Fundamentals | Respiratory system | Physiology

Physiology of the respiratory passages: Functionally, the respiratory passages are divided into the conducting and respiratory passages. The conducting airways extend from the nose up to the terminal bronchioles. Their function is facilitation of air passage and to warm, filter and moisten the inspired air. The respiratory airways are primarily involved in gas exchange and extend from the respiratory bronchioles to the alveolar sacs.

Smooth muscle is present in large and small airways of the conducting zone and in the walls of the alveolar ducts. They are innervated by both the sympathetic and parasympathetic nervous system. Beta 2 receptors are present throughout the lung, including the alveolar airspace where it facilitates alveolar fluid clearance. Distal airways and alveoli have the greatest amount of beta 2 receptors. Beta 2 receptor stimulation causes bronchodilation by relaxing the smooth muscle. Both M2 and M3 parasympathetic receptors are present. M2 are postganglionic, parasympathetic autoreceptors which are activated by ACh and inhibit further ACh secretion. M3 are present on smooth muscle. Parasympathetic i.e. vagal stimulation results in bronchoconstriction, glandular secretion and vasodilation of bronchial vessels. Vagal stimulation causes constriction of the major bronchi but does not affect the respiratory bronchioles and alveoli as they are not innervated. In asthmatics the resting airway tone is higher, airways are hyperreactive and there is dysfunction of the M2 receptors. Major basic protein of eosinophils and viral enzymes like neuraminidase can antagonize the M2 receptor function.

Thin alveolar walls and large surface area helps in gas exchange and diffusion. Macrophages or “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 refer to the volumes present in the lung 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 the residual volume, functional residual capacity or FRC and total lung capacity or TLC. FRC can be measured by body plethysmography, helium dilution or nitrogen washout methods.

Tidal volume (TV): It is the volume of air moving in or out of the lungs during quiet breathing. On average TV is 500 ml.
Inspiratory reserve volume (IRV): It is the maximum volume of air than can be inspired above the TV. It is about 3000ml.
Expiratory reserve volume (ERV): It is the maximum volume of air that can be forcefully expired from the lungs below the TV. It is about 1200 ml. Both the IRV and ERV help when the ventilatory requirements increase as in exercise.
Residual volume (RV): It is the volume of air remaining in the lungs after maximum expiration. It averages 1200 ml. This volume keeps the lungs from collapsing.
Inspiratory capacity (IC): It is the TV + IRV. It is the maximum volume of air that can be inspired after reaching the end of a normal, quiet expiration.
Functional residual capacity (FRC): It is RV + ERV. It is the volume of air remaining in the lungs after a normal, quiet expiration. It is determined by the opposing forces of elastic recoil of the lung, which pulls inward and elastic recoil of the chest wall, which tends to pull outwards. FRC is the equilibrium point between these two recoil forces.
Vital capacity (VC): It is derived as 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): It is 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): It is 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): It is 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 is the TLC. It can be derived by multiple formulas e.g. sum of VC + RV or IC + FRC or, TV + IRV + FRC, or TV + IRV + ERV + RV.

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 the following equation:

Ve = TV X RR where TV is the tidal volume and RR is the respiratory rate.

However, as not all inspired air reaches the alveoli on account of the anatomical dead space, actual ventilation is given by alveolar ventilation (Va) as follows -

Va = (TV - Vd) X RR, where Vd is the anatomic dead space volume.