Week 6 - Blood pattern analysis

What is blood made up of?

Plasma within which are cells including red blood cells, platelets, white blood cells and other substances like alcohol, illicit drugs, medicines

What is the purpose of blood?

To deliver nutrients and oxygen to cells, to take waste products from the cells (i.e. carbon dioxide), to fight infection/viruses with antibodies

How is blood special compared to other fluids?

It is variably viscous and elastic, droplets tend to hold together in flight, similar consistency to paint and behaves in a similar way, is slightly thicker than water

How much blood do we have?

Males have 5-6 litres while females have 4-5 litres

What is included in the circulatory system

Arteries which carry blood away from the heart and because of this, the pressure is higher, veins bring blood back to the heart, so pressure is lower, arterioles, capillaries

Ways in which we can find blood

• Through visual cues, e.g. through examination with lights, magnification

• Chemical tests for haemoglobin

• Spraying with luminol

How can luminol be problematic?

Can cause false positives as bleach can also cause a glow when sprayed with luminol. Considering that many perpetrators use bleach to clean up blood, this can be problematic

What is blood pattern analysis?

A specialised branch of forensic science in which the investigator deduces evidence from shape and distribution of bloodstains

Low velocity spatter

Dripping/large stains that might have been there for a while and seaped

Medium velocity spatter

Usually caused by something like a beating

High velocity spatter

Usually caused by shooting, looks like small, fine, mist-like stains, blood travels a long distance and may hit an object with force

Passive

Gravity only, relatively slow moving and controlled

Active

When blood has travelled under a force other than gravity

Transfer

Movement of blood through the movement of objects/assailants, third parties

Passive stains

Slow, steady and gentle, e.g. drops falling under gravity, passive flows of blood from a wound

Passive drip patterns

Size of the drip pattern determined by:

• Amount of blood

• Nature of object from which blood drips

• The height from which it falls

Spatter mechanisms

• Impact mechanisms - bloodstain resulting immediately from impact (gunshot, bleeding etc.)

• Projection mechanisms - coughing, cast off, arterial spurt etc.

• Secondary mechanisms - blood interacting with another liquid, e.g. someone bleeding in a body of water, or blood vs. rain

The greater the spatter marks…

The higher/further away the wound probably happened in relation to the trace, whereas if there are no spatter marks and just a pool of blood, the wound likely happened lower to the stain/closer to the source

Altered stains

The process of clotting and drying, falling off the body, blood may clump with the stain having a lighter or straw coloured outer stain

What information can contact stains give us?

Smears can indicate the direction of movement, patterns of footwear/weaponry

Blood landing at an angle…

Has an elongated shape from the sphere of blood hitting the surface at an angle, the more acute the angle the longer and thinner the stain

Cast-off

A pattern of bloodstains caused when an object is swung or moved in motion, causing blood to be flung from the object

Cessation cast-off

When a bloody object comes to a sudden halt (end of swinging, hits something)

What can blood cast away from the assailant tell us?

Indication of assailant’s movement, the timings involved

What is a tell-tale sign of a cast-off?

The substantial lump/blob at the top

Exhalation

Where blood is inhaled and breathed/coughed/vomited out. This usually happens as a result of chest damage or injury to the mouth/nose. Where the blood lands from this can give indications as to events. The blood may have surfactant (substance from the lungs) in it

Arterial damage

Large volumes of blood characterised by a high pressure arterial spurting driven by the heart, may produce W or V shapes on walls, arterial spurting at a near vertical angle may produce “arterial rain”

Punching

Spatters are consistently found around the cuff of the punching arm which indicates repeated punching with large amounts of force, may also be on the upper arm and across the upper chest

Kicking and stamping

Blood within seams and crevices of footwear indicates forceful contact, blood found on bottom of trousers

Firearm spatter

Characterised by mist and fine spread spatter 

Voids

When there is a shape/blank space in the blood spatter, showing something was removed after the event that caused the blood

Clotting

Depending on how long the blood has been there, it will clot and the colour will change slightly. Gives us indication of how long the blood has been there, e.g. if it has started to clot it won’t be fresh

Flyspots

Insects will walk through bloodstains and distribute this around a scene, investigators must specify this has been caused by flies rather than a cast-off or primary bloodstains

What can blood pattern analysis indicate?

• Lines of blood might indicate how many times struck/stabbed, force used

• Shape and orientation of blood drops may indicate placement of victim during the attack

• Movement of persons can be indicated through blood footprints

• Pools of blood can indicate time since event through process of clotting/drying

• What weapons/tools have been used

Downsides to blood pattern analysis

• Relies on the individual assessment of the expert

• Imprecise nature of practice - will attempt to re-create blood patterns in the lab to establish how it was caused

• Is a subjective matter of interpretation

What is the value of blood pattern analysis?

The value is presumptive and only serves as a reconstructive purpose

When is blood pattern analysis used?

When there is a large volume of blood at the scene, a substantial enough amount to make it worthwhile investigating