This series of illustrations shows how respiratory acidosis develops at the cellular level.

1. Pulmonary ventilation decreases, and retained carbon dioxide (CO₂) then combines with water (H₂O) to form carbonic acid (H₂CO₃) in larger-than-normal amounts. H₂CO₃ dissociates to release free hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-). Excessive H₂CO₃ causes a drop in pH. Look for a PaCO₂ level above 45 mm Hg and a pH level below 7.35.
   
2. As the pH level falls, hemoglobin (Hb) release oxygen (O₂). The altered Hb, now strongly alkaline, picks up H⁺ and CO₂, thus eliminating some of the free H⁺ and excess CO₂. Look for decreased arterial O₂ saturation.
   
3. Whenever PaCO₂ increases, CO₂ builds up in all tissues and fluids, including cerebrospinal fluid and the respiratory center in the medulla. The CO₂ reacts with H₂O to form H₂CO₃, which then breaks into free H⁺ and HCO₃^-. The increased amount of CO₂ and free H⁺ stimulates the respiratory center to increase the respiratory rate. An increased respiratory rate expels more CO₂ and helps to reduce the CO₂ level in blood and tissues. Look for rapid, shallow respirations and a decreasing PaCO₂.
   
4. Eventually, CO₂ and H⁺ cause cerebral blood vessels to dilate, which increases blood flow to the brain. That increased flow can cause cerebral edema and depress central nervous system (CNS) activity. Look for a headache, confusion, lethargy, nausea, diaphoresis, tachycardia, and vomiting.
   
5. As respiratory mechanisms fail, the increasing PaCO₂ stimulates the kidneys to conserve HCO₃^- and sodium ions (Na) and to excrete H⁺, some in the form of ammonium (NH₄). The additional HCO₃^- and Na combines to form extra sodium bicarbonate (NaHCO₃^-), which is then able to buffer more free H⁺. Look for increased acid content in the urine; increased serum pH and HCO₂^- levels; and shallow, depressed respirations.
   
6. As the concentration of H⁺ overwhelms the body's compensatory mechanisms, the H⁺ move into the cells, and potassium ions (K) move out. A concurrent lack of O₂ causes an increase in the anaerobic production of lactic acid, which further skews the acid-base balance and critically depresses neurologic and cardiac functions. Look for hyperkalemia, arrhythmias, increased PaCO₂, decreased partial pressure of arterial oxygen (PaO₂), decreased pH, and decreased level of consciousness (LOC).