Bloodstain Pattern Analysis 101: Basic Properties of Blood

Bloodstained Knife

Blood is a fascinating liquid to study in relation to forensics.  About 55% of blood is plasma. Plasma is the “liquid” component made up of proteins, salts, acids, and water. Actually a little over 90% of plasma is water. The other 45% of blood is made up of the cellular material (often called the “formed elements”).  There are three types of blood cells; red, white, and platelets. The majority are red blood cells. Interestingly, red blood cells contain no nuclei and therefore have no value for DNA testing.  In fact, when you consider all properties of blood (plasma and cells) only 1% of that material contains DNA; and that material is randomly distributed throughout the blood volume.

Blood is about 4-5 times more viscous (thicker) than water.  Another interesting quality of blood is that it is somewhat adhesive.  You might be thinking “so what?” but there is a reason these characteristics are important to bloodstain analysts. Thick, sticky blood means that it will adhere to other objects. Those objects may transfer a blood pattern to another object. The more this happens, the more we have to analyze.  A suspect with blood on his/her hands may leave bloody finger or palm prints at the crime scene. Blood on their shirt may transfer to the seat of the getaway car. Blood on their shoes may seep into seams and remain there for months. Even if the blood is cleaned up, trace amounts may still exist.

Bloody transfer of ring on T-shirt

If you are writing a homicide scene, you may want to consider how any blood on your suspect might be transferred during or  after the crime.  If they transport a victim in their car will blood leak out onto the floorboards or into the trunk? If the victim is wrapped in something, what will the killer do with those bloodstained wrappings? In that disposal will any of the blood transfer to the suspect, vehicle, or other objects? As long as blood is somewhat “wet” it can transfer to other objects. Remember, even “invisible” trace amounts of blood can be valuable to the forensic analyst.