The lungs bring in oxygen and release carbon dioxide, providing the essential elements of life. The capacity of the lungs to exchange these gases in the blood is an indicator of the respiratory system's efficiency and directly impacts all body systems and disease.
Each breathing cycle has two parts: inhalation and exhalation, also known as inspiration and expiration. These steps are controlled by the nervous system and respiratory muscles, with expiration being a passive event requiring no energy or exertion under normal conditions.
Pressure plays an important role in breathing. There are three pressures of interest in lung efficiency and function: atmospheric, intrapleural, and intrapulmonic. The atmospheric pressure is the pressure of the external environment.
The intrapleural pressure is the pressure within the potential pleural space between the membranes surrounding the lungs, the parietal and visceral pleura. This is a potential space because there should not be a space there and the pressure should always be slightly below atmospheric, referred to as negative pressure. If injury causes a space, by wound or internal hemorrhage, this pressure increases and compresses the lungs, causing restrictive breathing and possibly asphyxiation. This is a collapsed lung.
The intrapulmonic pressure is the pressure within the lungs. This is the pressure that fluctuates above and below atmospheric, resulting in breathing. This is also the pressure that determines lung capacity, which varies with age and the size of the body.
As one ages, the elasticity of the alveoli diminishes, resulting in a lower capacity. Illness or injury also affects the capacity to bring in and exhale air due to obstruction by phlegm, inflammation, swelling, infection, and foreign objects. The vital capacity of the lungs, the amount of air that can be taken in and blown out with the most forceful inhalation and exhalation is, on average, 3.5 to 5 liters of air.
Tidal volume is the amount of air involved in one normal inhalation and exhalation. The average tidal volume is 500 mL, but is less for shallow breathing. The minute respiratory volume is the amount inhaled and exhaled in one minute. It is determined by the tidal volume multiplied by the respirations per minute. The average is 6000 mL (or 6 liters) per minute.
The inspiratory reserve is the amount of air beyond the tidal volume that can be taken in with the deepest inhalation. This averages 2 to 3 liters. The expiratory reserve is the amount of air beyond tidal volume that can be exhaled forcefully, which is beyond the normal passive exhalation. The average is 1 to 1.5 liters. The vital capacity is the sum of the tidal volume and reserves.
There is always air in the lungs. This is called residual air. Even after the most forceful exhalation, 1 to 1.5 liters of air remain in the lungs. This ensures efficient and consistent gas exchange.
Reference:Scanlon and Sanders. Essentials of Anatomy and Physiology, 4th edition. Saunders.