Five things you might not have known about blood

Something New Every Day – Lecture 18 – Blood Patterns

  • How coagulation works – when a blood vessel is damaged or exposed, the lining produces proteins which attract the platelets which make up 1% of blood. The platelets turn sticky and clag together to plug the small wound. Once they’re all wedged in there, they themselves form a protein which reacts in the bloodstream to form insoluble protein strands called ‘fibrin’ and these (like fibreglass strands) bond together to seal the wound.
  • What makes a blood pattern? Blood generally has a fixed surface tension and a particular viscocity that makes it tend to cling to itself if left unchanged (kind of like the classic ball of mercury). But when gravitational forces or impact forces are sufficient, they can break blood’s inherent surface tension/viscosity and cause the blood to break into multiple droplets, the size and number of which create different patterns that forensic serologist can use to determine what happened at a crime scene.
  • Types of blood patterns
    • Passive – generally the result of gravity (dripping, pouring, pooling, streaking) or transfer
    • Spatter – droplets scattered on a surface. They result from a force in addition to gravity (eg: blunt force trauma, gunshot, knife wounds etc)
    • Altered – changed through natural clotting, insect activity, mixing with organic material, attempted clean-ups or the presence of an obstruction that interferes with the natural patterning (eg: a void)
    • Sattelite droplets are secondary droplets caused when a passive droplet impacts something (like the floor) and spatters off
    • Cast-off is the particular name given to the droplets that have come off the weapon and not the victim. These are used to determine the minimum number of strikes and the order that the strikes happened.
    • All splatter is spatter but not all spatter is splatter – Spatter is the term used when droplets of blood are scattered on a surface by the forces of a crime/incident. ‘Splatter’ is a type of spatter but it is only one configuration of spatter
    • How to make blood that both looks and feels real – 500ml powdered milk, slowly add 325ml water (stir until smooth), 1.5ounces red food colour, 25 drops green, 5 drops blue.

‘Trails of Evidence: How Forensic Science Works” is a The Great Courses DVD lecture series