Editing Respiratory system (section)

===Ventilatory volumes===
[[File:Lungvolumes Updated.png|thumb|550px|'''Fig. 3''' Output of a 'spirometer'. Upward movement of the graph, read from the left, indicates the intake of air; downward movements represent exhalation.]]{{Main |Breathing|Lung volumes}}
The lungs expand and contract during the breathing cycle, drawing air in and out of the lungs. The volume of air moved in or out of the lungs under normal resting circumstances (the resting [[tidal volume]] of about 500&nbsp;ml), and volumes moved during maximally forced inhalation and maximally forced exhalation are measured in humans by [[spirometry]].<ref name=tortora8 /> A typical adult human spirogram with the names given to the various excursions in volume the lungs can undergo is illustrated below (Fig.&nbsp;3):

Not all the air in the lungs can be expelled during maximally forced exhalation ([[Expiratory reserve volume|ERV]]). This is the [[Lung volumes|residual volume]] (volume of air remaining even after a forced exhalation) of about 1.0–1.5&nbsp;liters which cannot be measured by spirometry. Volumes that include the residual volume (i.e. [[functional residual capacity]] of about 2.5–3.0&nbsp;liters, and [[total lung capacity]] of about 6&nbsp;liters) can therefore also not be measured by spirometry. Their measurement requires special techniques.<ref name=tortora8>{{cite book |last1= Tortora |first1= Gerard J. |last2=Anagnostakos|first2=Nicholas P.| title=Principles of anatomy and physiology |url= https://archive.org/details/principlesofanat05tort |url-access= registration |pages=[https://archive.org/details/principlesofanat05tort/page/570 570–572]|edition= Fifth |location= New York |publisher= Harper & Row, Publishers|date= 1987 |isbn= 0-06-350729-3 }}</ref>

The rates at which air is breathed in or out, either through the mouth or nose or into or out of the [[alveoli]] are tabulated below, together with how they are calculated. The number of breath cycles per minute is known as the [[respiratory rate]]. An average healthy human breathes 12–16 times a minute.

{| class="wikitable"
|-
! Measurement  !! Equation !! Description
|-
| [[Respiratory minute volume|Minute ventilation]] ||  tidal volume * respiratory rate|| the total volume of air entering, or leaving, the nose or mouth per minute or normal respiration.
|-
| Alveolar ventilation || (tidal volume – [[Dead space (physiology)|dead space]]) * respiratory rate || the volume of air entering or leaving the alveoli per minute.
|-
| [[Dead space (physiology)|Dead space ventilation]] || dead space * respiratory rate ||  the volume of air that does not reach the alveoli during inhalation, but instead remains in the airways, per minute.
|}