Cerebrospinal fluid is found in the brain and spinal cord. It is a clear fluid that provides a barrier to absorb shock and prevent injury to the brain. It is useful for diagnosing neuro-degenerative diseases such as Alzheimers. There are various substances in the cerebrospinal fluid that can be measured including urea, glucose, potassium, chloride, sodium, protein, creatinine, calcium, alkaline phosphatase, and cortisol. Different things can be learned about the person or how the died by looking at the concentrations of some of these substances. For example, high levels of urea can indicate kidney damage. High levels of cortisol, the hormone released under stress, could indicate a violent death. Creatinine is stable post-mortem, so the concentration at death is preserved. This is also helpful to determine the kidney function of an individual. Sodium and Potassium can also be measured in the cerebrospinal fluid to predict the time since death, but it is not as accurate as it would be if the vitreous humor was used, since it has a lower correlation.