Biological Systems MCAT Practice Exam

What is the primary trigger for increased ventilation rate?

1. Increased blood oxygen levels

2. Decreased pH

3. Decreased heart rate

4. Increased body temperature

The correct answer is: Decreased pH

The primary trigger for an increased ventilation rate is decreased pH, which is often associated with an increase in carbon dioxide (CO2) levels in the blood. When CO2 levels rise due to metabolic processes, it leads to the formation of carbonic acid, which dissociates into bicarbonate ions and hydrogen ions, resulting in a lower pH (more acidic) environment in the blood. The body senses this change through chemoreceptors located in the brain and blood vessels. In response to the decrease in pH, the respiratory center in the brain signals the respiratory muscles to increase the rate and depth of breathing. This enhances the exchange of gases in the lungs, allowing for more CO2 to be expelled and for blood pH levels to return to normal. Thus, the mechanism of stimulating ventilation in response to lower pH is a critical physiological response to maintain homeostasis during states of metabolic acidosis or other conditions that may lead to increased CO2 production. Other options do not serve as primary triggers for ventilation in the same manner; for example, increased blood oxygen levels can stimulate ventilation but typically as a secondary response, and decreased heart rate is more related to cardiovascular response rather than directly influencing respiratory drive. Similarly, increased body temperature can influence