Module 4: Physiology of the Respiratory System - Volume/Time And Flow/Volume Curves.

A Volume/Time Curve:

  • The figure on the right shows a recording of a single forced expiration into a spirometer.
  • The x-axis shows time. The y- axis shows volume.
  • FEV1 is the volume measured after one second of exhalation.
  • FVC is the total volume exhaled.
  • Exhaled volume accumulates rapidly in the first part of exhalation as air moves out of the larger, conducting airways and then slows with the increased resistance to flow from small airways.

A Flow/Volume Curve:

  • On the left, you can see a flow/volume curve that corresponds to the volume/time curve in the previous illustration.
  • Flow = Volume/Time.
  • By integrating volume and time with volume, the flow volume curve shows how flow varies from the beginning to the end of expiration.
  • Flow/volume curves enable an appreciation of peak flow and mid-expiratory flow rates. It is capable of discerning changes in mid- and end-expiratory flow that cannot be seen on a volume/time curve. Flow/volume curves are therefore more sensitive in detecting abnormalities in the smaller conducting airways, a useful sign in the early detection of disease.

Flow/Volume Curves and Effort:

  • The first part of the F/V curve is effort dependant. A poor effort (curve A in the illustration) results in underestimation of the peak flow achieved in a full effort (curve D).
  • The middle to end part of the curve is effort independent, so that better efforts do not result in very different mid- to end-expiratory flow rates. This is because the resistance in the small airways is such that greater effort cannot overcome it, and may result instead in premature closure of the airways.
  • The last part of the F/V curve depends on sustained effort to accurately estimate FVC.

Creative Commons License
Postgraduate Diploma in Occupational Health (DOH) - Modules 3 – 5: Occupational Medicine & Toxicology by Prof Rodney Ehrlich & Prof Mohamed Jeebhay is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Permissions beyond the scope of this license may be available at http://www.healthedu.uct.ac.za/