Human bodies are leaky. If pierce to any depth they tend to gush or spray blood aimlessly.
This is hardly surprising since the average-sized adult male has between five and six quarts of blood gurgling through their system (females and children have slightly less).
It can gush, it can drip, or spatter across the air forming a pattern from a wound.
Crimes involving violent contact between individuals are frequently accompanied by bleeding and resultant bloodstain patterns.
Crime-scene analysts have appreciated the analysis of bloodstain patterns that were deposited on floors, walls, ceilings, bedding, and other relevant objects for providing valuable insights into the events that might occur during the commission of a crime.
What Information Bloodstain Pattern Analysis Provide?
- About the surface texture
- Directionality of angle of impact
- Dropping distance
- Angle of impact
- Impact of bloodstain spatter pattern
- Classification of the Impact pattern
- Origin of the impact pattern
- Types of spatter
Surface Texture And Bloodstain Patterns
Surface texture is of paramount importance in the interpretation of bloodstain patterns; comparisons between standards and unknowns are valid only when identical surfaces are used.
- Spatter on the hard, smooth, and less porous surface results in; smooth surfaces like glass, smooth tiles, etc.
- Rough surfaces result in irregularly shaped stains with serrated edges; surfaces like hardwood, carpet, cloth, paper, etc.
Note 1: Satellite spatters are small drops of blood that are distributed around the perimeter of a drop or drops of blood and were produced as a result of the blood impacting the target surface.
Directionality of Angle of Impact
The direction of blood striking an object is determined using the pattern it makes. The distorted or disrupted edge of an elongated stain indicates the direction of travel of the blood drop.
In general, satellite spatter around parent stains will have the pointed end facing against the direction of travel. As the stain becomes more elliptical in shape, its direction becomes more detectable because the pointed end of a bloodstain faces its direction of travel.
Angle of Impact of Bloodstain Patterns
The impact angle of blood on a flat surface can be determined by measuring the degree of circular distortion.
The formula is given by,
Sin A= (Width of bloodstain)/ Length of bloodstain
where A= the angle of impact.
- Length always should be longer than the width and the measurement should be done with a ruler, micrometer, or photographic loupe.
- If the angle of impact is 90˚, the resulting bloodstain generally will be circular in shape and less than 90˚ will form elliptical bloodstain.
For example, the width of a stain is 1.75cm and the length is 3cm,
- Sin A= 1.75cm/ 3cm = 0.5833.
- Sin ̵ 1 0.583 = 35.7˚
In general, bloodstain diameter increases as height increases.
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Impact Bloodstain Spatter Pattern
Spattered blood is defined as a random distribution of bloodstain that varies in size that may be produced by a variety of mechanisms.
- Forward Spatter: Spatter projected outward and away from the source, such as an exit wound, is called forward spatter.
- Back Spatter, sometimes called blowback spatter, is blood projected backward from a source, such as an entrance wound, and potentially deposited on the object or person who created the impact.
Moreover, as the force of impact on the source of blood increases, the velocity of blood droplets leaving the source of blood also increases. And the resultant blood drop size decreases.
On the basis of the velocity of the force impacting a bloody object spatter is classified as follows:
1. Low-Velocity Spatter
These are large separate or compounded drops caused by an impact of fewer than five feet per second; blood cast off from a fist, shoe, weapon, dripping, or splashing.
This kind of spatter is normally produced by gravity alone, by a minimal force. The resulting blood spots are mostly 4 to 8 mm in diameter.
2. Medium-Velocity Spatter
Caused by an impact of 5 to 25 feet per second; blows with a baseball bat, hammer, axe, or similar instrument.
These are small drops with diameters of 1 to 4 millimeters.
3. High-Velocity Spatter
Caused by an impact of 100 feet per second; almost always gunshot, but occasionally produced by contact with high-velocity machinery.
The smallest of all, these blood spots are typically less than 1 mm in diameter, producing a fine mist-like spray, much like an aerosol.
Origin and Analysis of Impact Bloodstain Patterns
A. Area of Convergence
Area of convergence is obtained by:
- The 2-Dimensional plane from which the drops originated.
- Established by drawing straight lines through the long axis of several individual bloodstains.
- The point where these lines meet is the point of convergence.
Note 2: An object hitting a source of blood numerous times will never produce exactly the same pattern each time.
B. Area of Origin
To depict the area of origin,
- 3-Dimensional space from which the blood was projected.
- Shows the position of the victim or suspect in space when the stain-producing event took place.
- Must use the area of convergence and angle of impact for each bloodstain.
Types of Bloodstain Spatter
A. Gunshot Spatter
- It may produce minute spatters of blood less than 0.1mm in diameter referred to as mist-like dispersions (characteristic of high-velocity spatter).
- May be characterized by both forward spatters from an exit wound and back spatters from an entrance wound.
- The presence of the back spatter on a firearm or a shooter is dependent upon the distance between the firearm and the victim.
- This mist pattern is not observed in spatter associated with stabbing, or beating.
- The size of the spatter depends upon a number of factors like the caliber of the weapon, the quantity of available blood, the location and the number of shots, and impeding factors, such as hair, clothing, etc.
- Some back spatter may strike the gunman and enter the gun muzzle; this is called the drawback effect (distance-dependent phenomenon).
