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.

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

                  CO2 + H2O « H2CO3 « HCO3- + H+

Question 1:

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.

Normal blood bicarbonate levels should be 22 – 26 mM.

Question 2:

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 3:

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 4:

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 5:

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 6:

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

Answer

1. The bicarbonate buffer system includes the regulation and balancing of bicarbonate ion, carbonic acid, and carbon dioxide. They help in order to maintain optimum pH in the blood, transport CO2 in the blood to liberate excess H+ ions and the CO2 reaches lungs for exhalation.

Therefore, all the options are correct. The correct answer is option (D).  

2.  Molly consumed a large number of aspirin tablets which leads to low bicarbonate levels in the blood resulting in metabolic acidosis. In this condition the body is unable to form a normal level of bicarbonate ions thus the blood bicarbonate levels are below normal.

Therefore, the correct answer is option (C).

3.  Molly is suffering from metabolic acidosis. The physician administered activated charcoal through a nasogastric tube and IV bicarbonate to raise the pH greater than 7.20. When aspirin is consumed in high doses, the HCO3 levels will cause a reduction in the pH so in order to maintain a stable pH, activated charcoal is given.

Therefore, the correct answer is an option (C).

4.  In case of metabolic and respiratory acidosis, the most common alkylating agent sodium bicarbonate is given as IV. After giving IV, the sodium bicarbonate will dissociate into bicarbonate ions which will acts as a conjugate base in order to restore the acid-base balance and maintain equilibrium.

Therefore, the correct answer is option (C).

5.  A drastic change in the pH levels will denture proteins altering the metabolic processes and leads to tissue damage. When the blood pH is lower than the normal range, the activity of the glycolytic enzyme phosphofructokinase will also decrease and will ultimately result in impaired glucose utilization by brain cells. When the pH level decreases, the H+ ion concentration and the CO2 level increases. This will affect the binding affinity of oxygen in hemoglobin and leads to decreases oxygen-carrying ability of hemoglobin.

Therefore, all the answers are correct. The correct option is (D).          

6. Apart from bicarbonate buffering system, the buffers which are capable of other buffering acid-base balances include Phosphate buffer system, hemoglobin, and deoxyhemoglobin. As none, these buffer systems are mentioned the answer is none of the above.

Therefore, the correct option is (F).

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