RESPIRATION AND ACID–BASE BALANCE
As you have just seen, respiration affects the pH of body fluids because it regulates the amount of carbon dioxide in these fluids. Remember that CO2 reacts with water to form carbonic acid (H2CO3), which ion-izes into H+ ions and HCO3– ions. The more hydro-gen ions present in a body fluid, the lower the pH, and the fewer hydrogen ions present, the higher the pH.
The respiratory system may be the cause of a pH imbalance, or it may help correct a pH imbalance cre-ated by some other cause.
Respiratory acidosis occurs when the rate or effi-ciency of respiration decreases, permitting carbon dioxide to accumulate in body fluids. The excess CO2 results in the formation of more H+ ions, which decrease the pH. Holding one’s breath can bring about a mild respiratory acidosis, which will soon stimulate the medulla to initiate breathing again. More serious causes of respiratory acidosis are pulmonary diseases such as pneumonia and emphysema, or severe asthma. Each of these impairs gas exchange and allows excess CO2 to remain in body fluids.
Respiratory alkalosis occurs when the rate of res-piration increases, and CO2 is very rapidly exhaled. Less CO2 decreases H+ ion formation, which increases the pH. Breathing faster for a few minutes can bring about a mild state of respiratory alkalosis. Babies who cry for extended periods (crying is a noisy exhalation) put themselves in this condition. In general, however, respiratory alkalosis is not a common occurrence. Severe physical trauma and shock, or certain states of mental or emotional anxiety, may be accompanied by hyperventilation and also result in respiratory alkalosis. In addition, traveling to a higher altitude (less oxygen in the atmosphere) may cause a tempo-rary increase in breathing rate before compensation occurs.
If a pH imbalance is caused by something other than a change in respiration, it is called metabolic acidosis or alkalosis. In either case, the change in pH stimulates a change in respiration that may help restore the pH of body fluids to normal.
Metabolic acidosis may be caused by untreated diabetes mellitus (ketoacidosis), kidney disease, or severe diarrhea. In such situations, the H+ ion concen- tration of body fluids is increased. Respiratory com-pensation involves an increase in the rate and depth of respiration to exhale more CO2 to decrease H+ ion formation, which will raise the pH toward the normal range.
Metabolic alkalosis is not a common occurrence but may be caused by ingestion of excessive amounts of alkaline medications such as those used to relieve gastric disturbances. Another possible cause is vomit-ing of stomach contents only. In such situations, the H+ion concentration of body fluids is decreased. Respiratory compensation involves a decrease in res-piration to retain CO2 in the body to increase H+ ion formation, which will lower the pH toward the normal range.
Respiratory compensation for an ongoing meta-bolic pH imbalance cannot be complete, because there are limits to the amounts of CO2 that may be exhaled or retained. At most, respiratory compensation is only about 75% effective.