Physiology of the respiratory passages: Functionally, the respiratory passages are divided into conducting and respiratory passages.
The conducting airways extend from the nose to the terminal bronchioles. Their functions are to:
The respiratory airways are primarily involved in gas exchange. They extend from the respiratory bronchioles to the alveolar sacs.
Smooth muscle is present in the large and small airways of the conducting zone and in the walls of the alveolar ducts. These smooth muscles are innervated by both the sympathetic and parasympathetic nervous systems.
Beta 2 receptors are present throughout the lung, including the alveolar airspace, where they facilitate alveolar fluid clearance. The distal airways and alveoli have the greatest density of beta 2 receptors. Beta 2 receptor stimulation causes bronchodilation by relaxing airway smooth muscle.
Both M2 and M3 parasympathetic receptors are present:
Parasympathetic (vagal) stimulation results in bronchoconstriction, glandular secretion, and vasodilation of bronchial vessels. Vagal stimulation constricts the major bronchi but does not affect the respiratory bronchioles and alveoli because they are not innervated.
In asthma, resting airway tone is higher, the airways are hyperreactive, and there is dysfunction of the M2 receptors. Major basic protein from eosinophils and viral enzymes such as neuraminidase can antagonize M2 receptor function.
Thin alveolar walls and a large surface area support efficient gas exchange and diffusion. Macrophages (“dust cells”) engulf debris and dust particles and migrate up to the bronchioles, from where they are ultimately coughed out. In chronic smokers and city dwellers, carbon-laden macrophages are commonly seen.
Lung volumes and capacities: Lung volumes are the volumes present in the lungs at various phases of respiration. Lung capacities are derived by adding two or more lung volumes.
All lung volumes can be measured by spirometry except:
FRC can be measured by body plethysmography, helium dilution, or nitrogen washout methods.
Tidal volume (TV): The volume of air moving in or out of the lungs during quiet breathing. On average, TV is 500 ml.
Inspiratory reserve volume (IRV): The maximum volume of air that can be inspired above the TV. It is about 3000 ml.
Expiratory reserve volume (ERV): The maximum volume of air that can be forcefully expired from the lungs below the TV. It is about 1200 ml. Both IRV and ERV help when ventilatory requirements increase, such as during exercise.
Residual volume (RV): The volume of air remaining in the lungs after maximum expiration. It averages 1200 ml. This volume helps keep the lungs from collapsing.
Inspiratory capacity (IC): TV + IRV. This is the maximum volume of air that can be inspired after reaching the end of a normal, quiet expiration.
Functional residual capacity (FRC): RV + ERV. This is the volume of air remaining in the lungs after a normal, quiet expiration. It is determined by opposing elastic recoil forces:
FRC is the equilibrium point between these two recoil forces.
Vital capacity (VC): TV + IRV + ERV. It can be measured by spirometry. It is the maximum volume of air that can be expired after maximal inspiration. It decreases with age.
Forced vital capacity (FVC): The volume of air that can be exhaled forcefully after a maximal inspiration. In healthy individuals, there is little or no difference between VC and FVC. However, FVC < VC in asthma, COPD, and obstructive lung disorders due to air trapping, small airway collapse, and obstruction to airflow.
Forced expiratory volume in one second (FEV1): The maximum volume of air that can be breathed out of the lungs in one second. Normally it is more than 80% of FRC. It is decreased in both obstructive and restrictive lung diseases. FEV1 is assessed to monitor the severity and progression of lung disease.
Forced expiratory flow at 25-75% (FEF 25-75): The forced expiratory flow between 25-75% of FVC. It has been related to small airway disease. Lower FEF 25-75 has been associated with increased severity of asthma.
Total lung capacity (TLC): The volume of air contained in the lungs at the end of a maximal inspiration. TLC can be derived using multiple formulas, for example:
Minute ventilation and alveolar ventilation: Minute ventilation (Ve) is the total volume of gas entering or leaving the lung per minute. It is given by:
Ve = TV X RR where TV is the tidal volume and RR is the respiratory rate.
However, not all inspired air reaches the alveoli because of anatomic dead space. The actual ventilation of alveoli is given by alveolar ventilation (Va):
Va = (TV - Vd) X RR, where Vd is the anatomic dead space volume.
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