Total Lung Capacity

Lung volumes and lung capacities refer to the volume of air associated with different phases of the respiratory cycle. Lung volumes are directly measured; Lung capacities are inferred from lung volumes.

The average total lung capacity of an adult human male is about 6 litres of air,[1] but only a small amount of this capacity is used during normal breathing.

Tidal breathing is normal, resting breathing; the tidal volume is the volume of air that is inhaled or exhaled in only a single such breath.

The average human respiratory rate is 30-60 breaths per minute at birth,[2] decreasing to 12-20 breaths per minute in adults.[3]

Factors affecting volumes

Several factors affect lung volumes; some can be controlled and some cannot. Lung volumes vary with different people as follows:

Larger volumes Smaller volumes
taller people shorter people
people who live at higher altitudes people who live at lower altitudes
non obese obese[4]

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 partial pressure of oxygen is lower at higher altitude which, as a result means that oxygen less readily diffuses into the bloodstream. In response to higher altitude, the body's diffusing capacity increases in order to process more air.

When someone living at or near sea level travels to locations at high altitudes (e.g., the Andes; Denver, Colorado; Tibet; the Himalayas) that person 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.)

Specific changes in lung volumes also occur during pregnancy. Functional residual capacity drops 18–20%,[5] typically falling from 1.7 to 1.35 litres, due to the compression of the diaphragm by the uterus. The compression also causes a decreased total lung capacity (TLC) by 5%[5] and decreased expiratory reserve volume by 20%.[5] Tidal volume increases by 30–40%, from 0.5 to 0.7 litres,[5] and minute ventilation by 30–40%[5][6] giving an increase in pulmonary ventilation. This is necessary to meet the increased oxygen requirement of the body, which reaches 50 mL/min, 20 mL of which goes to reproductive tissues. Overall, the net change in maximum breathing capacity is zero.[5]

Values

Average lung volumes in healthy adults[7]
Volume Value (litres)
In men In women
Inspiratory reserve volume 3.3 1.9
Tidal volume 0.5 0.5
Expiratory reserve volume 1.0 0.7
Residual volume 1.2 1.1
Lung capacities in healthy adults[7]
Volume Average value (litres) Derivation
In men In women
Vital capacity 4.8 3.1 IRV plus TV plus ERV
Inspiratory capacity 3.8 2.4 IRV plus TV
Functional residual capacity 2.2 1.8 ERV plus RV
Total lung capacity 6.0 4.2 IRV plus TV plus ERV plus RV

The tidal volume, vital capacity, inspiratory capacity and expiratory reserve volume can be measured directly with a spirometer. These are the basic elements of a ventilatory pulmonary function test.

Determination of the residual volume is more difficult as it is impossible to "completely" breathe out. Therefore measurement of the residual volume has to be done via indirect methods such as radiographic planimetry, body plethysmography, closed circuit dilution (including the helium dilution technique) and nitrogen washout.

In absence of such, estimates of residual volume have been prepared as a proportion of body mass for infants (18.1ml/kg),[8] or as a proportion of vital capacity (0.24 for men and 0.28 for women)[9] or in relation to height and age ((0.0275*AgeInYears+0.0189*HeightInCentimetres-2.6139) litres for normal-weight individuals and (0.0277*AgeInYears+0.0138*HeightInCentimeters-2.3967) litres for overweight individuals).[10] Standard errors in prediction equations for residual volume have been measured at 579ml for men and 355ml for women, while the use of 0.24*FVC gave a standard error of 318ml.[11]

Restrictive and obstructive

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

Type Examples Description FEV1/FVC
restrictive diseases pulmonary fibrosis, Infant Respiratory Distress Syndrome, weak respiratory muscles, pneumothorax volumes are decreased often in a normal range (0.8 - 1.0)
obstructive diseases asthma or COPD volumes are essentially normal but flow rates are impeded often low (Asthma can reduce the ratio to 0.6, Emphysema can reduce the ratio to 0.78 - 0.45)

See also

References

External links

  • "Lung Function Fundamentals" at anaesthetist.com
  • RT Corner (Educational Site for RT's and Nurses) at rtcorner.net
  • Volume of Human Lungs
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