Chapter 2: Crime Scene Bloodstain Pattern Analysis

# 2.1: Basic Biological Properties of Human Blood

* **Blood** is a bodily fluid circulating within the body.
* An average adult has a blood volume of approximately 8% of his or her body weight.
* Blood consists of a cellular portion as well as a liquid portion known as **plasma**.
* **Cellular Portion** consists of blood cells and platelets.
* **Plasma** is mostly composed of water and other substances such as proteins, inorganic salts, and other organic substances.
* Blood can form clots (or thrombi) that are the result of **blood coagulation**.
* Coagulation begins after an injury occurs, stopping blood loss from a damaged vessel.
* The normal coagulation time for 1 mL of venous blood in a glass tube is 5–15 min.
* The coagulation time can be affected by many factors such as blood volume and mechanical disturbance.


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# 2.2: Formation of Bloodstains

* Blood is viscous, and blood viscosity is a measure of the blood’s resistance to flow.
* The viscosity of blood is approximately five times greater than that of water.
* During the formation of a drop of blood, blood leaks out from a blood source.
* The surface tension of the blood causes it to hang from the opening of a blood source and to form a pendant drop of blood.
* The molecules of a blood drop are held together by the cohesion force to maintain the shape of a blood drop.
* Surface tension causes liquids to minimize their surface, making the formed blood drop spherical.
* As the volume of the drop gradually increases and exceeds a certain size, it detaches itself and falls. The falling drop is also held together by surface tension.
* A falling blood drop is influenced by the downward force of gravity acting on the drop and the air resistance that acts in the opposite direction as the drop is in motion.
* When a bloodstain lands on a surface, the shape and size of the bloodstain is affected by the texture of the target surface.
* Bloodstains that land on porous or rough surfaces usually have more distortion around the edges of the stains than those that land on smooth surfaces.


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# 2.3: Chemical Enhancement and Documentation of Bloodstain Evidence

* Many chemical reagents react with blood to exhibit a color, a chemiluminescent light, or a fluorescent light. These tests are extremely sensitive and thus are used as chemical enhancement reagents for detecting bloodstains.
* For bloodstain pattern analysis, the enhancement reagent is primarily used for detecting latent bloodstains such as diluted bloodstains that are visible on enhancement.
* **Luminol:** Used for locating bloodstains at the scene.
* Other reagents such as phenolphthalein, leucomalachite green, and tetramethylbenzidine are not often used as enhancement reagents but rather as *presumptive tests for blood*.
* In bloodstain pattern analysis, special attention must obviously be given to bloodstains.
* The photographic documentation of bloodstains may be performed by multiple means, including film and digital photography as well as videotaping.
* Photographs should be taken with an overall view followed by a medium-range and a close-up view of the bloodstain patterns.
* A scale of measurement must be included in the photograph, which is critical for bloodstain analysis.
* To avoid any distortion, the photographs should be taken with the camera lens parallel to the target surface where the bloodstains are located.
* An overall photograph provides an overall view of the scene including the bloodstain evidence.
* A midrange photograph provides more details of the bloodstain pattern compared with that of the overall photograph.
* Single bloodstains should be visible in midrange images.
* A close-up photograph, usually taken with a macro lens, provides a detailed image of single bloodstains, which is useful for spatter pattern analysis.


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# 2.4: Analyzing Spatter Stains

* **Spatter Stain:** A bloodstain resulting from a blood drop dispersed through the air due to an external force applied to a source of liquid blood.
* The patterns of spatter stains, including the shape and the size of the stains, are affected by the direction and the angle of impact of the spatter stains that are projected.

## Velocity of Blood Droplets

* **Low Velocity Impact Spatter**:
* Formed when a blood droplet is traveling at