Analyzing Blood Splatter
• Blood spatterns can vary from a few drops on a bed to pools on the floor or sprays across the wall.
Properties of Blood Volume • On average, blood accounts for 8 % of total body weight • 5 to 6 liters of blood for males 4 to 5 liters of blood for females • A 40 percent blood volume loss, internally or/and externally, is required to produce irreversible shock (death). • A blood loss of 1. 5 liters, internally or externally, is required to cause incapacitation.
Properties of Blood • Why does a drop of blood have a curved surface when it lands on a flat surface instead of spreading out flat? • Why doesn’t it separate in the air before it hits the ground? Gravity, cohesion, adhesion, surface tension
Properties of Blood • Gravity: pulls it to ground – Droplet becomes longer than wide • Cohesion: blood mixture is attracted to similar blood mixtures and sticks together, not separates, as it falls – Causes droplet to stay together • Adhesion: attractive forces of two different objects – starts as teardrop because of adhesion • Surface Tension: the elastic like property of the surface of the liquid that makes it tend to contract, caused by the forces of attraction between the molecules of the liquid. – Formation of a sphere
Density is defined as mass per unit volume. The density of water is 1000 kg/m 3. The density of blood is proportional to the total protein concentration or cellular component of blood and is influenced only to a minor extent by other ions, gases etc. that are dissolved in the plasma. The density of blood plasma is approximately 1025 kg/m 3 and the density of blood cells circulating in the blood is approximately 11 25 kg/m 3. The average density of whole blood for a human is about 1060 kg/m 3.
Surface tension is important in bloodstain pattern analysis • the gravitational force must overcome • the surface tension of blood before • a drop of blood can fall, and • drops of blood remain intact as they move • through the air due to surface tension.
Blood Droplets • Flat surface – edge of blood drop appears smooth and circular – Glass, marble • Porous surface – edge of drop of blood may form small spikes (extensions) or satellites – Spikes – attached to make droplet – Satellites – not attached to main droplet
Six Patterns Blood can be classified: 1. Passive Fall - Blood falling directly to floor at 90 -degree angle will produce circular drops, with secondary satellites being more produced if surface hits is textured
• When blood is present at a crime scene, investigators will examine it and ultimately confirm that it is human blood by testing it in the laboratory. A test strip is used at the scene and a positive result will turn it green.
• In some cases, the offender will have cleaned up the crime scene and removed all visible traces of blood. This is commonly done by washing the area, using bleach or removing stained furnishings.
Luminol • Crime scene investigators may search the area using a highpowered light that reveals tiny specks of blood or they may use luminol. • Luminol is a reagent that reacts with the iron found in hemoglobin to cause luminescence. It is sprayed across the crime scene and will show up the finest traces of blood.
Basic facts about blood spatterns: • Medium velocity spatter is caused by fist fights, boots and baseball bats • Medium velocity droplets are caused by knives • High velocity spatter is caused by bullets When a blood spot falls straight down to the floor, it will be circular A blood spot falling at an angle will be more elongated like an exclamation mark
• When a blood spot falls straight down to the floor, it will be circular A blood spot falling at an angle will be more elongated like an exclamation mark The tail of the exclamation mark will always point in the direction of travel
• Arterial bleeding causes spurts of blood caused by the contractions of the heart. This can cause a curtain-like patterns on walls • Cast-off patterns are created when blood is flicked off a moving object such as a baseball bat
A skilled investigator can analyze a crime scene and enter facts and figures into a computer program to determine things such as the height of the killer and the type of bleeding. Here are some of the facts that can be revealed through careful investigation: • Whether the victim was sitting, lying or standing during the attack • The type of weapon used and the type of injury caused • Where in the room the victim was attacked and if he was moved afterwards • The track of the blood curve can reveal if the assailant was right or left-handed • The amount and pattern of blood can reveal if the attack was short and quick or long and frenzied
Whatever calculation is used the steps are the same • Identify stains that have a common area of convergence. • Draw lines through the central long axis of the stain away from the direction of travel. • Identify the area of convergence. • Measure the distance (cm) from the back edge of the stain to the area of convergence. • Calculate the angle of impact of each of the stains (measure width and length of stains in mm and apply the formula). • Use a minimum of 3 stains.
Angle of Impact The angle of impact can be calculated using the formula: angle of impact = arcsin (opposite side/hypotenuse) The investigator has to do the following to obtain the data: • 1. Measure the length and width of the splatter. • 2. Divide the width of the splatter by its length. • 3. Determine the arcsin of that number, typically using a calculator with an arcsin function.
Identifying Blood Trail Motion • Droplets dripping from a moving object or person do not • drop straight down. As they are in motion themselves, they • fall to the ground at an angle.
Direction of Travel
Point of Convergence • The point of convergence of all the blood spatters can be located by drawing an extended line and locate the point of intersection of all the lines. • A string can be pinned on each blood spot and raise it following its angle of impact. • A three-dimensional model can be created and the point of convergence can be shown
Area of Convergence
When multiple angles are analyzed
Once you have determined the area of convergence, you will measure the distance from the area of convergence to the edge of the drop of blood when it first impacted a surface. The distance is indicated by the dotted line.
Next determine the angle of impact for each droplet of blood. a. Select one of the blood droplets and determine the angle of impact for that drop of blood. b. To calculate the angle of impact, you will need to use the Law of Sines. c. Remember, when you measure the length of the blood droplet, do not include thin extension of the leading edge (tail).
Point of Origin
The base of each stain’s present position, the point in two dimensional space where the paths converge (c), and their point of origin (o), define another right angle
Using the Law of Tangents to solve for height. a. Going back to the right triangle and adding the angle of impact, we can determine the height from where the blood originated. b. The height of the source of blood is the side opposite the angle of impact. c. To solve for the height (or side opposite the angle of impact), we apply the Law of Tangents.
Tangent of angle of impact = opposite / adjacent = height / distance Height = tangent of angle distance
Computer Programs Used