Saturday, July 14, 2012

The Science Behind Blood Splatters

Are you a Dexter fan, the Showtime series about a serial killer who is also a blood spatter expert,   if so you might enjoy this report on the science of blood spatter by the University of California Davis.
With the guidance of chemical engineer William Ristenpart, UC Davis forensic science graduate students research bloodstain patterns to precisely determine whether and how a crime may have occurred.


Bloodstain pattern analysis (BPA) is one of several specialties in the field of forensic science. The use of bloodstains as evidence is not new; however, the application of modern science has brought it to a higher level. New technologies, especially advances in DNA analysis, are available for detectives and criminologists to use in solving crimes and apprehending offenders.

The science of bloodstain pattern analysis applies scientific knowledge from other fields to solve practical problems. Bloodstain pattern analysis draws on the scientific disciplines of biology, chemistry, mathematics and physics. If an analyst follows a scientific process, this applied science can produce strong, solid evidence, making it an effective tool for investigators.

Blood is a tissue that is circulated within the body to assist other parts of the body. This connective tissue has specialized cells that allow it to carry out its complex functions. For a healthy person, approximately 8% of their total weight is blood. For a 70 kg (154 lb.) individual, this equates to 5.6 L (12 US pints).

Biological considerations

Blood contains three components suspended within plasma. The three components are erythrocytes, leukocytes, and platelets.

Erythrocytes, also known as red blood cells, are transporters. The role of erythrocytes is to transport oxygen. To do this it produces great quantities of hemoglobin, which gives it the distinct red colour. Blood that has passed through the heart and been oxygenated (in the arteries) tends to have a brighter shade of red as opposed to blood that is returning to the heart (in the veins). There are about 30 trillion erythrocytes circulating in the human blood at any given time.

Leukocytes, also known as white blood cells, are defenders. The role of leukocytes is to defend the body against harmful bacteria and microorganisms. There are five different types of leukocytes all having different sizes, shapes, structures, and functions. Leukocytes fight infection and disease. There are about 430 billion leukocytes circulating in the human blood at any given time (~1 per 700 erythrocytes).

Platelets are pieces of larger cells that have broken off in the bone marrow. These bits of cytoplasm are enclosed by a membrane and do not have a nucleus. They play a major role in haemostasis (control of bleeding) by plugging up a breach in a vessel.

Plasma is a yellowish fluid that carries the suspended erythrocytes, leukocytes, and platelets. It is composed of water (92%), proteins (7%), and other materials such as salts, waste, and hormones, among others. Plasma makes up about 55% of blood. The remaining 45% is blood cells and platelets. Because plasma is lighter than the blood cells and platelets, it can be easily separated. Plasma does not separate from blood cells in the body because it is in a constant state of agitation.

Physical considerations

In physics there are two continuous physical states of matter, solid and fluid. Once blood has left the body it behaves as a fluid and all physical laws apply.

Gravity acts on blood (without the body's influence) as soon as it exits the body. Given the right circumstances blood can act according to ballistic theory.

Viscosity is the amount of internal friction in the fluid. It describes the resistance of a liquid to flow.

Surface tension is the force that maintains the shape of a drop of liquid, such as blood. When two fluids are in contact with each other (blood and air) there are forces attracting all molecules to each other.

Blood spatter flight characteristics

Experiments with blood have shown that a drop of blood tends to form into a sphere in flight rather than the artistic teardrop shape. This is what one would expect of a fluid in freefall. The formation of the sphere is a result of surface tension that binds the molecules together.


Contacts and sources:
University of California Davis
Wikipedia
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