Respiratory bronchiole
A diagram of respiratory bronchiole
showing the alveoli across which gas
exchange takes place

The human respiratory system consists of a series of tubes that branch and terminate as clusters of small membranous air sacs called alveoli. Oxygen and carbon dioxide diffuse across the walls of the alveoli between the air and the blood. Factors that influence diffusion include
  • Surface area
  • Diffusion distance
  • Concentration gradient
In the human lung, the alveoli provide a large surface area (about the size of a tennis court) and their thin walls provide a short diffusion distance. A high concentration gradient is insured by:
  1. Directing blood with low oxygen and high carbon dioxide levels to the lungs.
  2. Pulmonary ventilation (breathing), which maintains a high level of oxygen and a low level of carbon dioxide in the alveolar air.
Thus, the alveoli and associated blood supply are well suited for the diffusion of oxygen into the blood and carbon dioxide into the air in the alveoli.

Ventilation of the human lung is produced by muscular contraction, which changes the volume of the thoracic cavity and the elastic lungs. Inspiration is achieved by contraction of the diaphragm and the intercostal muscles, both of which increase the volume of the thoracic cavity. In the resting individual, expiration is usually passive since muscle relaxation, the elasticity of the thoracic cavity and gravity act to decrease thoracic volume. Under certain circumstances, such as during exercise, forced expiration is produced by contraction of intercostal muscles.

The amount of air that moves in or out of the lungs during any one breathing cycle is called the tidal volume. This is not the maximum amount of air that can be moved through the lungs (the vital capacity), since there are reserve volumes (inspiratory and expiratory) that can be tapped to increase the tidal volume. Even if the expiratory reserve volume is fully expelled from the lungs, there is still air in the lungs. Clearly, the tidal flow of air through the lungs results in the mixing of the fresh air with residual air. Since this residual air has been in the lungs it has a lower oxygen level and higher carbon dioxide level than fresh air. It is, therefore, not surprising that air in the alveoli is a mixture of stale and fresh air.

While the depth of breathing can be altered the rate of breathing can also be changed. These two parameters are controlled by the respiratory control center which is located in the medulla of the brain. The center insures that the exchange of oxygen and carbon dioxide at the lungs takes place at a rate that matches the body's requirements. This is a dynamic process since the body's requirements change over time. During times of increased demand the rate and depth of breathing can be changed. In this lab you will measure these parameters in a volunteer at rest and immediately after exercise when the body's demands for oxygen have been elevated.
Before Starting
  1. Breathing will be recorded from a volunteer. It is important that the volunteer is healthy and has no history of respiratory or cardiovascular problems.
  2. The tubes on the flow head should always be in the upright position to avoid problems with condensation.
  3. The spirometer is set for 35 mV/liter for Channel 3 -- the integral of the signal from the spirometer (Introduction).

Proceed to Equipment Setup.