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.
Several factors affect lung volumes; some can be controlled and some cannot be controlled. Lung volumes vary with different people as follows:
|Larger volume||Smaller volumes|
|taller people||shorter people|
|people who live at higher altitudes||people who live at lower altitudes|
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. Also, due to the lower environmental air pressure at higher altitudes, the air pressure within the breathing system must be lower in order to inhale; in order to meet this requirement, the thoracic diaphragm has a tendency to lower to a greater extent during inhalation, which in turn causes an increase in lung volume.
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.)
Lung function development is reduced in children who grow up near motorways although this seems at least in part reversible. Air pollution exposure affects FEV1 in asthmatics, but also affects FVC and FEV1 in healthy adults even at low concentrations.
Specific changes in lung volumes also occur during pregnancy. Functional residual capacity drops 18–20%, 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% and decreased expiratory reserve volume by 20%. Tidal volume increases by 30–40%, from 0.5 to 0.7 litres, and minute ventilation by 30–40% 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.
|In men||In women|
|Inspiratory reserve volume (IRV)||3.0||1.9|
|Tidal volume (TV)||0.5||0.5|
|Expiratory reserve volume (ERV)||1.1||0.7|
|Residual volume (RV)||1.2||1.1|
|Volume||Average value (litres)||Derivation|
|In men||In women|
|Vital capacity||4.6||3.1||IRV + TV + ERV|
|Inspiratory capacity||3.5||2.4||IRV + TV|
|Functional residual capacity||2.3||1.8||ERV + RV|
|Total lung capacity||5.8||4.2||IRV + TV + ERV + RV|
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 (18.1 mL/kg), or as a proportion of vital capacity (0.24 for men and 0.28 for women) or in relation to height and age ((0.0275* Age [Years]+0.0189*Height [cm]-2.6139) litres for normal-mass individuals and (0.0277*Age [Years]+0.0138*Height [cm]-2.3967) litres for overweight individuals). Standard errors in prediction equations for residual volume have been measured at 579 mL for men and 355 mL for women, while the use of 0.24*FVC gave a standard error of 318 mL.
Online calculators are available that can compute predicted lung volumes, and other spirometric parameters based on a patient's age, height, weight, and ethnic origin for many reference sources.
How much does a breath of air weigh? The mass of one breath is approximately a gram (0.5-5g). A Litre of air weighs about 1.2 grams (1.2 kg per cubic metre). A half Litre ordinary tidal breath weighs just over half a gram; a maximal four Litre breath weighs almost five grams.
The results (in particular FEV1/FVC and FRC) can be used to distinguish between restrictive and obstructive pulmonary diseases:
|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 or Emphysema||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)|
Lung capacity can be expanded through flexibility exercises such as yoga, breathing exercises, and physical activity. A greater lung capacity is sought by people such as athletes, freedivers, singers, and wind-instrument players. A stronger and larger lung capacity allows more air to be inhaled into the lungs. In using lungs to play a wind instrument for example, exhaling an expanded volume of air will give greater control to the player and allow for a clearer and louder tone.