Blood is viscous, and blood viscosity is a measure of the blood’s resistance to flow.
During the formation of a drop of blood, blood leaks out from a blood source.
When a bloodstain lands on a surface, the shape and size of the bloodstain is affected by the texture of the target surface.
Bloodstains that land on porous or rough surfaces usually have more distortion around the edges of the stains than those that land on smooth surfaces.
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Directionality: The characteristic of a bloodstain that indicates the direction blood was moving at the time of deposition.
Parent Stain: The resulting spatter stain is an elongated ellipse.
Satellite Stain: A smaller bloodstain that originated during the formation of the parent stain as a result of blood impacting a surface.
Angles of Impact: The acute angle (alpha), relative to the plane of a target at which a blood drop strikes the target.
When a blood drop lands on a surface at a perpendicular angle (90°), a circular parent stain is formed, where the length and the width of the stain are equal.
When a blood drop is projected onto a surface at an angle of between 0° and 90°, the stain is elongated.
As the impact angle decreases, the shape of the spatter stain is more elongated in which the length of the stain is greater than the width.
Area of Origin: The three-dimensional location from which spatter originated.
In the String Method, multiple well-formed, elongated spatter stains are selected for analysis.
In the Tangent Method, the directionality of a single stain is determined first.
A line is then back projected through the major axis of the bloodstain.
For a single impact event, approximately two dozen stains are processed to determine the area of convergence.
Area of Convergence: The area containing the intersections generated by lines drawn through the long axes of individual stains that indicates in two dimensions the location of the blood source.
It is formed due to bleeding from wounds, and the blood is deposited on a surface by the influence of the force of gravity alone.
Drip Stain: Formed when a falling drop of blood from an exposed wound or a blood-bearing object lands on a surface.
Drip Pattern: Formed when a liquid drips into another liquid, where one or both of the liquids are blood.
Splash Pattern: Formed when a volume of blood spills onto a surface.
Flow Pattern: Caused by the movement of a large volume of blood on a surface either due to gravity or to the movement of the target such as a victim or postmortem disturbance.
Pool: A bloodstain resulting from the accumulation of liquid blood on a surface.
Transfer Bloodstain: A bloodstain resulting from contact between a blood-bearing surface and another surface.
Swipe Pattern: A bloodstain pattern resulting from the transfer of blood from a blood-bearing surface onto another surface, with characteristics that indicate relative motion between the two surfaces.
Wipe Pattern: An altered bloodstain pattern resulting from an object moving through a preexisting wet bloodstain.
Perimeter Stain: A type of wipe pattern, is a bloodstain that is disturbed before it is dried but it maintains the peripheral characteristics of the original stain.
Projected Bloodstain: Formed when a volume of blood is deposited on a surface under pressure or a force that is greater than the force of gravity.
Impact Pattern: Formed when an object strikes liquid blood.
Cast-Off Pattern: Formed when blood drops are released from a moving blood-bearing object.
Forward Spatter: Formed when blood drops travel from an exit wound in the same direction as a projectile.
Back Spatter: Formed when blood drops travel from an entry wound in the opposite direction of a projectile.
Expiration Pattern: Formed when blood is forced by airflow through the trachea and out of the nose or mouth.
Arterial Spurt Pattern: Associated with wounds damaging arterial blood vessels where blood stains are driven by arterial pressure.
At a crime scene, if the projectile of bloodstains is blocked by an object, a void pattern is formed, which exhibits an area where there is an absence of blood surrounded by continuously distributed bloodstains.