Case Study Section 1 – pH Based on “The Case of the Mortified Mom: Acids, pH and

Question

Case Study Section 1 – pH

Based on “The Case of the Mortified Mom: Acids, pH and Buffers” by Terry Platt (National Center for Case Study Teaching in Science)

The Patient:

Paramedics were called to the home of the Mathews family because their 3-year old daughter, Molly, had gotten into the medicine cabinet and consumed a large number of aspirin tablets. When the paramedics arrived Molly had vomited several times, with bits of undissolved tablets visible, but seemed sleepy, almost lethargic. She was rushed to the nearest Emergency Room. When she reached the hospital she was unarousable and was breathing rapidly and deeply. She was examined and lab samples were obtained.

Above is the structure of aspirin, or acetylsalicylic acid, which is a weak acid with a pKa of 3.5. The active, and toxic at high doses, form is the protonated form.

Question 1:

What percent of the ingested aspirin is in the active form in the stomach (remember that gastric juice has a pH of ~1.5)?

A.              0.01%

B.              1%

C.              50%

D.             99%

Question 2:

What percent of ingested and absorbed aspirin is in the active form in the blood (average pH of 7.4) under normal conditions?

A.              0.00013%

B.              0.013%

C.              50%

D.             100%

Molly’s lab analyses came back and she had a blood pH of 6.8.

Question 3:

By what factor does the [H+] of Molly’s blood differ from normal blood?

A.              0.25X

B.              0.5X

C.              0.6X

D.             2.0X

E.              4.0X

Blood pH can be impacted by the carbonic acid/bicarbonate system, which utilizes the following formula:

                  CO2 + H2O « H2CO3 « HCO3- + H+

Question 4:

Under normal conditions the carbonic acid/bicarbonate system functions to:

A.              Maintain blood pH near physiological pH.

B.              Remove excessive H+ generated during metabolism.

C.              Transport CO2 from the tissues to the lungs for exhalation.

D.             All of the above.

Question 5:

Why is Molly breathing so rapidly and deeply when she arrives at the Emergency Room, despite being nearly comatose?

A.              The aspirin has inhibited her ability to use oxygen effectively.

B.              Her body is trying to rid itself of CO2.

C.              She is out of breath from all she has been through.

D.             Her hemoglobin can’t deliver oxygen at low pH.

Normal blood bicarbonate levels should be 22 – 26 mM.

Question 6:

Which of the following conditions would you expect to be the situation in Molly’s case?

A.              Blood bicarbonate above normal.

B.              Blood bicarbonate within the normal range.

C.              Blood bicarbonate below normal.

The physician intubated Molly’s trachea and carried out hyperventilation, which he explained to the family was “to avoid hypoventilation and a worsening of her metabolic acidosis.” They also administered activated charcoal through a nasogastric tube and IV bicarbonate.

Question 7:

What is the purpose of the activated charcoal?

A.              Absorb the residual aspirin to prevent it from entering her bloodstream.

B.              Increase the rate of absorption of the aspirin to shorten the duration of the overdose.

C.              Increase the pH of the stomach.

D.             Settle her stomach after the vomiting.

Question 8:

What effect would the IV bicarbonate have on the carbonic acid/bicarbonate reaction above?

A.              Shift it to the left.

B.              Shift it to the right.

C.              Settle it closer to equilibrium.

D.             No effect.

A few hours later Molly was breathing more regularly and her blood pH was returning to normal.

Question 9:

What would be the consequence(s) of lower than normal blood pH?

A.              Cause proteins to denature

B.              Decrease in enzyme activities

C.              Limit hemoglobin’s ability to transport oxygen

D.             All of the above

E.              None of the above

Question 10:

Bicarbonate was an effective buffer in Molly’s situation, what else could/does serve as buffers in the body?

A.              Free amino acids

B.              Phosphate compounds

C.              Proteins

D.             Urea

E.              All of the above

F.              None of the above

Explanation / Answer

Question 1:

What percent of the ingested aspirin is in the active form in the stomach (remember that gastric juice has a pH of ~1.5)?

The pH equation

HA <-> H+ + A-

pKa = 3.5

pH= pKa + log(A-/HA)

pH = 3.5 + log(A/HA)

1.5 = 3.5 + log(A/HA)

A-/HA = 10^-(1.5-3.49)

A/HA = 97.72

A = 97.72*HA

1 mol of A = 97.72 mol of HA

fraction of HA = mol of HA / total mol = 1/(97.72+1) = 0.01012965 = percet = 0.01012965*100% = 1.012%

Note that 99% is HA- or the conjguate base

1% is the molecular Aspirin

Therefore, choose 1 %

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