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The average pair of human lungs can hold about 6 litres of air, but only a small amount is used during normal breathing. Different lung volumes and capacities measure various features about the lungs. These volumes vary with the age and height of the person, the values here are for a 70 kg, average-sized adult male:
- Total lung capacity (TC), about six litres, is all the air the lungs can hold.
- Vital capacity (VC) The maximum volume of air that can be expelled at the normal rate of exhalation after a maximum inspiration
- Tidal volume (TV) is the amount of air breathed in or out during normal respiration. It is normally from 450 to 500 mL.
- Residual volume (RV) is the amount of air left in the lungs after a maximal exhalation. This averages about 1.5 L.
- Expiratory reserve volume (ERV) is the amount of additional air that can be breathed out after normal expiration. This is about 1.5 L.
- Inspiratory reserve volume similarly, is the additional air that can be inhaled after a normal tidal breath in. About 2.5 more litres can be inhaled.
- Functional residual capacity (ERV + RV) is 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 space is the volume of the airways.
There is approximately 10% replacement of alveolar air per normal breath. This maintains very constant O2 and CO2 levels in the alveoli.
Factors affecting lung volume
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.
The tidal volume, vital capacity, inspiratory capacity and expiratory reserve volume can be measured directly with a spirometer. Determination of the residual volume is more difficult (a given percentage of inert gas is added to the breathed air, and the amount by which it is diluted is used to calculate the volume in which it was diluted.)
These are the basic elements of a ventilatory pulmonary function test. The results can be used to divide pulmonary diseasese into restrictive diseases, in which the volumes are decreased, and obstructive disease, 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.
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