The Healthy Lung
Healthy lungs feature airways, known as bronchi and bronchioles, that are clear and open, allowing unobstructed air passage. The walls of these airways are thin and flexible, containing smooth muscle that maintains their shape. This structure supports efficient exchange of oxygen and carbon dioxide within the tiny air sacs called alveoli, where the lung tissue is also thin and pliable. Air flows freely and quietly through this network, facilitating normal breathing.
The lining of these airways, called the mucosa, is smooth and produces a minimal amount of thin mucus. This mucus traps small particles and is moved out by tiny hair-like structures called cilia, keeping the airways clean. The healthy lung is designed for efficient gas exchange, ensuring the body receives sufficient oxygen. This state provides a baseline for understanding changes in conditions like asthma.
Changes During an Asthma Flare-Up
During an asthma flare-up, the lungs undergo physical alterations that impede airflow. One primary change is bronchoconstriction, where the smooth muscles encircling the airways tighten. This muscular tightening narrows the internal diameter of the bronchial tubes, making it difficult for air to pass through and causing the characteristic wheezing sound. The rapid constriction reduces the space available for breathing.
The lining of the airways, or mucosa, becomes inflamed. Immune cells, responding to triggers, release substances that cause the airway walls to swell. This inflammation leads to a thickening of the airway lining, further constricting the passage and contributing to breathlessness. The swollen tissues reduce the overall volume within the tubes, making breathing feel labored.
The airways also produce an excessive amount of mucus. This mucus is thicker and stickier than usual, forming plugs that can further block the already narrowed bronchial tubes. The combination of tightened muscles, swollen linings, and obstructed passages due to mucus restricts airflow, leading to coughing, shortness of breath, and chest tightness.
Long-Term Structural Alterations
In individuals with persistent asthma, the airways can undergo chronic changes even between flare-ups, a process known as airway remodeling. One alteration is the thickening of the airway walls. Continuous inflammation and repair processes can lead to the deposition of structural proteins, making the walls less flexible and more rigid over time. This persistent thickening means the airways may never fully return to their original open state, even when symptoms are controlled.
An increase in the mass of the smooth muscle surrounding the airways is another change. Chronic inflammation can stimulate the growth of these muscle cells, leading to a thicker muscle layer. This enlarged muscle mass makes the airways more prone to constriction, contributing to heightened airway reactivity and a greater likelihood of future flare-ups. The increased muscle bulk can also reduce the overall capacity of the airways.
Subepithelial fibrosis can also develop, which involves the deposition of connective tissue, similar to scarring, just beneath the airway lining. This process contributes to the overall thickening and stiffening of the airway walls. The presence of this fibrous tissue reduces the elasticity of the airways, impairing their ability to expand and contract normally. The airway walls may also develop an increased number of blood vessels, contributing to ongoing inflammation and swelling.
How Doctors Visualize These Changes
Doctors utilize various methods to understand the functional and structural changes occurring in asthmatic lungs. Spirometry is a common diagnostic tool that measures how much air a person can exhale and how quickly they can do it. While spirometry does not directly show the physical appearance of the lungs, the reduced airflow measurements it provides are a direct consequence of the narrowed and obstructed airways observed in asthma.
Imaging techniques, such as chest X-rays or computed tomography (CT) scans, can reveal indirect signs of asthma’s impact. A chest X-ray might show signs of hyperinflation, indicating that air is trapped in the lungs due to airway obstruction. High-resolution CT scans can visualize thickened bronchial walls, which reflects the chronic inflammation and remodeling described earlier. These scans provide a structural snapshot of the lungs.
A bronchoscopy may be performed, though it is not a routine procedure for diagnosing asthma. During a bronchoscopy, a thin, flexible tube with a camera is inserted into the airways, allowing a direct visual inspection of the internal lining. This view can show inflamed, swollen airway walls and the presence of excess mucus, providing a real-time assessment of acute changes.