Blood is a substance with characteristics you probably don't think about. Tap a blood drop and it's sticky, walk through a blood stain and it's slippery. It is a non-Newtonian fluid and behaves much like ketchup. During a violent encounter, blood will stick to surfaces in patterns called blood spatter. The correct term is spatter, not splatter. These patterns can be read by someone who has been trained to do so, and they may discern the story of a violent crime as easily as you read this. Blood spatter occurs when blood is impacted by a violent attack. Droplets fly through the air and deposit on surfaces. The shape of a droplet changes depending on the type of surface it hits. Drops on glass will be very different from drops that land on wood. The angle of impact changes the shape of blood drops as do the velocity and distance traveled. Stains will either be round or elliptical depending on the angle of impact. If you measure the width and length of a droplet, you can determine the angle of impact. From this information, the location of the victim and perpetrator during the attack may be determined.
Blood spatter has been traditionally organized into three categories: low velocity, medium velocity, and high velocity. The first is usually blood drops resulting from the normal gravitational pull. The drops are large, measuring 4mm, and round. Medium velocity patterns are most commonly seen in blunt force trauma and stabbings. They range in size from 1-4mm and are elongated for the most part. High velocity is produced by gunshot or in some cases industrial machinery. It results in a fine mist of blood spatter and droplets are less than 1mm. Blowback patterns occur when a gun is fired into a victim. As the bullet enters the body, blood drops project back towards the shooter. The amount of the pattern is directly related to the size or caliber of the bullets, as well as the distance between perpetrator and victim. Forward spatter is blood that exists with the bullet as it leaves the victim.
Cast-off is another type of spatter pattern. It occurs when centrifugal force causes blood to leave a bloody weapon and land on a surface such as a wall or ceiling, usually in a straight line or curve. Picture a victim being struck on the head with a blunt object. The first contact draws no blood into the stain because the victim isn't bleeding yet. The next blow will carry copious amounts of blood, but each successive blow will draw less blood. This is because less blood can now stick to the weapon. Arterial blood stains happen when an artery is cut during a sharp weapon attack, such as stabbing. The artery spurts blood, because it is under pressure, in a recognizable pattern. It will be a brighter, redder color because the blood is oxygenated. The areas where this wounding takes place are the heart/aorta, the femoral artery of the leg, the radial artery of the wrist, the brachial artery of the arm, the carotid artery in the neck, and the temples on the head. The resulting blood pattern will have an up and down appearance, as the spurting follows the beating of the heart. Variation in the stain can be produced if the victim is moving during the episode and the extent of the wound. Small punctures leave smaller stains then large punctures.
These are just the very basic parts of bloodstain analysis. It takes a lot of training to learn the subtleties among the different stains. Because blood has both a liquid and a solid component, drying of blood can cause a list of crime scene information all of its own. There are transfer stains, swiping stains, and saturation stains all with their own interpretations which must be studied. Experience is the greatest teacher when it comes to blood spatter and its peculiar characteristics.