Bloodstain Pattern Analysis
Bloodstain pattern analysis examines the relationship between the size and shape of the droplets and the forces that created them. Passive drops fall under their own weight as the surface tension of the liquid is overcome by gravity. They result in the largest of drops and are usually circular when landing on a flat surface. As the amount of force increases, the size of the droplets decreases. Drop size and force are inversely proportional. High force events such as bullet impacts and compressed air from a sneeze produce a mist like appearance as the force tears the droplets apart.
Blood has three major components; plasma, red blood cells, and cholesterol. These can be easily separated by spinning in a centrifuge. Blood is a suspension as long as it remains circulating through the body. A single drop of blood is normally .05 ml in volume. Adult males typically have 5 to 6 liters of blood while females typically have only 4 to 5 liters.
Not all patterns can be reconstructed. A cast off pattern is created when an object in motion casts off individual droplets in sequence along the travel path. Each droplet has its own starting point and direction of travel. It is valuable in establishing the motion and direction of the object that produced the stain.
ANGLE OF IMPACT - The acute angle formed between the direction of a blood drop and the plane of the surface it strikes.
BLOODSTAIN - Evidence that liquid blood has come into contact with a surface
BUBBLE RINGS - Rings in blood that result when blood containing air bubbles dries and retains the bubble's circular configuration as a dried outline.
CAST-OFF PATTERN - A bloodstain pattern created when blood is released or thrown from a blood-bearing object in motion.
FLOW PATTERN - A change in the shape and direction of a bloodstain due to the influence of gravity or movement of the object.
PASSIVE STAIN - Bloodstain drop pattern created by the force of gravity acting alone.
POINT OF CONVERGENCE - The common point, on a two dimensional surface, over which the directionality of several blood drops can be retraced.
PERIMETER STAIN - A bloodstain that consists of only its outer edge, the center area having been removed by wiping after liquid blood has partially or completely dried.
POINT OF ORIGIN - The common point in three dimensional space to which the trajectories of several blood drops can be retraced.
PROJECTED BLOOD PATTERN - A bloodstain pattern that is produced by blood released under pressure.
SATELLITE SPATTER - Small droplets of blood that are launched outward as a result of the blood impacting the target surface.
Surface Tension- The tendency of a liquid to retain contact with molecules of a like material which forms the smallest shape possible, a sphere.
TARGET - A surface upon which blood has been struck by blood in flight.
TRANSFER/CONTACT PATTERN - A bloodstain pattern created when a wet, bloody surface comes in contact with a second surface. A recognizable image of all or a portion of the original surface may be observed in the pattern.
VOID - An absence of stains in an otherwise continuous bloodstain pattern.