Blood is one of the most significant and frequently encountered types of physical evidence associated with death and violent crime investigations. Bloodstain evidence has two primary categories of examination important to crime scene investigators: genetic marker typing and bloodstain pattern interpretation
There are four primary parts of blood.
1. Blood consists of 55 percent plasma. This is also known as the fluid portion.
2. Erythrocytes are the red blood cells.
3. Leukocytes are the white blood cells.
4. Platelets aid in clotting when an open wound occurs.
An adult has 5 to 6 liters of blood. Blood loss of 1.5 liters will incapacitate a person. Blood loss of 40% is lethal.
Bloodstain pattern analysis is the examination of the shapes, location and distribution patterns of bloodstains in order to provide an interpretation of the events that gave use to their origin. Bloodstain pattern analysis can determine the origin of the bloodstains and the type and direction of impact that produced the bloodstains.
Different objects produce certain bloodstain patterns. The number of blows, shots, etc. and the position of the victim during the bloodshed can influence bloodstain patterns.
The analysis of bloodstains can support or contradict a statement made by witnesses or suspects. It can also assist in determining the cause of death homicide, suicide, accidental.
Age and gender do not affect the bloodstain patter nor does alcohol content of the blood or atmosphere conditions. It is also important to note that every kind of blood, animal and human alike, will distribute itself in the same manner.
There are three primary impact levels of blood splatter.
1. Low velocity, which is caused by the force of gravity. This is free falling, dripping blood.
2. A force between 5 and 25 feet per second causes medium velocity blood splatter. This is most often observed in beatings or stabbings.
3. High velocity blood spatter occurs when the force is greater than 100 feet per second. This kind of blood spatter is often seen in shootings, explosions and power tool accidents.
One simple rule one can learn about blood splatter is that smaller blood marks are the result of greater force.
Blood striking a surface at an angle produces a tear-shaped pattern. The greater the angle of impact, the more elongated the tear-shape will be. Teardrop stains always point in the direction of travel.
By drawing a line through the long axis of a group of bloodstains, the point of convergence can be determined. The bloodstains will not point to one exact point, however. Rather, they will point to an area on the ground roughly the size of a basketball.
The point of origin is the height at which the impact occurred. This is a tedious process of measuring the width and length of each bloodstain and then using a simple equation to determine the point of origin.