A 14-year-old girl with cystic fibrosis has complained of an increased cough productive of green sputum over the last week. She also complained of being increasingly short of breath, and she is noticeably wheezing on physical examination. Arterial blood was drawn and sampled, revealing the following values:
pH 7.30
pCO2 50 mm Hg
pO2 55 mm Hg
Hemoglobin - O2 saturation 45 %
[HCO3-] 24 meq/liter

1. How would you classify this girl's acid-base status?
2. How does cystic fibrosis cause this acid-base imbalance?
3. How would the kidneys try to compensate for the girl's acid-base imbalance?

1. This girl is in respiratory acidosis, as is evident from her decreased arterial blood pH, elevated arterial
blood pCO2, and normal arterial HCO3- concentration.
2. The pooling of excessively thick mucus obstructs the small and large airways. This reduces the
patient's minute ventilation, causing hypoventilation. As she hypoventilates, the pCO2 level rises and
the excess CO2 reacts with H2O in the bloodstream to produce carbonic acid (H2CO3), which
dissociates into H+ ions and HCO3- ions
3.the kidneys would compensate for this primary respiratory acidosis by increasing the rate of tubular secretion of hydrogen ions into the renal tubules. Incidentally, the fact that this patient's blood HCO3- levels are still normal means that the kidneys have not yet begun to compensate for her respiratory acidosis