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. Lung volumes refer to physical differences in volume, while lung capacities represent different combinations of lung volumes, usually in relation to inspiration and exhalation. These values vary with the age and height of the person; the values that follow are for a 70 kg, average-sized adult male Palsson, et al. Tissue Engineering (2003). CRC Press. ISBN 0-8493-1812-2. page 7-7. :

• Total lung capacity (TLC) = 6 L. The volume of gas contained in the lung at the end of maximal inspiration.
• Vital capacity (VC) = 4.8 L. The amount of air that can be forced out of the lungs after a maximal inspiration.
• Tidal volume (TV) = 500 mL. The amount of air breathed in or out during normal respiration.
• Residual volume (RV) = 1.2 L. The amount of air left in the lungs after a maximal exhalation.
• Expiratory reserve volume (ERV) = 1.2 L. The amount of additional air that can be breathed out after normal expiration. (At the end of normal a breath, the lungs contain the residual volume plus the expiratory reserve volume, or around 2.4 liters. If one then goes on and exhales as much as possible, only the residual volume of 1.2 liters remains).
• Inspiratory reserve volume (IRV) = 3.6 L. The additional air that can be inhaled after a normal tidal breath in.
• Functional residual capacity (ERV + RV) = 2.4 L. The amount of air left in the lungs after a tidal breath out.
• Inspiratory capacity (IC) = is the volume that can be inhaled after a tidal breath out.
• Anatomical dead volume (or dead space) = 150 mL. The volume of the conducting airways.

Factors affecting lung volume

The breathing mechanism in mammals is called Tidal Breathing. Tidal breathing means that air goes into the lungs the same way that it comes out. The total lung capacity depends on the person's age, weight, sex and the degree of physical activity. For example, females tend to have a 20–25% lower capacity than males. Tall people tend to have a larger total lung capacity than shorter people. Heavy smokers have a drastically lower capacity than nonsmokers. Lung capacity is also affected by altitude. A person who is born and lives at sea level will have a smaller lung capacity than a person who spends their life at a high altitude. This is because there is less oxygen in the air at altitude, so the lungs gradually expand to process more air. When someone from sea level travels up to the higher parts of the earth (eg. the Andes, Mexico City, Tibet and the Himalayas) they will often develop a condition called altitude sickness because their lungs cannot process enough oxygen for their body's needs.

Measurement

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. The results can be used to divide pulmonary diseases into restrictive diseases, in which the volumes are decreased, and obstructive diseases, where volumes are essentially normal but flow rates are impeded.

There are four basic measured lung volumes:

• Tidal volume (TV): the volume of air an individual is normally breathing in and out.
• Inspiratory reserve volume (IRV): the maximum volume of air that can be inspired in addition to the tidal volume.
• Expiratory reserve volume (ERV): the maximum volume of air that can be expired in addition to the normally expired air.
• Residual volume (RV): 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).

From these volumes, several important capacities are also calculated:

• The total lung capacity (TLC): the total volume of the lung (i.e.: the volume of air in the lungs after maximum inspiration).

$TLC\; =\; IRV\; +\; TV\; +\; ERV\; +\; RV$

• The functional residual capacity (FRC): the amount of air that stays in the lungs during normal breathing.

$FRC\; =\; ERV\; +\; RV$

Respiratory quotient = RQ = rate of CO₂ production/rate of O₂ consumption. RQ can vary with diet and exercise, but under typical conditions, RQ = 0.8.

Following is a list of average partial pressures of Oxygen gas (torr) for a human at rest:

• Outside air = 160
• Alveolar air = 105
• Arteriole blood = 100
• Venous blood = 40
• Cells << 40

Only ~ 7.5% of lung air is changed over on one breath at rest.

References