138 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.

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.

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.

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 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.