Lung Volumes, Capacities and Pulmonary Ventilation

Hello :) In this post we'll be going through the various lung volumes and capacities that exist. We'll take a look at the effects of compliance and resistance on ventilation as well as the effect of surfactant on alveolar surface tension, compliance and ventilation. I'll discuss the factors that affect airway resistance and differentiate between restrictive and obstructive lung disease.

Lung Volumes and Capacities

Spirometry is a non invasive test that is used to asses pulmonary function. It measures the volume of air that moves into and out of the lungs. It can be used to help diagnose and distinguish between pulmonary diseases. A typical spirometry reading is shown below, it shows various volumes and capacities:

 

A lung volume is measured directly through spirometry it is the volume of air associated with different stages of the respiratory cycle. Lung volumes include:

• Tidal Volume: the volume of air that moves in and out of the lungs during a single unforced breath. It occurs during quiet respiration.
• Inspiratory Reserve Volume (IRV): The maximum volume that can be inspired after the end of a normal inspiration. It measures how much extra you can breathe in by forced inspiration.
• Expiratory Reserve Volume (ERV): the maximum volume that can be expired after the end of a normal expiration, it measures how much extra you can breathe out by forced expiration. 
• Residual Volume (RV): this is the volume of air remaining in the lungs after maximum expiration.

Tidal volume is the only lung volume that can be practically measured in animals while residual volume cannot be measured by spirometry in humans. This is because the spirometer only measures air that is breathed in or out.

A lung capacity is the sum of two or more lung volumes. Lung capacities include:

• Inspiratory Capacity (IC): this is the maximum volume that can be inspired at the end of resting expiration. It equals the sum of the tidal volume and the inspiratory reserve volume. ie:
  

  IC = VT + IRV

• Vital Capacity (VC): this is the maximum volume of air that can be expired following a maximum inspiration. The vital capacity is the sum of the tidal volume, inspiratory reserve volume and expiratory reserve volume. ie:
  

  VC = VT + IRV + ERV

• Functional Residual Capacity (FRC): this is the volume of air that is remaining in the lungs at the end of quiet expiration. It is the sum of the expiratory reserve volume and the residual volume. ie:
  

  FRC = ERV + RV

• Total Lung Capacity (TLC): this is the total volume of air in the lungs at the end of maximum inspiration. It is the sum of the vital capacity and residual volume or the sum of the tidal volume, inspiratory reserve volume, expiratory reserve volume and residual volume. ie:

  TLC = VC + RV

  OR

  TLC = VT + IRV + ERV + RV

Factors Affecting Pulmonary Ventilation

Pulmonary ventilation is the movement of air into (inspiration) and out of (expiration) the lungs. It is affected by several factors which will be discussed in this section.

Lung Compliance

Compliance describes how easily the lungs can be stretched. It is the change is lung volume resulting from a given change in transpulmonary pressure. An increased compliance is favourable because it means that the lungs stretch more easily. Thus less work is required to attain a certain lung volume. Compliance is dependent on the elasticity of the tissues in the lung and thoracic cage as well as alveolar surface tension. The alveoli are lined with a fluid. Attractive forces between the water molecules exist and this creates surface tension and makes it harder for the alveoli and lungs to expand. This surface tension is reduced by surfactant.

Surfactant is secreted by Type II alveolar cells and disrupts the attractive forces between the water molecules. Surfactant decreases surface tension causing an increase in compliance which makes it easier to breathe.

Resistance

Resistance is a force that opposes airflow. An increased resistance means that a greater pressure gradient is required to produce the same amount of flow. When considering the respiratory system, the two most important factors which influence resistance is the radius of the tube and the airflow velocity.

As the radius of an individual airway decreases the resistance increases. This can be demonstrated be breathing normally and then comparing this to when you breathe through a drinking straw. It is much more difficult to breathe through a drinking straw because it has a smaller radius than your airways and so more effort is required to inhale or exhale.

 Lung Disease

Restrictive lung disease involves a decreased compliance, it is caused by a decreased ability for expansion due to a problem with the lungs, pleura or thoracic wall. Examples include pulmonary oedema, pulmonary fibrosis, or surfactant deficiency.

An obstructive lung disease involves an increase in resistance and is caused by a narrowing of the airways causing an obstruction to the flow of air. This causes a difficulty in expiring and the lungs may over inflate.  Examples include Asthma and Chronic Obstructive Pulmonary Disease (COPD).