Lung volumes

Lung Volumes

Lung volumes refer to physical differences in lung volume, while lung capacities represent different combinations of lung volumes, usually in relation to inhalation and exhalation.

The average pair of human lungs can hold about 6 litres of air, but only a small amount of this capacity is used during normal breathing.

Breathing mechanism in mammals is called "tidal breathing". Tidal breathing means that air goes into the lungs the same way that it comes out.

An average human breathes some 9-20 times per minute.

Factors affecting volumes

Several factors affect lung volumes; some can be controlled and some cannot. Lung volumes can be measured using the following terms:

A person who is born and lives at sea level will develop a slightly smaller lung capacity than a person who spends their life at a high altitude. This is because the atmosphere is less dense at higher altitude, and therefore, the same volume of air contains fewer molecules of all gases, including oxygen. In response to higher altitude, the body's diffusing capacity increases in order to be able to process more air.

When someone living at or near sea level travels to locations at high altitudes (eg. the Andes, Denver, Colorado, Tibet, the Himalayas, etc.) s/he can develop a condition called altitude sickness because their lungs remove adequate amounts of carbon dioxide but they do not take in enough oxygen. (In normal individuals, carbon dioxide is the primary determinant of respiratory drive.)

Values

These values vary with the age and height of the person. For males, the values that follow are for a 70 kg (154 lb), average-sized adult male ^[1]; for females, the editors of this article have not yet produced data from primary sources; until they do, the data listed are estimates obtained by reducing the values for males by 22.5% ^[2]:

Measurement	Value (Male/Female)	Calculation	Description
Total lung capacity (TLC)	= 6.0 / 4.7 L	= IRV + Vt + ERV + RV	The volume of air contained in the lung at the end of maximal inspiration. The total volume of the lung (i.e.: the volume of air in the lungs after maximum inspiration).
Vital capacity (VC)	= 4.6 / 3.6 L	= IRV + Vt + ERV	The amount of air that can be forced out of the lungs after a maximal inspiration. Emphasis on completeness of expiration. The maximum volume of air that can be voluntarily moved in and out of the respiratory system.[3]
Forced vital capacity (FVC)	= 4.8 / 3.7 L	measured	The amount of air that can be maximally forced out of the lungs after a maximal inspiration. Emphasis on speed.[4][5]
Tidal volume (Vt)	= 500 / 390 mL	measured	The amount of air breathed in or out during normal respiration. The volume of air an individual is normally breathing in and out.
Residual volume (RV)	= 1.2 / 0.93 L	measured	The amount of air left in the lungs after a maximal exhalation. The amount of air that is always in the lungs and can never be expired (i.e.: the amount of air that stays in the lungs after maximum expiration).
Expiratory reserve volume (ERV)	= 1.2 / 0.93 L	measured	The amount of additional air that can be pushed out after the end expiratory level of normal breathing. (At the end of a normal breath, the lungs contain the residual volume plus the expiratory reserve volume, or around 2.4 litres. If one then goes on and exhales as much as possible, only the residual volume of 1.2 litres remains).
Inspiratory reserve volume (IRV)	= 3.0 / 2.3 L	measured IRV=VC-(TV+ERV)	The additional air that can be inhaled after a normal tidal breath in. The maximum volume of air that can be inspired in addition to the tidal volume.
Functional residual capacity (FRC)	= 2.4 / 1.9 L	= ERV + RV	The amount of air left in the lungs after a tidal breath out. The amount of air that stays in the lungs during normal breathing.
Inspiratory capacity (IC)	= 3.5 / 2.7 L	= Vt + IRV	The maximal volume that can be inspired following a normal expiration.
Anatomical dead space	= 150 / 120 mL	measured	The volume of the conducting airways. Measured with Fowler method.[6]
Physiologic dead volume	= 155 / 120 mL		The anatomic dead space plus the alveolar dead space.

The tidal volume, vital capacity, inspiratory capacity and expiratory reserve volume can be measured directly with a spirometer. Determination of the residual volume can be done by radiographic planemetry, body plethysmography, closed circuit dilution and nitrogen washout.

These are the basic elements of a ventilatory pulmonary function test.

Restrictive and obstructive

The results (in particular FEV1/FVC and FRC) can be used to distinguish between restrictive and obstructive pulmonary diseases:

References

External links

• "Lung Function Fundamentals" at anaesthetist.com
• "Respiratory Calculations" - University of St. Thomas by Rex Njoku and Dr. Anthony Steyermark
• RT Corner (Educational Site for RT's and Nurses) at rtcorner.net
• Volume of Human Lungs