blood drops

2008 HowStuffWorks

Basics of Blood

As unpleasant as it is to deal with, when a crime results in bloodshed, the blood left behind functions as evidence for investigators. A bloodstain pattern analyst can't simply glance at drips and smears of blood and immediately tell you the who, what and when of a crime scene. Blood spatter analysis takes time and is just one piece of the puzzle when investigators are putting together the elements of a crime. However, bloodstain pattern analysis can corroborate other evidence and lead investigators to seek additional clues. After close analysis, blood spatters can indicate important information such as:

  • Type and velocity of weapon
  • Number of blows
  • Handedness of assailant (assailants tend to strike with their dominant hand on the opposite side of the victim's body)
  • Position and movements of the victim and assailant during and after the attack
  • Which wounds were inflicted first
  • Type of injuries
  • How long ago the crime was committed
  • Whether death was immediate or delayed

Blood spatters can lead to the recreation of a crime because of how blood behaves. Blood leaves the body as a liquid that follows the laws of motion and gravity. It travels in spherical drops due to surface tension. Blood molecules are very cohesive, or attracted to each other, so they squeeze against each other until they form a shape with the smallest area possible. These drops behave in predictable ways when they strike a surface or a force acts upon them.

Imagine what happens when you spill water droplets on the floor. The water falls slowly to the ground, making a circular puddle. The shape and size of the puddle depends on how much water you pour, how high you hold the water glass and whether you're spilling it on carpet, wood, linoleum or some other surface. A lot of water makes a larger puddle. If the water falls from a distance, the puddle will be smaller in diameter. A hard surface will retain more of a circular shape, while carpet absorbs some of the water and makes the edges spread.