Emergency Medical Technician (EMT) Intermediate Practice Exam

What is the primary respiratory stimulus in a healthy adult?

1. Decreased arterial oxygen level

2. Increased arterial carbon dioxide level

3. Increased respiratory rate

4. Decreased vascular resistance

The correct answer is: Increased arterial carbon dioxide level

The primary respiratory stimulus in a healthy adult is indeed based on the levels of carbon dioxide in the blood. The respiratory drive is primarily regulated by the concentration of carbon dioxide, rather than oxygen. When carbon dioxide levels increase, it results in a corresponding increase in hydrogen ion concentration due to the formation of carbonic acid. This increase in acidity (lower pH) is detected by chemoreceptors located in the brain and in the blood vessels, which then stimulate the respiratory centers to increase the rate and depth of breathing. This compensatory response helps to expel the excess carbon dioxide, thereby restoring normal levels. Oxygen levels do play a role in respiration as a secondary stimulus, but they are not the main regulator under normal physiological conditions for healthy adults. While changes in respiratory rate occur in response to these stimuli, they are a result of the body's regulation of carbon dioxide rather than a primary driver. Decreased vascular resistance is more related to cardiovascular responses and does not directly influence the respiratory stimulus.