Bloodstain Patterns - In-Depth Notes

Bloodstain Patterns

4.1 Bloodstain Pattern Evidence

  • Purpose: Used as a forensic tool to assist in investigations.
  • Corroborate statements from witnesses or suspects.
  • Help apprehend suspects based on evidence found.
  • Aid in interrogating suspects effectively.
  • Enable reconstruction of the crime scene.
  • Strengths and Weaknesses:
  • The validity of analysis depends on the quality of information available and the abilities of the examiner.

4.2 Human Blood

  • Biological Properties:
  • Provides crucial substances to tissues: oxygen, electrolytes, nourishment, hormones, vitamins, and antibodies.
  • Removes waste products from tissues.
  • Composed of plasma, red and white blood cells, and platelets.
  • Clotted plasma is known as serum; white blood cells contain DNA.
  • An average healthy adult has about 4.5 to 6.0 liters of blood.

4.3 Formation of Bloodstains and Bloodstain Patterns

  • Size and Shape:
  • Blood drops can vary in size due to several factors, including surface tension, height of drop, and type of surface.
  • Smooth surfaces produce less spatter; rough surfaces tend to create more.
Physical Properties of Blood
  • Molecular Forces: Blood is held together by molecular forces, with surface tension being slightly less than that of water.
  • Viscosity and Specific Gravity:
  • Viscosity refers to the resistance of fluid movement; higher viscosity means slower moving.
  • Specific gravity measures the weight of blood compared to water.
  • Terminal Velocity: The force exerted downward by gravity is countered by the forces acting upward.
Factors Affecting Bloodstain Formation
  • Source of passive drops, surface tension, height from which blood falls, type of surface, gravity, and air resistance all influence bloodstain formation.

4.4 Size, Shape, and Directionality of Bloodstains

  • Directionality:
  • Determined by the geometry and edge characteristics of the stain; the narrow end indicates the direction of travel.
  • Area of Origin:
  • Blood source location can be identified in both 2D and 3D perspectives.
  • 2D involves drawing lines through long axes of stains where they converge.
  • 3D estimates impact angles from trajectories leading back to a common axis.
  • Angle of Impact:
  • Calculated with the ratio of width to length:
    • Circular stains: ratio = 1 (90° impact).
    • Elliptical stains: ratio > 1 (>90° impact).

4.5 Altered Bloodstains

  • Factors Influencing Drying:
  • Drying time is affected by volume, surface texture, and environmental conditions (temperature, humidity, airflow).
  • Skeletonized Stain: Refers to cases where the center of a dried bloodstain flakes away, leaving a visible outer rim.
  • Void Areas: Absences of bloodstains amidst otherwise continuous blood patterns, often due to the removal of objects.

Implications of Absence of Blood Evidence

  • Exoneration and Implication:
  • The absence of blood on an assailant does not automatically exonerate or implicate them.
  • Possible reasons for absence can include protective outerwear or removal of clothing.

Summary of Chapter 4

  • Bloodstain patterns are a common feature at violent crime scenes.
  • They need to be found, documented, analyzed, and interpreted carefully.
  • Key Analysis Focus:
  • Size, shape, directionality, and impact angles are crucial in understanding the dynamics of blood at crime scenes.
  • Scientific analysis is invaluable for legal proceedings and investigations.