Respiring tissues produce carbon dioxide, which blood transports to the lungs. Carbon dioxide reacts with water in a reversible system involving carbonic acid, hydrogen ions and bicarbonate. Other conditions being comparable, an increase in carbon dioxide can increase hydrogen-ion concentration and lower pH. Lower pH means greater acidity, not a larger pH number.
Chemoreceptor information helps respiratory control centres adjust ventilation. Increased carbon dioxide, including its pH-related effects around central receptors, generally stimulates breathing so more carbon dioxide is removed. As carbon dioxide falls toward the usual range, the stimulating signal is reduced. Muscles such as the diaphragm are effectors in this pathway. Ventilation means movement of air into and out of lungs; cellular respiration is the metabolic process that supplies usable energy.
Example 1
CO2 rises while other acid–base factors are held constant. Predict pH and the usual breathing response.
pH tends to fall and ventilation tends to increase, helping remove additional CO2.
Example 2
A supplied CO2 reading drops from 48 to 42 mmHg. Find the decrease.
48−42=6 mmHg. This is a decrease in partial pressure, using the given units.
Watch out: Lower pH corresponds to greater hydrogen-ion concentration and greater acidity.