Blood cells

Plasma can be separated from the blood by centrifuge, a device which rapidly spins a test tube of blood causing cells to collect at the bottom and the plasma to form on top. Another part of the blood is called serum. This is the yellowish liquid remaining after the blood has clotted. The serum contains nearly all the proteins and enzymes but none of the cells or clotting factors as these are consumed when the blood clots.

Clotting Clotting is a helpful key to crime scene investigations as it can from an estimate guide to how much time has passed since the blood has left the body. Blood will start to clot between 3-15 minutes after it has left the body, but this can be affected by haemophilia, leukaemia and various medications. When the blood starts to clot it turns into a dark, shiny, jelly-like mass and after sometime it will start to contract and separate from the serum. If the blood is found in liquid form then the bleeding only happened a few minutes before, if it is a shiny, gelatinous pool, then bleeding occurred less than an hour ago and if the blood is separated into a clot and serum than several hours have passed.

Blood grouping Part of blood analysis is to determine what the blood type is of any stain and this is done by mainly looking at the RBC’s. These contain antigens which are designated as either A or B. People who have A antigens in their RBC’s have type A blood and the same with people who have B antigens have the blood type B. People who have A and B antigens have the blood type AB and those who have neither A or B antigens have the blood type O.

Another antigen in the blood is called the Rhesus and Rh factor. If the RBC contains the Rh factor then the blood is known as positive, or negative if doesn’t. So therefore if the blood type is A positive then it contains the A and Rh antigen and same with the blood type B positive. If the blood type is negative then it only contains the A or B antigens and people with O negative have neither A or B or the Rh antigens.

Why blood is important at crime scenes? Blood is the most common bodily fluid at crime scenes and is the most useful due to the way it moves and clots. Blood stains left at an accident, suicide or crime scene can help determine what happened or if a crime was even committed. Other than revealing blood types, blood can also reveal the presence of diseases, drugs or alcohol and is used to determine the identification of the victim and suspect through DNA analysis. The shape and location of the blood stains also provide clues about where the victim and suspect where when the crime took place and where they went afterwards.

Blood spatter A blood spatter is a group of blood stains resulting from one or more injury. Spatters are produced in several different ways such as stabbing, gunshots, beatings, arterial bleeding, cast-off blood, and splashing. A drop of blood is formed due to a smaller amount breaking away from a larger blood source and due to surface tension it remains spherical until they strick or are struck by an object or surface. When a drops hits a surface it creates a circular pattern around the point of impact. The shape and size of the circular patter all depends of the size of the drop, the speed in which it fell, the angle it hits the surface and the type of surface it strikes.

A blood drop will pick up speed as it falls, this is know as terminal velocity (its maxium free-fall speed) and this, with the distance the drop falls from an inch to 7 feet, will produce the sahpe and size of the circular patterns diameter. The diameter of the spatter pattern can vary from 13mm to 22 mm. When a drop strikes a surface from 90 degrees the spatter will form an even circle at the point if impact. If the blood falls from an angle smaller than this, it creates a longer oval pattern with a narrow end aiming in the drops direction of travel.

Blood will behave differently on certain types of surfaces. Hard smooth surfaces such as glass with produce a smaller and neater spatter than rough irregular surfaces like concrete. Projected blood spatters Projected blood spatters occur when a force other than gravity is applied. The size, shape, and number of resulting stains will depend on the amount of force applied to the strike. Projected blood is classified in one of two ways, by velocity; Low Velocity – A bloodstain pattern that is caused by a low velocity impact or force to a blood source.

This impact results in a fairly large spatter. This includes arterial bleeding, which is blood loss in the form of spurts or gushes when an artery is damaged. Arterial bleeding patterns normally result in a cascading spatter. Cast-off blood is another low velocity blood source. This is blood that is ‘flung’ from an object in a centrifugal force, this usually happens when a seris of arching blows is delivered and the patterns are usually found on walls and ceilings. These spatter patterns can help with estimating the height and whether the assailant is left or right handed it can also indicate the minimum number of blows to the victim.

Medium Velocity -A bloodstain pattern caused by a medium velocity impact or force to a blood source. These type of spatters are normally smaller than those from low-velocity droplets and tend to come from impacts with blunt or sharp objects which distribuate blood in all directions from the source of impact. Again these can help determine the point of origin. Sometimes a fine mist spatter is produced and this is caused by any wounds made to the troat, face or lungs as the blood mixes with exhaled air. The fine spray may be found on or around the victim and the attacker.

High Velocity – A bloodstain pattern caused by a high velocity impact or force to a blood source. The spatter produced is a mist like stain. These are normally associtaed with gun shot wounds as bullets travel at a high velocity. These patterns tend to produce an exit or entrence wound. A blood spatter showing an entrance woud is called blowback or back spatter and this is when the blood travels in the opposite direction to the path of the object. A bloodstain found near the exit wound is called forward spatter in which the blood follows the path of the object.

Transfer patters are the results of when blood soaked objects come into contact with another object. These can include bloody shoe prints, hand prints and finger prints. A transfer patter normally happens when someone brushes against or kneels in a bloodstain, or wipes and cleans their hands or weapons resulting in the victims blood being transferred on the suspects clothing or onto floors and walls. Matching the victim’s blood to a transfer stain found on the suspects clothing may help in solving the crime.


Forensic for dummies. D.P. Lyle, MD. Wiley publishing, Inc. 2004

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