Bloodstain Pattern Analysis (BPA)

• What happened
• How it happened
• Direction and force of bloodshed
• Position of victim and assailant

Principles of BPA

1. Physics of Blood Drops

• Blood behaves as a fluid and follows laws of physics
• Surface tension forms droplets
• Gravity and external force influence shape

2. Locard’s Exchange Principle

“Every contact leaves a trace”
Blood acts as transfer evidence between victim, suspect, and scene

Types of Bloodstain Patterns

 1. Passive Bloodstains

• Formed only by gravity
• Includes:
  • Drops
  • Pools
  • Drips
• Indicates minimal external force

2. Transfer Bloodstains

• Occurs when a bloody object contacts a surface
• Examples:
  • Handprints
  • Footprints
  • Smears (wipe/swipe)
• Helps identify movement or contact

3. Projected Bloodstains

• Caused by force greater than gravity
• Types include:
  • Impact spatter
    • Low velocity (large drops)
    • Medium velocity (beating/stabbing)
    • High velocity (gunshot mist-like)
  • Arterial spray (from damaged arteries)
  • Cast-off (blood flung from weapon)

Key Concepts in BPA

1. Angle of Impact

• Determines angle at which blood hits surface
• Formula:
  Angle = sin⁻¹ (width ÷ length)
• Smaller angle → elongated stain

2. Direction of Travel

• Tail or pointed end shows direction of movement
• Helps identify source and motion

3. Area of Convergence

• Point where blood originated (2D)
• Determined by tracing lines backward

4. Area of Origin

• 3D location of blood source
• Uses trigonometry and stringing method

5. Velocity of Blood