Definition : Lung capacity

Breathing is a vital process that ensures the supply of oxygen to our body and the elimination of carbon dioxide produced by our cells. Physiologically, respiration comprises several key stages: pulmonary ventilation, gas exchange in the lungs, and gas transport in the blood.

Pulmonary ventilation, or simply external breathing, begins with inhalation, the act of taking air into the lungs. When the respiratory muscles, such as the diaphragm and intercostal muscles, contract, the rib cage expands and the volume of the lungs increases. This creates negative pressure inside the lungs, causing air to enter the airways. The air passes through the nose or mouth, traverses the trachea and bronchi, and reaches the pulmonary alveoli, where gas exchange takes place.

Pulmonary alveoli are tiny air sacs located at the end of the bronchioles. These structures are surrounded by blood capillaries, forming an interface between air and blood. When inhaled air reaches the alveoli, the oxygen it contains diffuses through the walls of the alveoli and capillaries, and binds to the hemoglobin present in the red blood cells. At the same time, carbon dioxide, a metabolic product, diffuses from the blood into the alveoli, ready to be exhaled.

Blood gases are transported by hemoglobin, a protein found in red blood cells. Once oxygen has been bound to hemoglobin, the red blood cells transport it to the body's tissues, where it is released for use in metabolic processes. At the same time, the carbon dioxide produced by the cells is transported by the red blood cells to the lungs, where it is released during exhalation.

The breathing process is controlled by the respiratory system, which comprises the respiratory center in the brain stem and the respiratory receptors in the lungs and blood vessels. The respiratory center automatically regulates the rate and depth of breathing according to the body's need for oxygen and the removal of carbon dioxide.

Lung capacity is measured using a technique known as spirometry, which assesses respiratory air volumes and flows. Spirometry is commonly used clinically to assess lung function, diagnose respiratory disorders and monitor the progression of these conditions. Here are the general steps involved in measuring lung capacity in a professional manner:

• Preparation: The patient is informed of the procedure and, if necessary, of specific instructions such as stopping the use of bronchodilators before the test. Spirometry equipment is checked and calibrated to ensure accurate measurements.
• Positioning : The patient is invited to sit comfortably in a chair, with a straight back. It is important that the patient is relaxed and cooperative throughout the test.
• Explanation: The healthcare professional explains to the patient how to perform the required breathing maneuvers correctly. This includes a deep inhalation followed by a forced, full exhalation into the spirometer.
• Tidal volume measurement: The patient is asked to breathe normally for a few breaths, and the tidal volume (the amount of air inhaled and exhaled with each breath) is recorded.
• Measurement of lung volumes: The patient is then instructed to perform various breathing maneuvers, such as maximum inspiration followed by maximum forced expiration (slow vital capacity maneuver) and maximum forced expiration after maximum inspiration (forced vital capacity maneuver). These maneuvers are used to measure various lung volumes, such as inspiratory reserve volume, expiratory reserve volume and residual volume.
• Recording respiratory flows: In addition to lung volumes, spirometry also measures respiratory flows, such as peak expiratory flow and peak expiratory flow. These measurements provide information on the rate at which air can be exhaled from the lungs.
• Repeating measurements: It is often necessary to repeat maneuvers to obtain reliable, consistent measurements. Several attempts can be made for each maneuver, taking into account the quality criteria defined by professional recommendations.
• Evaluation of results: Once the measurements have been taken, the data are analyzed by the healthcare professional. The results are compared with specific reference values based on the patient's age, sex and size, to assess lung function and diagnose any abnormalities.

Spirometry is a widely used and well-established method of measuring lung capacity. However, it is important that the test is carried out by a qualified healthcare professional, trained in the interpretation of results, to ensure an accurate assessment of an individual's lung function.

Lung capacity can be influenced by a variety of physiological, anatomical and pathological factors. Here is a professional description of the main factors that can have an impact on lung capacity:

• Body size and morphology: Lung capacity is partly determined by a person's body size and thoracic morphology. In general, taller individuals have a higher lung capacity than shorter ones, as their rib cage offers more space for the lungs to expand during inspiration.
• Age: Lung capacity tends to decrease with age, due to anatomical and physiological changes associated with aging. Lung tissue gradually loses its elasticity, and respiratory muscle strength may decline. However, the age-related decline in lung capacity varies from person to person.
• Gender: Men generally have slightly more lung capacity than women, due to anatomical differences such as the size of the thorax and diaphragm. However, these differences can be offset by other factors such as body size and level of physical activity.
• Fitness level: A good level of fitness, particularly high cardiorespiratory endurance, is associated with greater lung capacity. Regular exercise strengthens the respiratory muscles, improves pulmonary ventilation and promotes efficient gas exchange.
• Lung pathologies: Certain pulmonary conditions can lead to an alteration in lung capacity. For example, asthma, chronic bronchitis, emphysema and pulmonary fibrosis can reduce the lungs' ability to expand and contract normally. Restrictive diseases can lead to a reduction in total lung capacity, while obstructive diseases can affect maximum airflow during exhalation.
• Lifestyle: Factors such as smoking, exposure to air pollutants, including indoor and outdoor air pollution, and occupational exposure to toxic substances can damage the lungs and reduce lung capacity.
• Genetic factors: certain genetic diseases, such as cystic fibrosis, can alter the structure and function of the lungs, resulting in reduced lung capacity.

