Forensic Science Unit 2

Bloodstain Pattern Analysis (BPA)

The study of the size, shape, and distribution of bloodstains to  reconstruct events at a crime scene (e.g., force involved, victim position, sequence of events).

Surface Tension

Blood behaves as a fluid with predictable surface tension. A falling drop forms a  sphere in mid-air. 

 Impact Factors

Stain shape depends on speed, impact angle, drop volume, height, and target surface  texture. 

 Smooth surfaces

 Preserve drop shape with clean, rounded edges. 

Rough/Porous surfaces:

Cause drops to break up, creating irregular scalloped edges and  satellite spatter. 

Passive Stains 

Formed primarily by gravity acting on a  relatively still source. Passive drops (90° impact = circular),  pools, flows.

Projected stains

Formed when external force/energy is  applied to a blood source. Impact spatter, cast-off (swung objects), arterial spray.

Transfer stains

Formed when a wet, bloody surface  contacts another surface,Shoe prints, handprints (transfers),  swipes, wipes.

Directionality

The tail of an elongated bloodstain always points in the direction the drop was  traveling when it landed.

90° Impact

Perfectly circular stain with no tail. 

Angled Impact (< 90°):

Elongated ellipse; smaller angles yield longer ellipses. 

Angle of Impact Formula: 

_{sin(α )=}Width/Length. ^α=arcsin(^Width/_Length ) 

 Region of Convergence:

The 2D point on a surface where directional lines drawn through individual  stains intersect. 

3D Origin Determination:

Traditional manual stringing vs. digital laser scanning/software. Modern software accounts for gravity's downward pull (parabolic paths), providing greater accuracy.

 Key Case Study: Dr. Sam Sheppard Case (1954/1966)

Re-examination of bloodstain patterns by Dr. Paul Kirk overturned Sheppard's conviction, demonstrating the necessity of standardized, objective  methodology in BPA. 

Medical Examiner (ME):

A licensed physician, typically board-certified in forensic pathology,  appointed based on medical qualifications. Performs autopsies directly.

Coroner:

An elected official (rooted in historic English tradition) who may or may not be a physician. Frequently relies on contracted forensic pathologists to perform autopsies. 

 Cause of Death (COD):

The specific injury or disease that initiates the lethal chain of events (e.g.,  Gunshot wound to the torso). 

Mechanism of Death:

The physiological or biochemical disruption produced by the cause (e.g.,  Massive hemorrhage / blood loss). 

 Manner of Death (MOD):

The broad circumstances surrounding the death.

NASH +  Undetermined: 

1. Natural (disease/aging) 

2. Accident (unintentional injury) 

3. Suicide (self-inflicted with intent) 

4. Homicide (caused by another person) 

5. Undetermined (insufficient evidence)

Change Algor

Body cooling toward ambient temperature. ~1.5°C drop per hour for the first 8  hours.

Livor Mortis

Gravitational pooling of blood in lowest body regions (discoloration).