The Body’s pH Balance
The body maintains a stable internal environment, including its acid-base balance, measured by pH. This precise balance is important for cells, tissues, and organs to function correctly. Even small deviations from the ideal pH range can disrupt biological processes, affecting enzyme activity and cellular communication. Maintaining pH within a narrow range is a continuous and regulated process.
The pH scale ranges from 0 to 14, where 0 is highly acidic, 14 is highly alkaline (or basic), and 7 is neutral. The human body’s blood pH is tightly regulated within a narrow, slightly alkaline range of 7.35 to 7.45. This narrow range is optimal for most biological functions, though other body fluids, like stomach acid, have different pH levels for specialized purposes.
The Body’s pH Balance and Carbon Dioxide
Metabolic activities continuously generate various byproducts, some of which are acidic. Carbon dioxide (CO2) is an example, produced as cells break down nutrients for energy. This CO2 acts as an acid-forming gas that needs to be managed to maintain pH.
When CO2 dissolves in the bloodstream, it reacts with water (H2O) to form carbonic acid (H2CO3). The reaction is CO2 + H2O ⇌ H2CO3. Carbonic acid then quickly dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-), following the reaction H2CO3 ⇌ H+ + HCO3-. The presence of these hydrogen ions directly contributes to the acidity of the blood.
How Breathing Controls pH
The respiratory system acts as a rapid-response mechanism for pH regulation, primarily by controlling the amount of carbon dioxide expelled from the body. Specialized sensory cells, known as chemoreceptors, constantly monitor the levels of carbon dioxide and hydrogen ions in the blood and cerebrospinal fluid. These chemoreceptors are located in areas like the aorta, carotid arteries, and the brainstem itself. Signals from these receptors are sent to the respiratory control centers within the brainstem, which is responsible for regulating breathing patterns.
If blood CO2 levels rise, indicating increasing acidity, the brainstem stimulates an increase in the rate and depth of breathing. This increased ventilation, or hyperventilation, expels more CO2 from the lungs, reducing the amount of carbonic acid and hydrogen ions in the blood, thereby raising the pH back towards its normal range.
Conversely, if blood CO2 levels fall too low, leading to increased alkalinity, the brainstem reduces the rate and depth of breathing. This decreased ventilation, or hypoventilation, retains more CO2 in the body, which then combines with water to form more carbonic acid and hydrogen ions. This process helps to lower the blood pH, bringing it back into the physiological range. This adjustment of breathing rate allows the body to quickly “blow off” or retain CO2 as needed to maintain acid-base balance.
When Respiratory pH Regulation Goes Awry
When the respiratory system’s ability to regulate carbon dioxide levels becomes impaired, the body’s pH balance can be disrupted. One such condition is respiratory acidosis, which occurs when the lungs do not expel enough CO2. This accumulation of CO2 in the blood leads to an increase in carbonic acid and hydrogen ions, causing the blood pH to drop below 7.35. Conditions that impair breathing, such as severe lung diseases or certain medications that depress the respiratory drive, can lead to respiratory acidosis.
On the other hand, respiratory alkalosis occurs when the lungs expel too much CO2. This excessive removal of CO2 reduces the amount of carbonic acid and hydrogen ions in the blood, causing the blood pH to rise above 7.45. Causes of respiratory alkalosis often involve hyperventilation, which can be triggered by anxiety, pain, or high altitudes.