Summary of Blood Evidence- Lesson 2 (Forensics)

Summary of "Blood Evidence - Lesson 2 (Forensics)"

This lesson continues the study of Blood Evidence in forensic science, focusing on blood spatter and blood stain interpretation. Unlike other forensic evidence such as fingerprints or DNA that directly link suspects to crimes, blood stains serve as a narrative tool to reconstruct the events that caused bleeding at a crime scene.

Main Ideas and Concepts

• Purpose of Blood Spatter Analysis:
  • To piece together the sequence of events that led to bloodshed.
  • Blood stains tell the story of what happened rather than directly identifying suspects.
  • Analysts correlate static blood stains with the dynamic forces that caused them.
• Types of Blood Stains:
  • Impact Spatter
  • Cast-off Spatter
  • Transfer Stains
  • These are analyzed by size, shape, and distribution to recreate crime scene events.
• Verification of Blood: Investigators first confirm the presence of blood using Presumptive Blood Tests before analysis.
• Key Analytical Techniques:
  • Determining trajectory (origin and path of blood).
  • Measuring stain shape to infer direction of movement and speed of blood at impact.
  • Understanding velocity categories:
    • Low velocity spatter: Slow-moving blood.
    • Medium velocity spatter: Moderate speed.
    • High velocity spatter: Very fast, producing tiny droplets (e.g., gunshot spatter).
• Physical Properties of Blood Drops:
  • Blood falls as spheres due to surface tension.
  • A passive drop falls vertically (90° angle) creating a circular stain.
  • Stains may have spikes and satellites (small droplets around the main stain) which provide clues.
  • Drop size inversely correlates with velocity/energy of impact (higher energy = smaller drops).
• General Rules:
  • The fastest free-falling blood drop (passive) travels at about 25 feet per second.
  • Blood always travels in the direction of the applied force.
• Importance of Expertise: Blood stain pattern analysis requires trained professionals due to the complexity and physics involved.

Methodology / Instructions for Blood Spatter Analysis

• Step 1: Confirm material is blood using Presumptive Blood Tests.
• Step 2: Collect and document blood stain patterns at the crime scene.
• Step 3: Classify stains by type (impact, cast-off, transfer).
• Step 4: Measure size and shape of stains.
• Step 5: Determine trajectory and direction of blood travel.
• Step 6: Estimate velocity of blood drops (low, medium, high).
• Step 7: Use stain characteristics (shape, satellites, spikes) to reconstruct events.
• Step 8: Compare Blood Spatter Analysis with witness or suspect statements to confirm or disprove accounts.

Additional Resources

The lesson recommends watching a detailed 17-minute video titled "Forensic Expert explains how to analyze bloodstain patterns" by Wired on YouTube for visual examples and further explanation.

Speakers / Sources Featured

• Primary Speaker: Unnamed instructor or forensic educator delivering the lesson.
• Referenced Source: Wired YouTube channel (for recommended supplementary video).

Category

Educational

Video