In modern medicine, it is common to find different healthcare awareness programs and patient-centered learning models centered on chronic respiratory conditions in humans. On the global scale, these conditions are considered the leading cause of mortality, morbidity, economic burden, and disability-adjusted life years in many people. Chronic respiratory diseases (CRDs) primarily affect the respiratory system's biological architecture, including the lungs, the pulmonary vasculature, the trachea, and the parenchyma. Over the years, the focus of care on these conditions has shifted rapidly to include the design of optimal treatment and/or prevention plans in addition to different methods to alleviate or control symptomatology. Although the number of conditions that directly falls under the general description of CRDs is quite plenty, asthma, COPD, interstitial lung diseases, lung cancer, and cystic fibrosis are commonly described. Tuberculosis, a major communicable respiratory disease, has also been discussed recently in light of an increasing antibiotic resistance expressed by the bacterial pathogen -Mycobacterium tuberculosis.
Since CRDs primarily attack the biological architecture of the respiratory tracks, understanding the physiology and morphology of the respiratory system better informs of the healthcare burden of CRDs. The respiratory tract is subdivided into two major parts – the upper and lower respiratory tract. The upper respiratory tract consists of the nose, mouth, and the initial portion of the trachea. The lower respiratory tract, on the other hand, is composed of the bronchioles, the later part of the trachea that progressively terminates into the bronchi, and the functional respiratory units known as the alveoli. Functionally, the complete processes of gas exchange occur with the support of the pulmonary vasculature as the airways derive consistent support from the lung parenchyma (Mikolasch et al., 2022). An essential function of the respiratory system is the exchange of gases – a process described exclusively as the intake of oxygen (O2) and the disposal of carbon dioxide (CO2). The single membranes of the pulmonary alveolus assume a significant role in this process as the exchange of gases occurs in their thin films.
The process of breathing includes inhalation, that is, the primary expansion of chest volume for the intake of air. The inhaled air then moves through the smaller "conductive" airways, and exhalation occurs. Exhalation is the release of air from the lungs as the chest volumes progressively contract. Two main muscle types facilitate the breathing process – the diaphragm and intercostal muscles. (Thomen et al., 2020). On average, the weight of a normal human lung is estimated at 1 kg, with around half (40%–50%) of that volume attributed to blood in the pulmonary vasculature. To establish the presence of obstructive lung diseases or a progressive lung impairment, conducting a measurement of the volume and action of breathing is important; this principle is essential in diagnosing lung diseases like asthma and COPD.
The complete path of pulmonary mechanics includes the efficient exchange of gas and requires biological input from the multiple components of the respiratory tract. Although it looks automated, efficient gas exchange depends directly on the consistency and accuracy of different measurements unutilized in monitoring the normal functioning of the lungs. The means and aids for conducting these measurements have been consistently overhauled and improved over the last few decades. In modern medicine, these measurements are conducted using the principles of spirometry. Spirometry is a safe, practical, and reproducible test to determine the ventilatory capacity of the lungs and identify any changing trends in the functionality of the lung concerning air capacity and gas exchange efficiency. Modern spirometers have the added advantage of generating prompt computer-aided feedback to the operator on the quality and repeatability of the test. Results can also include a real-time, reproducible digital flow chart of the flow-volume curve.
Modern spirometry measurements are directly explored in the assessment and care of chronic respiratory conditions, including tidal volume (TV), inspiratory reserve volume (IRV), and residual volume (RV). Other standard lung capacities that are routinely measured are inspiratory capacity (IC), vital capacity (VC), expiratory reserve volume (ERV), functional residual capacity (FRC), and total lung capacity (TLC) (Rocha et al., 2021).
Fig. 1. Example of a spirometry measurement.
To an extent, the value of these measurements depends directly on the healthiness of the lungs and the associated respiratory apparatus. A normal lung has several cell types that have highly specialized functions. The major cell types found in a healthy one include airway epithelial cells, goblet cells, ciliated cells, and Clara cells. Others may include neuroendocrine, basal, and type I and II alveolar cells. In addition, there are immune, stem, and adipose cells in the lungs. In different CRDs, the biological functionalities of one or more of these cells are completely or partially impaired. For instance, research reports have established a link between lung carcinomas and glandular cells in the lung, especially in subjects with multiple risk factors.
In addition, the pathology of respiratory diseases, such as neonatal respiratory failure, interstitial lung disease (ILD), alveolar proteinosis, and other rare lung diseases, seems to have a link with impairments in the secretion of surfactant proteins and lipids. Changes in the pulmonary vasculature, including smooth muscle hypertrophy and intimal hyperplasia, have been linked to severe hypoxic vasoconstriction of the small pulmonary arteries (Dekkers et al., 2021). Changes to the pulmonary architecture greatly impair respiratory function, forcing the basal cells in the airway epithelium to initiate a series of processes that permanently change the airway architecture. The airway's physical barriers and host-environment defense arrangement are important factors in pulmonary inflammation and the subsequent activation of immune cells; this may also be implicated in the complicated process of pulmonary remodeling and repair (Eenjes et al., 2022).