Blood stain patterns

When researching my last post I came across a website explaining what else blood stains can tell us, and decided that I would also like to research into this.

From blood stains we can find out many things, including:

1. When it happened
2. Type of weapon
3. Movements of those involved
4. What injuries occurred/ whether death was immediate

Due to surface tension of blood causes it to have a spherical shape, as it pulls itself in, and keeps this shape until it hits a surface. This means that we are more easily able to tell how the blood has got there.

Blood stains are categorized in terms of velocity:

• Low-velocity impact spatter= <1.5 m/s

This is blood that has fallen at the speed of gravity, so are therefore usually large circular spatter where blood has fallen from an open wound.

• Medium-velocity impact spatter= 1.5 m/s- 7.5 m/s

The force of this impact means that there are smaller splatters, and this is the type usually seen in stabbings.

• High-velocity impact spatter= >30m/s

At a higher impact velocity, the blood which spatters is of a much smaller size. This type is generally due to gunshot.

There are also other types of blood staining that can occur, including:

• Cast-off stains- where blood off an object is transferred onto another surface.
• Shadowing- where there is an empty space in the blood spatter where there must have been an object.
• Swipes- where blood off an object/ person has been smeared against a surface.
• Transfers- where a bloody surface comes into contact with another surface and a pattern can be seen.

Scientists can also tell the angle at which the blood fell, by measuring the length and width of the spatter and dividing the smaller number by the larger number and using the arcsine rule. The more spherical (and not elongated) that the blood spatter is, the closer to 90 degrees the source of blood was. As well as this, the direction of the 'tail' on a blood spatter shows which direction the blood has traveled in. 

REFERENCES
http://www.finalanalysisforensics.com/docs/BasicBloodstainPatternAnalysisTEXT.pdf
http://www.crimescene-forensics.com/Blood_Stains.html
http://dsc.discovery.com/life/csi-knowledge-how-bloodstain-pattern-analysis-works-infographic.html
http://forensicsciencecentral.co.uk/bloodstains.shtml