important to take notes of the heights and find where any inconsistency may lay. Knowing the height of the victim we may deduce that a certain blood stain originated from the perpetrator or the victim.
To calculate the area of origin, we use the tangent function:
TANi= H/D i– the angle of impact
H–distance above the area of convergence (which is unknown at this point)
D– the distance from the bloodstain to the area of convergence
**When put into a right triangle, D represents the leg adjacent to the angle and H represents the leg opposite of the angle. (See Figure 14)
Since everything is known except for distance of the area of convergence, (H) the formula changes to:
H=TANi *D
Let us say that we find a blood stain 10ft. away from the area of convergence and we have determined that the angle of impact is 80 ̊. To determine the height we construct the following. Now we calculate:
STEP 1: tan 80 ̊= D/15
STEP 2: 15(tan 80 ̊) = 85.1ft.
The height at which the blood fell was 85.1ft.
Graphing The Area of Origin
A graph in this case has an X- axis and a Z-axis. The X-axis represents the target plane (the sight of the bloodstains) and graphs the distance from the back-edge (using the long axis) of the stain to the area of convergence. The Z-axis represents the height above the target plane.). To graph:
1. Mark on the X-axis of the graph the distance of each stain to the Area of Convergence.
2. Draw a line from the mark on the X-axis, at the calculated angle of impact, to the Z-axis.
3. Repeat each stain.
4. The area where the lines drawn from the X-axis meet on the Z-axis is the probable Area of Origin. (See Figure 25)
Velocity of Blood
Low velocity blood spatter is caused by a low velocity force, usually the result of blood dripping from a person who is still, walking or running. Blood drops are falling freely and only moving because of the pull of gravity on it. At low velocities, larger bloodstains are produced. Usually, low velocity bloodstains are a result of blood dripping from a victim. Dripping blood often falls at a 90 angle. This causes a circular bloodstain to form on the surface that hits it lands upon. The droplets range from 4mm-10mm. However, this usually happens if the victim is not moving and completely still. If droplets are falling from a moving object or person, (walking or running) they fall to the ground at an angle (see angle of impact) and the direction of the movement can be established. Spines often appear on bloodstains left behind and can be caused by drops repeatedly landing in the same place, by the distance the drop falls (larger spines indicate larger distance), or by the surface upon which the blood lands (rough surfaces cause more spines to form). Low velocity blood may also be found in the trail of a person who is bleeding and larger pools of blood may indicate where the person paused. (See figure 15)
Medium velocity blood spatter is caused by a velocity that moves blood between five and 50 meters per second and the resulting bloodstains are between one and three millimeters in size. The size of the bloodstain depends on the angle of impact with the receiving surface. An slanted stain can be greater than 3mm but would belong and thin. Medium velocity blood spatter might result from blunt force trauma. For example, beating with fists, baseball bats, whips, bricks or hammers. (See figure 16)
High velocity blood spatter is a high velocity force moves blood greater than 50 meters per second and the bloodstains are usually smaller than mm and appear as fine spray or misting.

High velocity blood spatter can be caused by high-speed machines, most commonly caused by guns. (See figure 17)

Types of Stains

Bloodstains are classified into three basic types: passive stains, transfer stains and projected or impact stains. Passive stains include drops, flow and pools, and typically result from gravity acting on an injured body.(See figure 18) Transfer stains result from objects coming into contact with existing bloodstains and leaving wipes, swipes or pattern transfers behind such as a bloody hand print or a smear from a body being dragged.(See Figure 19) Impact stains result from blood jutting out into the air and are usually seen as spatter, but may also include gushes, splashes and spurts. (See figure 20)

Gunshot spatter includes spatter from the exit wound and spatter from the entrance wound. Gunshot spatter will differ depending on the capacity of the gun, where the wound is on the victim, if the bullet exits the body or not, distance between the victim and the gun and location of the victim relative to permanent objects at the crime scene (walls, floors.) (See Figure

21)
Cast-off are tails often found on bloodstains. Analysts can tell the direction the direction the blood was moving in by the shape of the spatter (tails point in the direction of motion). (See Figure 22)
Arterial spray refers to the spurt of blood released when a major artery is cut. The blood comes the broken blood vessel often forms an arcing pattern consisting of large, individual stains, with a new pattern created for each time the heart pumps . (See Figure 23)
Expiratory spatter is usually caused by blood from an internal injury mixing coming out through the nose, mouth or an injury to the airways or lungs. Expiratory spatter tends to form a mist due to the pressure applied by the lungs. Small air bubbles in the drops of blood are typically found in this type of spatter (See Figure 24)