The human body is a complex system of fluid dynamics, and once life processes cease, these dynamics transition into physical and chemical changes. The common idea that “blood and water separate” after death is not a literal chemical split, but rather a simplified observation of profound post-mortem fluid movement. When circulation stops, the body’s various fluids—from the blood within vessels to the water held inside cells—begin to obey the laws of gravity and biochemistry, leading to a visible redistribution of both components. The scientific changes that occur in the blood and the body’s total water content are distinct processes, each starting immediately and progressing along a measurable timeline.
Immediate Changes to Blood Composition
Upon the cessation of heart function, the circulatory system loses the motive force that kept blood components suspended and moving. Blood is a suspension composed of plasma, which is roughly 92% water, and formed elements like red blood cells, white blood cells, and platelets. The immediate lack of oxygen causes metabolism to switch to anaerobic glycolysis, which leads to a buildup of lactic acid and a drop in blood pH.
This changing chemical environment is often accompanied by the activation of fibrinolysin, an enzyme that works to dissolve blood clots. This is why blood can remain remarkably fluid inside the vessels for up to six to eight hours post-mortem. While the main body of blood remains liquid, some coagulation does occur, forming soft, gelatinous post-mortem clots known as cruor, which are distinct from the firm clots formed in a living person.
How Gravity Separates Blood (Livor Mortis)
Livor Mortis, or post-mortem lividity, is the physical settling of blood components due to gravity. This phenomenon is a direct consequence of the lack of circulation. Without the heart actively pumping, the heavier, cellular elements of the blood—primarily the red blood cells containing hemoglobin—sink downward through the lighter, water-based plasma.
This gravitational pooling causes a purplish-red discoloration on the areas of the body closest to the ground, known as the dependent areas. Livor mortis typically begins as small patches within 30 minutes to two hours after death. It becomes fully developed, or “fixed,” between eight and twelve hours. Fixation occurs when the pooled blood in the capillaries loses its fluidity and begins to coagulate in place, preventing the discoloration from shifting if the body is moved. This process is a physical stratification of the blood’s components within the closed circulatory system.
Post-Mortem Fluid Movement and Dehydration
The body’s overall water content also begins to move and change state. A process called autolysis begins almost immediately, in which cells break down and release their internal contents, including intracellular fluid, due to the action of their own digestive enzymes. This release of fluids from within the cells and tissues contributes to the overall post-mortem fluid shifts.
While autolysis releases internal fluids, the body simultaneously begins to lose water externally through evaporation, a process called desiccation or dehydration. Evaporation occurs most rapidly from exposed surfaces, such as the eyes and skin, and is heavily influenced by environmental factors like temperature, humidity, and air currents.
In dry, arid conditions, this external water loss can outpace decomposition, leading to the preservation of tissues through mummification. The movement of this interstitial and cellular fluid is a distinct process from the gravitational settling of blood, demonstrating that the body’s water content is subject to both internal release and external loss after death.