The air as it enters the lungs meets with two types of resistances: 1. Elastic Resistance 2. Non-Elastic Resistance.
Type # 1. Elastic Resistance:
Elastic resistance is exerted by the elastic tissue of the lung parenchyma and surface tension of intra-alveolar fluid. Elastic tissues of the lungs obey Hooke’s law which means that change in volume of lungs is directly proportional to the force applied that is to intrapleural pressure. Lung compliance has been defined as the change in lung volume in litre per cm change of H2O pressure.
This is measured under static conditions to avoid frictional resistance to air flow which occurs during movement of air. After a known volume of air has been inspired and respiratory movement is ‘zero’ – the change in intrapleural pressure is noted. The change in lung volume (ΔV) divided by change in pressure (ΔP) is the measure of the lung compliance or stretchability.
Normal value ranges from 0.09 to 0.26 litre/cm H2O. Diminution in the lung compliance indicates increased rigidity of the lungs, i.e. diminished elasticity as may happen in congestion, fibrosis, and emphysema and in various other conditions.
Type # 2. Viscous or Non-Elastic Resistance:
This is the resistance offered to the air flow into the lungs due to friction between moving column of air and the branching bronchial tubes which get narrower and narrower at each branching (airway resistance) and also the frictional resistance between the muscle fasciculi of the chest wall and tissues.
Unlike the elastic resistance which is to be measured only when air flow is ‘zero’ frictional resistance comes into play during air flow and can be measured only during active air flow. It is also to be noted that the flow of air in the bronchial system is turbulent and the turbulency increases during broncho constriction.
Turbulency is responsible for increase in viscous resistance – the relationship between the ‘resistance’ and ‘rate of flow’, i.e., velocity is given below:
The relationship between frictional resistance (R) and velocity (V) of air flow is given by the formula:
R = K X V where K is a density dependent constant and ‘V’ is the velocity of air flow. Note that if the velocity of air-flow is doubled the resistance increases 4 times. Typical asthma patient breaths slowly and any attempt to breath rapidly increases their distress.
The diagram (Fig. 8.11) has been constructed by integration of the volume of air moved with that of intrapleural pressure from moment to moment during one respiratory cycle. The data have been obtained from previous diagram (Fig. 8.5).
Line AO of the ‘ellipse’ OFAE represents elastic resistance. The elliptical nature of the pressure-volume curve is due to frictional resistance to air flow. The area OFAB is the total work of inspiration. The area OAB is work done against elastic forces. Area OFA is work against viscous resistance during inspiration.
The energy OAB stored in the lungs during inspiration is available to do the work OAE necessary to overcome the frictional resistance during expiration. The pressure exerted against ‘viscous’ or non-elastic resistance (DQ) is obtained by subtracting the elastic pressure (DQ) from the total pressure (CD) exerted at any point of the respiratory cycle. At the point C this is CD – CQ = DQ.
Work of Lungs:
It can be calculated that the lungs perform work of the order of 0.4 kg-m per minute during quiet breathing of which 60% is spent to overcome the elastic resistance and about 40% is spent to overcome the non-elastic or viscous resistance.