breathe, stretch, shake

the reason we inhale is to acquire oxygen to fuel our cells, and the reason we exhale is to expel the carbon dioxide that our cells have byproduced. in particular, carbon dioxide (CO2) is a curious gas--found in trace amounts in the air, it's often maligned as a greenhouse gas but is otherwise fairly harmless. dissolved in water, however, CO2 becomes an acid, which is why plain seltzer water tastes mildly sour. this is partially why CO2 must constantly be pumped out of our cells and our blood, so that our bodies' systemic pH does not get so low as to become harmful to normal cellular function.

let's say that, for some unfortunate reason, your lung function has suddenly diminished, and you can't breathe as quickly or as strongly as you normally should. this is hypoventilation. as a result, carbon dioxide is backing up in your blood, and this is lowering your blood's pH (i.e. making it more acidic). how does your body compensate for low blood pH? it has two options: 1) eliminate the CO2 via the lungs, which it currently can't, or 2) neutralize the acid. option two is where your kidneys come in (and where a few of my comp exam questions are sure to attack).

in the event of respiratory acidosis--like the case just described--the kidneys provide a buffer to ameliorate the downward swing in pH: bicarbonate. bicarb is the same compound found in baking soda and antacids, and its action in all uses is similar--to neutralize acid. the kidneys know to produce bicarb because they can sense both the increase in dissolved arterial CO2 and the lowered pH of arterial blood (the kidneys also pump out and excrete acidic hydrogen ions). dissolved bicarbonate equilibrates with the hydrogen ions in solution freed by CO2, and normal pH is restored.

now, let's say that your lungs are working fine again, but for an unrelated yet still unfortunate reason, you decide to shoot vinegar into one of your veins. as this example shows, not all acidoses are caused by pulmonary insufficiency. lung function--and corresponding amount of dissolved CO2--may be perfectly normal when another type of acidosis sets in. this is termed metabolic acidosis, and the body's compensatory response in this situation is slightly different.

in metabolic acidosis, the kidneys still secrete extra bicarbonate to neutralize acidic blood. however, the amount of arterial CO2 in metabolic acidotic patients is below normal. this seems counterintuitive because raised--not lowered--arterial CO2 levels stimuate the kidney cells to excrete hydrogen ions, as described in the respiratory acidosis case. however, because the lungs are functional in this case, they attempt to correct the acidosis by hyperventilating (i know the causation here is muddled but just bear with me), resulting in the decreased dissolved CO2. this, in turn, means that less bicarb is ultimately needed to neutralize blood acid.

all this serves to answer the following prototypical "renal regulation of hydrogen ion balance" test question:

if a patient presents with an extracellular pH of 7.32, an arterial CO2 pressure of 60 mmHg, and an arterial bicarbonate concentration of 30 mEq/L, what type of acid-base disorder does he have?

answer: respiratory acidosis

likewise, if a patient presents with an extracellular pH of 7.21, a pCO2 of 25 mmHg, and a bicarb of 10 mEq/L, what type of acid-base disorder does he have?

answer: metabolic acidosis

(normal values approximately are: pH=7.4, pCO2=40, [HCO3-]=24)