Is respiratory control more sensitive to small changes in arterial PO2 or in arterial PCO2?
Respiratory control is generally more sensitive to small changes in arterial PCO2 (partial pressure of carbon dioxide) rather than arterial PO2 (partial pressure of oxygen).
To understand why, we need to consider the key factor that drives the respiratory response – the regulation of pH in the body. The concentration of hydrogen ions (H+) in the body, which determines pH, is primarily influenced by the levels of carbon dioxide. When carbon dioxide is dissolved in the blood, it forms carbonic acid (H2CO3), which then dissociates into bicarbonate ions (HCO3-) and hydrogen ions.
When the arterial PCO2 level increases (indicating higher carbon dioxide concentration), it leads to an increase in hydrogen ion concentration and a subsequent decrease in pH, making the blood more acidic. On the other hand, when the arterial PCO2 level decreases, it results in lower hydrogen ion concentration and an increase in pH, making the blood more alkaline.
To maintain acid-base balance, the respiratory control system responds to changes in arterial PCO2 by adjusting the ventilation rate (breathing rate). When PCO2 levels rise, the brain signals the respiratory muscles to increase the rate and depth of breathing, which helps eliminate excess carbon dioxide. Conversely, if PCO2 levels decrease, the brain reduces the ventilation rate to retain more carbon dioxide.
In contrast, arterial PO2 levels exert less direct influence on respiratory control. While oxygen is crucial for sustaining cellular respiration, the primary regulation of blood oxygenation occurs through other mechanisms like the oxygen carrying capacity of hemoglobin in red blood cells and the peripheral chemoreceptors that monitor blood oxygen levels and can influence respiratory control to some extent.
Therefore, in summary, respiratory control is more sensitive to changes in arterial PCO2 because they directly affect pH and acid-base balance, leading to adjustments in ventilation rate to maintain homeostasis.