Nonrespiratory Lung Functions

Lung Capacity and Volume

Lung volumes and capacities refer to phases of the respiratory cycle; lung volumes are directly measured while capacities are inferred.

Learning Objective

Differentiate among tidal volume, inspiratory reserve volume, expiratory reserve volume, and vital capacity of lungs

Key Takeaways

Key Points

  • Lung capacity is a measure of lung volume inferred from the exhaled during the various cycles of breathing.
  • There is residual air leftover in the lungs during normal breathing.
  • Vital capacity is used to diagnose restrictive diseases, while the FEV1/FVC ratio is used to diagnose obstructive diseases.
  • FEV1/FVC ratio declines as someone ages, but declines faster in those who smoke due to damage caused by smoking.

Key Terms

  • FEV1/FVC ratio: The ratio between forced expiratory volume and forced vital capacity, which is used to measure the level of obstruction in the lungs.
  • vital capacity: The maximum volume of air that can be discharged from the lungs following maximum inspiration.


  • Pulmonary function tests (PFTs) may be used to help diagnose different pulmonary diseases. The two most often used measurements are FVC (forced vital capacity) and FEV1 (forced expiratory volume in one second).
  • An FEV1/FVC ratio of >80% indicates a restrictive lung disease like pulmonary fibrosis or infant respiratory distress syndrome.
  • An FEV1/FVC ratio of <70% indicates an obstructive lung disease like asthma or COPD.

Lung Capacity

Lung capacity generally refers to the total amount of air inside the lungs at certain phases of the respiratory cycle. It is usually measured as the amount of air that is exhaled after inhalation; this is measured with a device called a spirometer.

There are many different types and terms for the different components of lung capacity that all have different characteristics. In general, measuring lung capacity is important because it serves as the best indicator of lung health by quantifying the functional ability of the lungs to cycle air.

Vital Capacity

Vital capacity (VC) is the maximum amount of air that a person can exhale after inhaling as much air as possible. It is also the sum of tidal volume and the inspiratory and expiratory reserve volumes, which capture the differences between normal breathing and maximal breathing.

The inspiratory reserve volume is the extra space for air after a normal inspiration, and the expiratory reserve volume is the extra air that can be exhalaed after a normal expiration. VC tends to be decreased in those with restrictive lung diseases, such as pulmonary fibrosis, making VC a good diagnostic indicator of restrictive lung diseases.

Other important lung volumes related to lung capacity are residual volume (RV) and total lung capacity (TLC).

  • RV: The amount of air left in the lungs after a maximal expiration.
  • TLC: The volume of the lungs at maximal inflation, which is the sum of VC and RV.

FEV1/FVC Ratio

The most widely used diagnostic application for lung capacities is the ratio between forced expiratory volume (FEV1) and forced vital capacity (FVC).

  • FEV1: The volume of air exhaled in one second of forced expiration.
  • FVC: The total volume exhaled air during a forced expiration.

The FEV1/FVC ratio is an important indicator of lung health and is the standard approach for diagnosing COPD (chronic pulmonary obstructive disease), which includes emphysema and bronchitis, which are both caused by smoking. An FEV1/FVC ratio that is greater than .8 indicates a normal lung with generally healthy function, however a ratio below .8 indicates a significant degree of airway obstruction and suggests COPD.

This is a graph of lung capacity at the various stages of the respiratory cycle, which is one inhalation followed by an exhalation. The events during this cycle are labeled, from left to right: resting tidal volume, inspiratory reserve volume, residual volume, expiratory reserve volume, total lung capacity, and vital capacity.

Lung Capacity: Lung capacity at the various stages of the respiratory cycle, which is one inhalation followed by an exhalation.

The obstruction becomes worse the lower the ratio becomes, which increases the likelihood of respiratory failure and death. The FEV1/FVC ratio naturally falls as humans age, however smoking (the cause of COPD) will cause much larger decreases in FEV1/FVC ratio than what is normal.

Smoking causes this damage by initiating an inflammatory response in the lungs. Those who quit smoking will not experience a regain the FEV1/FVC ratio lost from smoking, however their rate of FEV1/FVC ratio decline will slow to normal, and their life expectancy will be less impacted.

