Postmortem lividity. Also known as livor mortis, this is caused by the pooling and settling of blood within the blood vessels from the effect of gravity. It appears as a purple discoloration of the skin. During life, the pumping action of the heart maintains a constant flow of blood through the numerous vessels of the body. Upon death, this pumping action ceases, and the blood pools within the dependent portions of the body. The location of livor mortis is determined by the position of the body after death. If the body is lying face down, livor will develop on the front of the body rather than on the back. The observation of lividity is important for two reasons:
It gives the investigator a general idea how long the body has been dead;
It tells you definitely whether or not the body was moved after death.
For example, if lividity is observed on the back of the body found lying face down, you can be sure that the body had originally been on its back. Lividity begins about 30 minutes after death with full development after 3 to 4 hours and becomes "fixed" in 8 to 10 hours. Fixed means that the livor has settled in one position for more than 8 hours and can no longer be significantly shifted by changing the position of the body. However, parts of the body that remain in direct contact with an object, such as the floor, a piece of pipe or wood, or even the weapon, will remain white, as the pressure will not allow the blood to settle into the dependent capillaries.