Where did the murder take place? How many blows were there with the weapon? How fast was the weapon moving? What does the pattern of blood on the suspects clothing actually show that he did? Analysing blood patterns answers all these questions.
Blood is made up of 55% plasma and 45% formed elements – blood cells and platelets. Plasma, a pale yellow liquid, contains dissolved proteins along with other chemical substances including hormones, antibodies, enzymes and glucose. There are three types of cells in the plasma – red cells, white cells and platelets. Red cells carry oxygen from the lungs to cells in the body and carbon dioxide back to the lungs. White cells protect the body against pathogens – bacteria, viruses and fungi. Platelets are small cell fragments that help blood clot.
The important properties of blood that affect blood patterns are firstly surface tension – a force on the molecules at the surface of a liquid which pulls them back towards the body of the liquid; and secondly viscosity – the ease with which a liquid flows. Blood has a significant viscosity about five times that of water. This is due to the fact there are attractive forces between blood components.
Forensic scientists can derive a lot of information from the size, shape and position of blood deposits. Force is required to break blood up into small droplets – the stronger the force, the smaller the drops – and so this is important in cases of assault. An area of concentrated very small or tiny blood spots indicates blood that has been spattered by a large force such as that produced by a fast-moving weapon.
Blood falling vertically onto a horizontal surface produces circular spots although the texture and wettability of the surface can modify the shape. The spots are oval in shape when the blood falls at an angle, the shape of the oval being characteristic of the angle. At low impact angles, a ‘wave cast off’ can occur – an elongated streak of blood in a ‘ten-pin’ pattern pointing in the direction of movement which is very useful when interpreting patterns.
The point of origin of blood deposits can sometimes be determined by constructing convergence lines along the direction of movement and angle of fall of individual deposits and finding out where the lines meet.
Blood thrown off a moving weapon (‘cast-off’ blood) can produce deposits having characteristic shapes. The blood travels tangentially to the swing of the weapon and often produces lines of individual deposits showing progressively changing impact angles. Smeared blood resulting from contact between two items is very different in appearance and can often indicate a direction of movement.
In our events, students are shown how to work out the positions of the parties to an assault, the angle of impact of blood spots, the approximate speed of a weapon, the minimum number of blows in an assault and the significance of blood smears.