Part of Lesson Plan: Blood Spatter - Understanding the Clues

Activity Overview / Details

Let's start with the obvious. Blood, in the body is liquid and it is under pressure. Blood can be easily categorized by blood type and it also contains DNA. Knowing the blood type can help the investigator limit the possible source of the blood to a segment of the population. The DNA in the blood can often be analyzed to identify to a mathematical certainty, the individual who left the blood. By sampling the blood stains that were left in our crime scene we can determine which individual left the blood on the phone, the couch, the hearth and Sid's hands.  Blood can also tell us whether or not there are drugs or poison present in the donor's system. This will take time however, and will most likely be done at the crime lab.

At the crime scene we have blood stains and they form patterns. John Glaister, a pathologist in Scotland broke down blood patterns into six categories. Considering violent crime scenes in terms of the categories of blood patterns present allows the investigator to begin assessing how the crime may have occurred. The categories are:

1) Blood drops – Gravity acts on the blood until it impacts a horizontal surface.

2) Blood splashes – Blood that has been thrown through the air until it struck a surface at an angle.

3) Pools of blood – These pools are next to the body and may indicate if the body has been moved.

4) Blood spurts – This is the result of arterial bleeding.

5) Blood smears – These happen when a bleeding person is moved.

6) Blood trails – Blood that is deposited when a wounded person walks or runs while dripping blood. It can also happen from carrying or dragging a body.

Let's see if we can associate these categories to a simple event. A person, let’s call him Ed, is trying to make a sandwich. While cutting the roll he accidently cuts his thumb enough that it hurts and bleeds a little. Startled, he quickly lifts his hand from the roll and shakes it. Some blood is flung from the wound and hits the side of the refrigerator. That is a "blood splash" category 2. Ed takes a close look at the cut. It isn't bad. As he examines the wound another small drop of blood forms. As it grows and becomes heavier gravity overcomes the blood’s surface tension and it falls striking the counter top. That is a "blood drop," category 1.  Well, Ed thinks, at least he didn't cut anything major, like an artery. If that had happened the blood would be spurting out under pressure. That would be category 4, blood spurt.

As Ed stands there thinking about what happened and trying to remember where he put the band aids, he continues to bleed. The drops begin to accumulate and form a puddle on the counter. This is becoming a "pool of blood," category 3.  

Ed leans forward and puts his hand on the counter as he opens the drawer where he keeps the band aids. His hand comes in contact with the small pool of blood and smears it; category 5. He starts to open the band aid but then remembers that he needs to wash the cut and put some disinfectant on it. The hydrogen peroxide is in the bathroom. As he walks to the bathroom a few drops of blood fall to the floor forming a "blood trail" which is category 6.

Ed's mother comes home and sees the mess. If she thinks about it for a few minutes in terms of the categories that we have discussed she will understand where Ed was when he hurt himself. She will see that he stood there for a few seconds that he leaned on the counter and then walked down the hall to the bathroom.

One further note should be made here. The faster the blood is moving, the smaller the drops will be. Speaking in generalities, blood spatter stains where the individual diameters are 4 mm or greater will be consistent with the velocity of the droplets moving at 5 feet per second or slower. This is called low-velocity impact spatter. When the diameters are in the 1 to 3 mm in size, the speed will have been in the range of 5 to 25 feet per second. When the stains are < 1 mm, the blood was traveling more than 100 feet per second. The resultant stain will look like a dried mist and will be consistent with a gunshot.

Now let's conduct an experiment.