Those with asthma, an acute form obstructive lung disease, will show a low FEV1/FVC ratio during an asthma attack, which returns to normal after the attack is over. Therefore, to diagnose asthma, many clinicians expose patients to methacholine or histamine to trigger mild asthma attacks to measure FEV1/FVC ratios.

Nonrespiratory Air Movements

The lungs have a number of metabolic functions in addition to their functions in gas exchange.

Learning Objective

Discuss the non-respiratory air movements of the respiratory system

Key Takeaways

Key Points

  • The lungs have a number of metabolic functions, such as the secretion of ACE (angiotensin converting enzyme), which converts angiotensin I to angiotensin II to stimulate changes in the renal system.
  • Higher levels of ACE lead to higher blood pressure. ACE inhibitors are used to treat hypertension by reducing ACE to reduce blood pressure.
  • Airway epithelial cells can secrete a variety of molecules—immunoglobulins (IgA), proteases, reactive oxygen species, and antimicrobial peptides—that all help protect the lungs and body from pathogens.
  • Non-respiratory air movements are mechanical functions that aren’t involved in gas exchange, such as voice production and coughing.

Key Terms

  • ACE: Angiotensin converting enzyme, which is secreted in the lungs and helps to increase blood pressure in the body through renal system feedback loops.
  • Airway epithelial cells: Airway epithelial cells can secrete a variety of molecules that aid in the immune system defense of lungs.


Non-respiratory air movements do not involve gas exchange. Examples are: sneezing, coughing, burping, laughing, singing, and talking.

While the primary function of the lungs is gas exchange, they have several other functions, which are both metabolic and mechanical. These include the secretion of many enzymes and proteins involved in other body systems and nonrespiratory air movements.

Metabolic Functions

The lungs secrete many enzymes and proteins that serve non-respiratory metabolic functions.

ACE (angiotensin converting enzyme) is an enzyme secreted by the endothelial cells of the capillaries in the lungs. ACE converts angiotensin I into angiotensin II, which are two important hormones in the renin-angiotensin feedback loop of the renal system.

This is a schematic diagram of the renin-angiotensin-aldosterone system. It shows how the renin-angiotensin system is dependent on ACE from the lungs to regulate blood pressure. ACE converts angiotensin I into angiotensin II, which are two important hormones in the renin-angiotensin feedback loop of the renal system. ACE activity results in increased blood pressure.

The renin-angiotensin-aldosterone system: The renin-angiotensin-aldosterone system is dependent on ACE from the lungs to regulate blood pressure. ACE activity results in increased blood pressure.

This system works to regulate blood pressure and blood volume by changing the amount of water retained by the kidneys. In general, more ACE leads to more angiotensin II, which leads to more aldosterone, which leads to more retained water through sodium reabsorption in the kidney, which leads to increased blood volume and blood pressure.

ACE inhibitors are a common treatment for those with hypertension, as it will reduce the amount of ACE, which will cause the kidney to excrete more water, which lowers blood volume and blood pressure.

The epithelial cells and macrophages of the lungs secrete many molecules that have immune system functions. In general these molecules have anti-microbial functions.

  • Immunoglobin A (IgA): An antibody that can attack pathogens and mark them for phagocytosis from macrophages and neutrophils.
  • Protease: Secreted from lung macrophages and neutrophils during inflammatory response to damage pathogens. A fibrinolytic that can break up thrombosis (blood clots) in the lungs.
  • Reactive oxygen species (ROS): Free radicals, which are any substance with an unpaired electron in the valence shell, can cause oxidative stress (damage) in cells. They are used to kill pathogens after being engulfed (phagocytized) by immune cells.
  • Anti-microbial peptides: Various chemokines and proteins that are secreted by the mucus membranes of the airways. They can damage and inhibit pathogens and are considered a barrier component of the immune system.

Mechanical Functions

There are several types of non-respiratory air movements that have important functions that are not primarily related to gas exchange. One example is voice production for speaking and singing, which involves fine control over the direction and flow of the air as it passes into the upper respiratory tract.

Other mechanical functions include sneezing and coughing, which protect the lungs and airways from irritants that could potentially cause damage. Coughing is a result of constriction from nervous stimulation in the trachea and larynx and also serves to dislodge mucus trapped inside the lungs.