What primarily controls the levels of PaCO2?

The primary control of the levels of PaCO2 in the blood is through minute ventilation (VE). Minute ventilation is defined as the amount of air that is inhaled or exhaled from a person's lungs in one minute and is a product of tidal volume and respiratory rate. By adjusting either the tidal volume (the depth of each breath) or the frequency of breaths, minute ventilation can be increased or decreased.

When minute ventilation increases, more carbon dioxide (CO2) is eliminated from the body, leading to a decrease in PaCO2 levels. Conversely, if minute ventilation decreases, less CO2 is exhaled, resulting in an increase in PaCO2 levels. Thus, minute ventilation is a critical determinant of the balance between CO2 production and elimination, making it the primary factor that controls arterial carbon dioxide tension.

Tidal volume can influence minute ventilation, but it is through the overall minute ventilation that the levels of PaCO2 are regulated. Other factors, like positive end-expiratory pressure (PEEP) and fraction of inspired oxygen (FiO2), have different effects on lung mechanics and gas exchange but do not primarily control the levels of PaCO2.