Summary of "Blood Spatter Analysis Lecture - part 2"

Blood Spatter Analysis — Lecture (Part 2)

Map spatter locations (examples shown on three walls and in an overhead view).
Droplet shape indicates direction:
- Round stains indicate impact perpendicular to the surface.
- Elongated/elliptical stains and “tails” point away from the source; the tail points in the travel direction.
By tracing stain shapes and angles, analysts can reconstruct where and how events occurred.

High-velocity: droplets ≤ 1 mm — associated with gunshot spatter (fine misting). Gunshot events can also produce back spatter and provide directional information related to bullet entry/exit.
Medium-velocity: droplets ~1–4 mm — associated with blunt-force trauma (beating), stabbing, and cast-off from a swinging weapon. Typical features include larger individual stains with satellite stains.
Low-velocity: droplets ≥ 4 mm — associated with gravity-driven drops, dripping while walking, and pooling.

When a weapon is swung, blood is flung in an arc; cast-off patterns radiate from the swing path and can appear on walls or ceilings.
Shape varies with angle: round when struck directly overhead, more elliptical at oblique angles.
These patterns help determine the origin and arc of movement.

Satellite stains: small drops surrounding a primary stain that provide clues about force and movement.
Back spatter: blood directed back toward the source (for example, toward a shooter).
First blow: often produces little or no spatter if no blood is exposed; subsequent blows produce patterns.
Arterial gush: a severed artery (e.g., throat slit) produces a spraying/pulsatile pattern; directionality and long drops that run are characteristic.
Expiratory/expelled blood: coughing, vomiting, or nasal bleeding can create a fine mist resembling gunshot misting; testing for saliva or nasal secretions distinguishes expiratory stains from gunshot spatter.

Transfer: blood moved from a pool or source onto another surface (examples: shoe prints, fingerprints, swipes).
Swipe: blood transferred by movement across a surface (for example, hair dragged across a stain).
These patterns help determine movement paths of victim and suspect.

Blood coagulates and dries over time; the degree of clotting/drying helps estimate when bleeding occurred relative to the crime.

Bloodstain pattern analysis (BPA) uses physics, biology, and chemistry to reconstruct positions, movements, and interactions of victim and suspect.
BPA focuses investigators on where to look for trace evidence (fingerprints, footwear, hairs, fibers), can reduce unnecessary DNA sampling, and provides courtroom visualizations to test witness statements and support testimony.

Bloodstain pattern analysis helps reconstruct events and directs investigators to relevant trace evidence and sampling points.

Document and map all stains across surfaces (include overhead and wall views).
Note stain shape and orientation at each location:
- Round = perpendicular impact.
- Elliptical/tail = oblique angle; tail points in travel direction.
Measure stain diameters to classify velocity:
- ≤ 1 mm → high-velocity (gunshot).
- 1–4 mm → medium-velocity (blunt/stab, cast-off).
- ≥ 4 mm → low-velocity (drips, pooling).
Look for satellite stains and back spatter to refine conclusions about force and direction.
Identify cast-off patterns and follow the arc to infer swinging motion and point of origin (note ceiling vs wall differences).
Check for expiratory origin when misting patterns are present (test for saliva or nasal secretions).
Identify arterial patterns (pulsatile sprays, directional run-downs).
Examine transfer/impression patterns (shoe prints, swipes) to determine movement and contact paths.
Assess clotting and drying stages to estimate timing of bleeding.
Integrate findings with other forensic evidence (fingerprints, fibers, DNA) and use reconstructions to evaluate witness/suspect statements and present findings in court.

Demonstration: bat striking blood-soaked foam to create medium-velocity/cast-off patterns.
Images referenced in lecture: cast-off patterns on a ceiling and overhead wall views.
Students were encouraged to read three documents on Moodle and prepare for lab exercises.

Lecture instructor/presenter (unnamed).
Referenced sheriff (unnamed) — described as a bloodstain pattern expert since 1984, quoted on the value of BPA.
Demonstration sources/imagery: soaked-foam + bat demo and ceiling cast-off photos used as visual aids.
Moodle course materials (three documents referenced).