B. Cast-off Spatter
- This is created when blood is flung from a blood-bearing object.
- Size of the spatter is directly proportional to the size of the point of the object; for example- the spatter produced by a bat will be larger in size than the spatter produced by a sharp knife.
- We can also determine the number of blows based on the number of cast-off spatters.
Example: Three casts off are found at a crime scene, the first blow will produce no cast off; the swing back will produce the first cast off then the second blow will produce the second cast off then the swing back will produce the third cast off.
- The pattern may also suggest whether the blow that caused the pattern was directed from right to left or left to right.
C. Arterial Spray Spatter (Projected Pattern)
- When an artery is breached, blood is projected from it in varying amounts.
- The pressure of the continuous pumping of blood causes blood to spurt out of the injured area.
- It is also called projected pattern blood spatter.
- This occurs mainly due to injury to a main artery or the heart.
- The size of arterial bloodstain varies from very large gushing or spurting patterns each time the heart pumps to very small spray types of patterns.
- Vertical arcs or wave inline show fluctuation in blood pressure, and the lineup of the stains shows the victim’s movement.
- Arterial patterns can also be differentiated because the oxygenated blood spurting from the artery tends to be a brighter red color than blood expelled from impact wounds.
D. Expirated blood Pattern
- Pattern created by the blood that is expelled from the mouth or nose from an internal injury is called an expirated blood pattern.
- The dimension of this spatter is somewhat similar to impact spatter that is associated with gunshots and beating, due to this resemblance case history will be the deciding factor.
- Expirated blood also may be lighter in color than impact spatters as a result of being diluted by saliva.
- If the blood has been recently expelled there may be visible air bubbles within the stains due to blood being mixed with air from the airway passages or lungs.
E. Void Patterns
- Void areas or patterns are absences of bloodstains in otherwise continuous patterns of staining.
- The blank space on the surface or object may give a clue to the size and shape of the missing object or person.
- This helps in establishing sequence and identifying alteration within a crime scene.
Types of Bloodstain Patterns
Not all bloodstains at a crime scene appear as spatter patterns. The circumstances of the crime often create other types of stains that can be useful to investigators.
A. Transfer/Contact Pattern
- When an object wet with blood comes into contact with another object or secondary surface, a blood transfer pattern is observed.
- Some examples of transfer patterns are tool prints, bloody fingerprints, footprints, palm prints, etc.
- Class or individual characteristics may be determined from distinct blood transfer patterns.
- These patterns help in determining the directionality of the victim, perpetrator, or any person present at the crime scene.
For example: If we find some blood-laden footprint on the crime scene, the first print produced will always be heavy or dark with blood; followed by some lighter prints. These prints will also indicate whether the person was walking or running as the running footprints have a large space gap between them
B. Flow Pattern
- Patterns made by drops or large amounts of blood flowing with the pull of gravity are called flows.
- Flows may be formed by single drops or large volumes of blood coming from an actively bleeding wound or blood deposited on a surface—from an arterial spurt.
- For example: The interruption of a flow pattern may be helpful in assessing the sequence and passage of time between the flow and its interruption.
- If a flow found on an object or body does not appear to be consistent with the direction of gravity, one may surmise that the object or body was moved after the blood had dried.
C. Swipe and Wipe Pattern
- Swipe pattern is formed by the movement of a bloody object across other objects containing no blood.
- For example: the blood is already present on the victim’s hand and she hit the wall or some surface it will lead to a swipe pattern
Note 3: the object with blood should be moving to form this pattern or else it will be known as a transfer pattern.
- Wipe pattern is formed when the blood on the floor or wall gets smeared with the object containing no blood.
D. Drip Patterns
- When multiple free-falling drops of blood are produced from a stationary source onto a horizontal surface, it is known as a drip pattern.
- It results from blood drops falling into previously deposited wet blood stains or small pools of blood.
- Drip patterns will be large and irregular in shape, with small satellite spatters around the periphery of the central parent stain on the horizontal and nearby vertical surfaces.
- A pool of blood occurs when blood collects in a level (not sloped) and undisturbed place.
- Blood pools on the absorbent surfaces may be absorbed throughout the surface and diffuse, creating a pattern larger than the original pool. This often occurs in pools on beds or sofas.
Note 4: Skeletonization: The process by which the edges of a stain dry to the surface in a specific period of time (dependent on environmental and surface conditions); skeletonization remains apparent even after the rest of the bloodstain has been disturbed from its original position. This may be important for classifying the source of the original stain.
- Evans, Colin, 1948- Criminal Investigations: Crime Scene Investigation, 1st ed. InfoBase Publishing. Copyright, 2009 [Link]
- James, S.H. & Nordy J.J, Eds. Forensic Science: An Introduction to Scientific and Investigative Techniques, 2nd ed. USA, FL: CRC Press, 2005.[Link]
- Saferstein R., Criminalistics: An Introduction to Forensic Science, 12th ed. Boston, Pearson Education, 2018.
- Siegel Jay A. & Kathy Mirakovits, Forensic Science the Basics, 3rd ed. Boca Raton, CRC Press, 2016.
- Eckert William G., Introduction to Forensic Science, 2nd ed. New York, Elsevier Science Publishing Co., Inc, 1992.
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