Important things to know about acid/base balance in the human body FOR THE MCAT:
The buffer system you need to know: HCO3-/CO2 (think HCO3- = base and CO2 = acid)
Chemical equation: H+ + HCO3- = H2CO3 = CO2 + H2O (imagine the equilibria arrows)
The lungs independently control CO2 (fast response minutes to hours) while the kidneys independently control HCO3- (slow response hours to days).
Physiologic pH: 7.4
When the pH goes out of whack, the body has a few different ways to deal with it: buffering, compensation, and correction. Buffering is automatic and instantaneous its a function of the chemistry shown above. When the challenge to the buffering system is too great and the pH becomes materially affected, chemoreceptors pick up this signal encourage the appropriate system to take action. The response depends on whether it is a metabolic problem (HCO3- too high or low) or a respiratory problem (CO2 too high or low). The kidneys adjust to compensate for respiratory problems (can do this by a variety of mechanisms but basically you just need to know that it affects HCO3- levels) while the lungs adjust the CO2 level by hyper- or hypoventilation to compensate for metabolic disorders. These compensatory mechanisms function best as a temporary stopgap (minutes to days) until the body can manage to correct the underlying reason for the acid/base imbalance. Of course, sometimes the problem is not correctable and people wind up living in a chronically compensated state.
Heres a handy list of the four main problems and what happens with them:
Respiratory acidosis caused by lung problems. Levels of CO2 are too high, causing pH to drop. The kidneys respond by increasing levels of HCO3-.
Respiratory alkalosis caused by lung problems. Levels of CO2 are too low, causing pH to rise. The kidneys respond by decreasing levels of HCO3-.
Metabolic acidosis not caused by lungs. Levels of HCO3- are too low, causing pH to drop. The lungs respond by decreasing levels of CO2 (hyperventilation).
Metabolic alkalosis not caused by lungs. Levels of HCO3- are too high, causing pH to rise. The lungs respond by increasing levels of CO2 (hypoventilation).
Just for fun, some causes of each (not an exhaustive list):
Respiratory acidosis: pulmonary embolism, cardiac arrest, pneumonia, COPD, airway obstruction you get the drift. Things that impair the lungs ability to move air or do gas exchange.
Respiratory alkalosis: pain, fever, things that cause respiratory rate and depth to increase.
Metabolic acidosis: diarrhea (loss of base via GI tract), ketoacidosis (from diabetes), lactic acid build-up, several drugs.
Metabolic alkalosis: vomiting (loss of acid via GI tract), occasionally gain of base (saw this once in a lady who drank multiple bottles of Maalox)