Bloodstain pattern analysis (BPA) is a specialized forensic discipline that involves interpreting the patterns left by blood at a crime scene. This field applies principles of biology, physics, and mathematics to understand how blood behaves and to reconstruct the events that led to bloodshed. By examining the size, shape, distribution, and location of bloodstains, analysts can draw conclusions about what occurred. BPA serves as a valuable tool in investigations, helping to piece together the sequence of events and corroborate witness statements.
The Appearance of a Vertical Bloodstain
When a blood droplet falls at a 90-degree angle onto a surface, it typically forms a circular stain. This occurs because the force of impact is distributed evenly around the point of contact, leading to a symmetrical pattern. The resulting circular shape exhibits little to no elongation. Minor irregularities, like small spines or scallops, may appear around the edges, especially on uneven surfaces. The shape remains circular, indicating a direct, vertical impact.
The Physics of Blood Droplet Formation
The formation of a blood droplet in freefall is governed by its inherent physical properties. Blood molecules exhibit strong cohesive forces, creating surface tension that acts like an invisible “skin,” causing the droplet to contract into a sphere. As gravity pulls the spherical blood droplet downwards, it maintains this shape until it encounters a surface. Upon vertical impact at 90 degrees, the kinetic energy of the spherical droplet causes it to flatten uniformly outwards from the point of contact. Surface tension and fluid dynamics ensure this symmetrical spread, resulting in a characteristic circular stain.
Understanding Impact Angle and Stain Shape
The angle at which a blood droplet strikes a surface significantly influences the shape of the resulting bloodstain. While a 90-degree impact produces a circular stain, any deviation from this vertical angle will result in a more elongated or elliptical shape. As the impact angle decreases, the bloodstain becomes progressively longer and narrower, with a distinct “tail” or “spines” pointing in the direction the blood was traveling. This elongation and tail serve as important indicators for analysts, determining the blood’s directionality. The circular appearance of a bloodstain from a near-vertical impact helps reconstruct the scene.