Changes that occur after death
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Changes that occur after death
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[ID] => 560153
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[post_date] => 2025-01-14 05:15:29
[post_date_gmt] => 2025-01-14 10:15:29
[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
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 (C3H5O3-) is the predominant form of lactic acid (HC3H5O3, Ka=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.
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[question] => Which of the following expressions can be used to find the pH of a 0.15 M aqueous solution of sodium lactate at 25°C?
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[description] => Reason for the Correct Answer:
Sodium lactate (NaC3H5O3) would dissociate and react with water as a base according to the following equation:
To find the pH, we use this expression to find the [OH–], which means we’ll need to find the Kb, the [HC3H5O3], and the [OH–]. From the equation above, we know that [HC3H5O3] will always equal [OH–], so we’ll set those both equal to x:
NOTE: When we find x, we will have found [OH–].
The passage does not provide us with Kb, but we can find it using Ka and Kw. Since Kw=Ka x Kb, then Kb = Kw/Ka:
When you plug-in:
Now you can solve for x, which is really [OH–], and take the negative log of it to find the pOH:
Now subtract from 14 to find the pH (14=pH+pOH): pH = 14 – pOH
And substitute in the value of pOH from above:
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[answers] => Array
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[each_answer] => A.
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[1] => Array
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[each_answer] => B.
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[2] => Array
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[each_answer] => C.
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[3] => Array
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[each_answer] => D.
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => How does in vivo [H+] in rat blood change during the first 96 hours post-mortem?
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[answer] => 2
[description] => Reason for the Correct Answer:
Remember that pH = -log[H+]. The lower the pH, the higher the [H+], and vice versa.
In the first 96 hours post-mortem, the pH of rat blood falls from ~7.3 to ~5.3. This means pH decreases by about 2 units.
pH is measured on a logarithmic scale, so every unit indicates a 10-fold change. Therefore, if pH decreases by 2 units, [H+] must increase 100-fold (10×10).
)
[answers] => Array
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[each_answer] => A.Increases by approximately 2-fold
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[each_answer] => B.Increases by approximately 100-fold
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[2] => Array
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[each_answer] => C.Decreases by approximately 2-fold
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[each_answer] => D.Decreases by approximately 100-fold
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[quiz_unique_key] => 83407773
[question] => What characteristic of post-mortem lactate levels presents the greatest difficulty in using lactate as an indicator of post-mortem interval?
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[answer] => 1
[description] => Reason for the Correct Answer:
All of the answer choices are true statements. You are looking for the reason that prevents lactate from determining post-mortem interval.
Imagine taking a lactate level from a rat corpse and finding that it is ~9mmol/L. This lactate level is consistent with multiple post-mortem intervals (9, 24, 48, 72, and 96 hours), making it impossible to determine how much time has passed since death.
In other words, lactate levels do not change in a regular, predictable fashion. This is the greatest difficulty in using lactate as an indicator of post-mortem interval.
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[answers] => Array
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[0] => Array
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[each_answer] => A.They do not change in a regular, predictable fashion.
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[1] => Array
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[each_answer] => B.They do not statistically correlate with pH levels.
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[2] => Array
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[each_answer] => C.They increase more than 5-fold in the first 96 hours post-mortem.
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[3] => Array
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[each_answer] => D.They differ significantly between pig and rat corpses.
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[quiz_unique_key] => 2377279144
[question] => How will the equilibrium ratio of lactate to lactic acid ([C3H5O3–]/[HC3H5O3]) be affected by post-mortem pH changes?
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[answer] => 1
[description] => Reason for the Correct Answer:
From Experiment 1, we know that blood pH drops after death. This means that [H+] increases.
Lactate is in equilibrium with its conjugate acid, lactic acid. The equation for this is HC3H5O3 ↔ C3H5O3– + H+.
Increasing [H+] would shift equilibrium to the left. This would increase [HC3H5O3], and decrease [C3H5O3–].
This would increase the denominator but decrease the numerator of the ratio ([C3H5O3–]/[HC3H5O3]), causing the overall ratio to decrease.
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[each_answer] => A.It will decrease.
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[each_answer] => B.It will decrease.
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[2] => Array
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[each_answer] => C.It cannot be predicted.
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[3] => Array
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[each_answer] => D.It will remain unchanged.
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[4] => Array
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[quiz_unique_key] => 2261298308
[question] => Why is the procedure described in Figure 2 a more appropriate choice for a lactate assay than an acid-base titration in this experimental context?
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[answer] => 2
[description] => Reason for the Correct Answer:
If acid-base titration were to be used to assess levels of lactate, we would add a strong acid to titrate the lactate anion, since lactate is present at much higher levels than lactic acid.
A strong acid will react with anything that can accept a proton, and there are many such species in blood. The passage specifically mentions formate and urate, which you know are anions that can accept protons because of the ending -ate.
In contrast to a titration, the procedure described in Experiment 2 utilizes the enzyme lactate dehydrogenase (LDH) and is highly specific for lactate. Enzymes are highly specific; LDH won’t react with anything else in the blood.
Therefore, the reason why the procedure in Experiment 2 is more appropriate than a simple acid-base titration is because “blood contains many basic anions in addition to lactate.” An acid-base titration is not specific enough.
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[each_answer] => A.Lactic acid is present in smaller amounts than lactate at normal blood pH.
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[each_answer] => B.Blood contains many basic anions in addition to lactate.
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[each_answer] => C.Blood contains many basic anions in addition to lactate.
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[each_answer] => D.The lactate anion is a carboxylate anion stabilized by resonance.
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