Final answer:
Following rebreathing, a person's spontaneous respiratory rate typically increases in response to elevated CO2 levels, triggering the respiratory center in the brain to quicken the breathing rate and decrease the air volume per breath as a negative-feedback mechanism.
Step-by-step explanation:
The volunteer's spontaneous respiratory rate is likely to have increased immediately following rebreathing. During rebreathing, an individual breathes from a closed system, which generally has a higher concentration of carbon dioxide (CO2) due to exhalation. This increase in CO2 leads to a more acidic environment in the blood, which is detected by chemoreceptors. In response, the respiratory center located within the medulla oblongata in the brain triggers an increase in respiratory rate and a decrease in the volume of air per breath, to expel CO2 more quickly and reduce its levels. This physiological change is an example of a dose-response, negative-feedback relationship where the body reacts to an increased stimulus (higher CO2 levels) with a greater response (increased breathing rate).
It's important to note that while the respiratory rate may increase, the volume of air per breath may decrease due to the shorter time for each respiratory cycle, resulting from the increased rate of breathing. The body's priority in this situation is to rid itself of excess CO2, and the immediate modification of respiratory function facilitates this process.