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25.4C: Regulation of H+ by the Lungs

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  • Acid–base imbalances in the blood’s pH can be altered by changes in breathing to expel more CO2 and raise pH back to normal.

    Learning Objectives

    • Describe the regulation of hydrogen ions by the lungs

    Key Points

    • Hydrogen ions (H+) are carried in the blood along with oxygen and carbon dioxide.
    • Sixty percent of the carbon dioxide is carried as dissolved bicarbonate.
    • A small amount of carbon dioxide is carried on the hemoglobin as carbaminohemoglobin, which is transported to the lungs for removal.
    • Following Le Chatelier’s principle, an imbalance in pH is returned to normal by increasing the rate of ventilation in the lungs.
    • To compensate for acidemia, more CO2 is expelled, while the opposite occurs for alkalemia.

    Key Terms

    • carbaminohemoglobin: A compound of hemoglobin and carbon dioxide. It is one of the forms in which carbon dioxide exists in the blood.
    • Le Chatelier’s principle: A principle that states that if a chemical system at equilibrium experiences a change in concentration, temperature, or total pressure, the equilibrium will shift in order to minimize that change.


    Since maintaining normal pH is vital for life, and since the lungs play a critical role in maintaining normal pH, smokers have yet another reason to quit smoking.

    Acid–base imbalance occurs when a significant insult causes the blood pH to shift out of its normal range (7.35 to 7.45). An excess of acid in the blood is called acidemia and an excess of base is called alkalemia.

    The process that causes the imbalance is classified based on the etiology of the disturbance (respiratory or metabolic) and the direction of change in pH ( acidosis or alkalosis). There are four basic processes and one or a combination may occur at any given time.

    1. Metabolic acidosis
    2. Respiratory acidosis
    3. Metabolic alkalosis
    4. Respiratory alkalosis

    Blood carries oxygen, carbon dioxide, and hydrogen ions (H+) between tissues and the lungs. The majority of CO2 transported in the blood is dissolved in plasma (60% is dissolved bicarbonate).

    This is a diagram of expiration that shows a person exhaling. When blood pH drops too low, the body compensates by increasing breathing to expel more carbon dioxide.

    Expiration: When blood pH drops too low, the body compensates by increasing breathing to expel more carbon dioxide.

    A smaller fraction is transported in the red blood cells that combine with the globin portion of hemoglobin as carbaminohemoglobin. This is the chemical portion of the red blood cell that aids in the transport of oxygen and nutrients around the body, but, this time, it is carbon dioxide that is transported back to the lung.

    Acid–base imbalances that overcome the buffer system can be compensated in the short term by changing the rate of ventilation. This alters the concentration of carbon dioxide in the blood, shifting the above reaction according to Le Chatelier’s principle, which in turn alters the pH. The basic reaction governed by this principle is as follows:


    When the blood pH drops too low (acidemia), the body compensates by increasing breathing to expel more CO2; this shifts the above reaction to the left such that less hydrogen ions are free; thus, the pH will rise back to normal. For alkalemia, the opposite occurs.