Bloodstain Pattern Analysis 101: Passive Flow

Passive blood flow

Bloodstain patterns are categorized into various “types” based on their physical characteristics or properties. This is the first in a series of articles I will do on the main types of bloodstain patterns. The simplest to understand is what we call passive flow.  Passive flow stains are commonly found at scenes of death with bleeding victims. By definition, passive flow patterns are influenced by the force of gravity alone. In simple terms think of a pool of blood under a victim.

Some people may think that these patterns are not very informative to the examiner but they would be wrong.  These patterns may be created by a change of the victim’s position; rolling or moving for example.  A victim’s position may change either from their own actions or by the actions of others. So if you find a pool of blood in one room and the victim is found in another room, you know (assuming the blood is from the victim) they changed locations.  The question is why. Bloodstain pattern analysts will evaluate a number of things including injuries and other bloodstain patterns to understand whether they changed locations under their own power.

Two big factors influencing passive flow are the location of the injury and the surface the blood flows onto. A victim laying inclined on a hard surface (like a sloping street) with a head wound pointing down slope may effectively “drain” more blood than the same victim laying with their feet down slope. Likewise, the surfaces the blood flows onto can greatly influence the appearance of the resulting bloodstain pattern.  For example, 2 liters of blood on carpeting may appear much smaller in size than 2 liters on tile or Linoleum.

There are a number of ways to use this in a story. Say your detective finds a woman killed in her home with a severe head injury. When looking at the blood pool the detective notices a change of direction in the flow like the victim was moved or rolled. This is further compounded by noticing a difference in the drying time of the two flow patterns (more on drying in a future article). Depending on the circumstances the death may be considered an accident or suicide until this discovery comes to light. Maybe the pathologist says the injury would have incapacitated the victim in a short amount of time. Whatever the circumstances, it is often that “one little clue” that has the potential to alter the course of an investigation by challenging the initial theory.

Passive blood pooling on vehicle seat