It's important to note that lung capacity is a global measure that can be influenced by several of these factors in combination. A thorough assessment of lung capacity must take these factors into account, and be interpreted in the light of each individual's specific clinical context.

Lung capacity is of vital physiological and medical importance. Here's a professional explanation of the importance of lung capacity:

• Respiratory function: Lung capacity is directly linked to respiratory function, which is essential for supplying our body with the oxygen it needs and for eliminating the carbon dioxide produced by our cells. Adequate lung capacity guarantees optimal ventilation and efficient gas exchange between the lungs and the blood, ensuring an adequate supply of oxygen for organ and tissue function.
• Detection of lung pathologies: The measurement of lung capacity, notably through spirometry, enables the detection and diagnosis of various pulmonary conditions such as asthma, chronic bronchitis, emphysema, pulmonary fibrosis and other restrictive or obstructive diseases. Decreased lung capacity can be an early indicator of these conditions, enabling early medical intervention and management.
• Lung disease monitoring: As part of lung disease monitoring, regular measurement of lung capacity enables us to assess the evolution of lung function over time, and the effectiveness of treatment. It can also detect any exacerbations or deterioration in lung function, enabling treatment strategies to be adjusted accordingly.
• Prevention and management of respiratory disorders: A precise assessment of lung capacity enables us to identify individuals at risk of developing respiratory disorders, and to implement appropriate preventive measures. In addition, as part of pulmonary rehabilitation, measuring lung capacity can help assess patient progress and adapt exercise programs and rehabilitation strategies.
• Fitness assessment: Lung capacity is an important indicator of overall fitness. High lung capacity is associated with better cardiorespiratory endurance and performance in physical activities. It is also used as a benchmark to determine fitness for certain activities, such as scuba diving or working in low-oxygen environments.

Lung capacity plays a crucial role in the normal functioning of the body, the detection and management of lung disease, and the assessment of physical fitness. Its assessment enables health professionals to diagnose lung conditions, monitor their evolution and adapt treatments accordingly, thereby helping to promote respiratory health and improve quality of life.

There are several medical conditions that can affect lung capacity, thus impairing respiratory function. Here's a professional explanation of the main medical conditions that can impact lung capacity:

• Chronic obstructive pulmonary disease (COPD): COPD is a term for conditions such as chronic bronchitis and emphysema. These conditions cause obstruction of the airways, limiting the flow of air in and out of the lungs. The airways can become obstructed by inflammation, excessive mucus production and narrowing of the bronchial walls. COPD leads to a progressive reduction in lung capacity, particularly peak expiratory flow.
• Asthma: Asthma is a chronic inflammatory disease of the airways, causing recurrent episodes of bronchospasm and airway obstruction. During asthma attacks, the airways narrow, making it difficult to exhale air from the lungs. This can lead to a temporary reduction in lung capacity.
• Pulmonary fibrosis: Pulmonary fibrosis is a condition in which healthy lung tissue is progressively replaced by scar tissue, making the lungs stiff and less elastic. This excessive scarring leads to a reduction in lung capacity, making it more difficult for the lungs to expand and contract normally during breathing.
• Restrictive lung diseases: Restrictive lung diseases are characterized by a decrease in total lung capacity due to stiffness or rigidity of the lungs or rib cage. This may be due to conditions such as severe pneumonia, systemic sclerosis, sarcoidosis, muscular dystrophy or certain neuromuscular diseases. Restrictive diseases lead to difficulty in breathing and reduced lung capacity.
• Pulmonary vascular diseases: Pulmonary vascular diseases, such as pulmonary hypertension, affect the blood vessels in the lungs. This can lead to increased resistance to blood flow in the pulmonary vessels, which can impact on the flow of oxygen in the blood and, consequently, lung capacity.
• Neuromuscular diseases: Certain neuromuscular diseases, such as muscular dystrophy, amyotrophic lateral sclerosis (ALS) and myasthenia gravis, can affect the respiratory muscles, including the diaphragm. This can lead to muscle weakness and reduced lung capacity.

These medical conditions can affect lung capacity in different ways, whether by reducing specific lung volumes, limiting airflow or disrupting gas exchange. It is essential that these conditions are diagnosed and managed by qualified healthcare professionals to ensure appropriate management and minimize the effects on patients' lung function and quality of life.