Ventilation and Lung Mechanics

1. What is Ventilation?
2. The Pleural Seal
3. Muscles of Ventilation
4. Spirometry
5. Dead Spaces
6. Energy of Breathing
   • Compliance
   • Surface Tension 
7. Airway Resistance
   • Changes to Airflow
8. Disorders of Lung Mechanics and Ventilation
   • Emphysema
   • Asthma
   • Interstitial Lung Disease
   • Pneumothorax
   • Atelectasis
9. Quiz

• Ventilation is the physical movement of air in and out of the lungs.
• The pleura of the lungs is composed of two serous membranes. The outer surface of the lungs and the inner surface of the thoracic cavity are held together by the surface tension between the pleural surfaces. The pleural seal allows the lungs to expand as the thoracic cavity expands.
• Compliance is the ability of the lungs to expand and stretch. This is determined by the elastic tissue in the lungs and the surface tension forces of the fluid lining the alveoli. The easier it is to expand the lungs, the higher the compliance.
• The alveolar surface is lined by fluid which generates a surface tension, and this decreases the compliance of the alveoli.
• Surfactant helps reduce the surface tension, particularly on smaller alveoli by being closer together. This molecule also helps small and large alveoli to expand until both the large and small alveoli have equal pressure, thus preventing the small alveoli from collapsing into the larger alveoli.
• Energy is required to force air through the airways of the respiratory system during inspiration and expiration. Resistance to airflow is affected by the radius of the airway and as the radius decreases, resistance increases.
• Airflow in the respiratory tract should be laminar and not turbulent as this increases resistance.
• Emphysema is the destruction of the elastin of the alveolar wall without fibrosis causing the permanent enlargement of air spaces.
• Asthma is a chronic inflammatory condition of the respiratory airways that leads to narrowing of the airways, particularly in the bronchi and bronchioles.
• Interstitial lung disease involves a greater amount of collagen and extracellular matrix deposited in the interstitial space.
• Pneumothorax is the presence of air in the pleural space. The elastic recoil of the lungs is no longer counteracted, and the lungs collapse inwards.
• Atelectasis is the complete or partial collapse of an entire lung or lobe. 

In this article, the process of ventilation and the mechanisms involved in inspiration and expiration will be discussed, alongside pathologies that can affect a patient’s ability to ventilate their lungs.

What is Ventilation?

Ventilation is the physical movement of air in and out of the lungs. It can easily be confused with the process of respiration, which is the gaseous exchange of oxygen and carbon dioxide across a membrane either at a cellular level or in the lungs.

Quiet inspiration and quiet expiration occur when a person is unaware of their breathing. There is no extra effort to breathe, just the subconscious drive for oxygen.

Quiet Inspiration

At the beginning of quiet inspiration, the external intercostal muscles contract causing the thoracic cavity to move outwards, whilst the diaphragm flattens. Both of these movements are active and increase the space in the thoracic cavity. As the volume increases in the thoracic space, the pressure drops below atmospheric pressure causing air to be drawn into the lungs through the mouth and nose.

Quiet Expiration

The diaphragm and external intercostal muscle cease contracting as the elastic recoil of the lungs leads to the thoracic cavity and lungs returning to their original position. The volume of the thoracic cavity decreases therefore the intra-thoracic pressure increases causing air to be pushed out of the lungs. The process of air moving out of the lungs is passive as the muscles relax.

The Pleural Seal

A very small amount of pleural fluid exists between the visceral and parietal pleura and forms a seal between the lung and the thoracic wall. The outer surface of the lungs and the inner surface of the thoracic cavity are held together by the surface tension between the pleural surfaces.

Diagram - The pleura of the lungs

Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

The pleural seal allows the lungs to expand as the thoracic cavity expands.

The pressure of the pleura is negative at rest. This is because the parietal pleura is pulled out by the chest wall, and the visceral pleura is pulled in by the elastic recoil of the lung. The pleural pressure becomes more negative during inspiration due to the expansion of the chest wall, and this increased negative pressure helps the lungs to expand with the chest wall.  

The respiratory expiratory level is a point at the end of expiration, just before inspiration begins, when the respiratory muscles are relaxed and the lungs are exposed to two equal but opposing forces. The first is the elasticity of the lung, which favours a small lung volume, and the second is the forces exerted by the rib cage, which favours a large thoracic volume. These are balanced at the respiratory expiratory level.

Muscles of Ventilation

During quiet inspiration, the main muscle involved in expanding the volume of the chest is the diaphragm, with help from the external intercostal muscles. In contrast, quiet expiration is a passive process and relies on the elastic recoil of the lungs.

When a patient is forcing inspiration, they will use their accessory muscles of the inspiration and these include sternocleidomastoid, scalene, serratus anterior and pectoralis major muscles. The accessory muscles used during forced expiration are the internal intercostal muscles and abdominal muscles.

Spirometry

Spirometry is a type of pulmonary function test and assesses a patient’s lung volume in a dynamic way, meaning it can track inspiration and expiration. There are lots of different terms used when describing a patient’s spirometry reading and some of these terms can sound very similar, meaning it is important to understand each term.

Diagram - A spirometry trace with lung volumes

Creative commons source by Vihsadas, modified by Rscottweekly [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Tidal Volume - the volume of air that enters and leaves the lungs in each breath.

Inspiratory Reserve Volume - the volume of air that is the difference between the volume of quiet inspiration, and the maximum inspiratory volume possible. It occurs when a patient takes a normal breath in and then increases the volume of the lungs to the maximum capacity possible.

Expiratory Reserve Volume - the volume of air that is the difference between the volume of quiet expiration, and the maximum expiratory volume possible. It occurs when a patient takes a normal breath out and then decreases the volume of the lungs to the minimum capacity possible.

Residual Volume - after forced expiration, the lungs are not completely emptied as air must exist in the lungs for them to stay expanded. The remaining air in the lungs is known as the residual volume.

Lung Capacity - unlike the lung volumes which can change with changes in tidal volume, lung capacities cannot and are fixed relative to points in the cycle of ventilation.

Inspiratory Capacity - from the end of quiet expiration to the maximum inspiration, and this can be calculated by the addition of inspiratory reserve volume and tidal volume.

Functional Residual Capacity - the volume of air in the lungs at the end of quiet expiration. This can be calculated by the addition of expiratory reserve volume and residual volume

Vital Capacity - the maximal volume of air that can be expelled from a patient’s lung after maximal inspiration. This can be calculated by inspiratory capacity added to expiratory reserve volume OR inspiratory reserve volume plus tidal volume and expiratory reserve volume.

Total Lung Capacity - the volume of air present in the lungs after maximal inspiration. This can be calculated by the addition of vital capacity and residual volume.

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