Blood drains from capillaries in the upper surfaces and collects in the blood vessels in the lower surfaces. Upper surfaces of the body become pale and the lower surfaces become dark. Cells cease aerobic respiration, and are unable to generate the energy molecules needed to maintain normal muscle biochemistry. Calcium ions leak into muscle cells preventing muscle relaxation. Muscles stiffen and remain stiff (rigor mortis) until they begin to decompose. Cells eventually die and the body loses its capacity to fight off bacteria. The cells' own enzymes and bacterial activity cause the body to decompose - muscles lose their stiffness. TIMING Brain cells can die if deprived of oxygen for more than three minutes. Muscle cells live on for several hours. Bone and skin cells can stay alive for several days. It takes around 12 hours for a human body to be cool to the touch and 24 hours to cool to the core. Rigor mortis commences after three hours and lasts until 36 hours after death. Forensic scientists use clues such as these for estimating the time of death