Introduction

The respiratory cycle of inspiration and expiration is controlled by complex mechanisms involving neurons in the cerebral cortex, brain stem, and peripheral nervous system, as well as central and peripheral receptors. These receptors respond to a variety of stimuli including chemicals and pressure. Central respiratory control (respiratory drive) occurs in the pons and medulla oblongata, which respond directly to chemical influences. Other input is received from stretch receptors in the lungs and chemoreceptors located in the carotid and aortic bodies.

At rest, the average adult male produces approximately 200 mL of CO₂ each minute, this rate may increase to over 2000 mL/minute with exercise or heavy work. Hyperventilation lowers CO₂ levels due to an increased opportunity for gas exchange in the lungs. Holding one’s breath or re-breathing air (such as breathing into a paper bag) raises CO₂ levels because there is less opportunity for gas exchange.Chemoreceptors respond most sensitively and rapidly to carbon dioxide, but they also respond to oxygen and pH (acidity). Constant adjustments in the respiratory cycle occur throughout the day to allow gas exchange in the lungs to maintain a steady level of CO₂ in the bloodstream. An increase in the CO₂ level stimulates breathing, while a decrease inhibits it. If the deviation from the “set point” is large enough, you may experience shortness of breath. The oxygen level can also influence the respiratory cycle, but larger deviations are required before its influence is felt.

In this experiment, you will alter CO₂ levels by holding your breath (hypoventilation), rapid breathing (hyperventilation), and exercise. You will compare the respiratory rate, tidal volume, and minute ventilation that result from each physiologic challenge to homeostasis.

Important: The equipment used in this experiment is for educational purposes only and should not be used to diagnose medical conditions.

Objectives