Florida State Paramedic Practice Exam

What is the normal stimulus to breathe?

1. Decreased blood pH

2. Increased PACO2 in arterial blood

3. Elevated oxygen levels

4. Low blood glucose

The correct answer is: Increased PACO2 in arterial blood

The normal stimulus to breathe is primarily controlled by levels of carbon dioxide in the blood, specifically the partial pressure of carbon dioxide (PACO2). When Paco2 rises, it leads to an increase in hydrogen ions and a subsequent decrease in blood pH, which stimulates the respiratory center in the brain to initiate breathing. This response is crucial because it ensures that carbon dioxide, a byproduct of metabolism, is effectively expelled from the body, thereby maintaining the acid-base balance. The other options present various physiological factors, but they do not serve as the primary triggers for respiration in the same way that elevated PACO2 does. For instance, decreased blood pH can result from increased PACO2 rather than acting as an independent stimulus. While low oxygen levels (hypoxia) can influence breathing, especially in patients with respiratory disorders, it is typically the elevation of carbon dioxide levels that acts as the most immediate and effective stimulus for increasing the respiratory rate in healthy individuals. Additionally, low blood glucose levels do not directly trigger the drive to breathe, making it an unrelated factor in respiratory control.