Acid/base chemistry: Changes that occur after death

Death results in extensive biochemical changes in all body tissues due to lack of circulating oxygen, altered enzymatic reactions, cellular degradation, and cessation of anabolic production of metabolites. These biochemical changes may provide chemical markers useful in accurately determining the time since death (post-mortem interval), the most sought after piece of information in a death investigation.

In a recent study, changes in blood pH and metabolite concentrations were examined in animal corpses post-mortem. Post-mortem changes in blood pH probably reflect accumulation of metabolites such as lactate, formate, and urate. Lactate (C${}_3$‍H${}_5$‍O${}_3$‍${}^{-}$‍) is the predominant form of lactic acid (HC${}_3$‍H${}_5$‍O${}_3$‍, K${}_a$‍=1.4x10${}^{-}$‍${}^4$‍) under normal physiological conditions. At a blood pH of 7.4, there are approximately 3,500 lactate molecules for every one molecule of lactic acid.

In the study, blood was taken from rat and pig corpses. Before death, the animals were sedated in order to obtain antemortem blood samples via cardiac puncture. Immediately post-mortem, a blood cannula was placed in the abdominal aorta for repeated sampling over the next 96 hours.

Figure 1: Post-mortem Changes in Blood pH. The results are shown as the average pH of blood samples from five rat and two pig corpses. Bars indicate the standard deviations of the samples at each time point. Ante indicates samples taken immediately before death. Blood pH was measured using a micro-electronic pH meter.

Figure 2: Post-mortem changes in lactate concentration. The results are shown as the average lactate concentrations from seven rat and two pig corpses. Bars indicate the standard deviations of the samples at each time point. Lactate concentrations were determined by enzymatic oxidation of lactate to pyruvate by LDH, coupled to the reduction of NAD${}^{+}$‍ to NADH. Spectrophotometric measurement of NADH was carried out at 340 nm, and values were compared against a standard curve to determine lactate concentration. Adapted from: Donaldson, A. E., & Lamont, I. L. (2013). Biochemistry Changes That Occur after Death: Potential Markers for Determining Post-Mortem Interval. PloS one, 8(11), e82011.