Compliance of the Lungs
The extent to which the lungs will expand for each unit increase in transpulmonary pressure . The total compliance of both lungs together in the normal adult human averages about 200 ml of air /cm of water transpulmonary pressure.Compliance diagram The characteristics of the compliance diagram are determined by the elastic forces of the lungs. These can be divided into two parts: (1) elastic forces of the lung tissue itself and (2) elastic forces caused by surface tension of the fluid that lines the inside walls of the alveoli and other lung air spaces
The elastic forces of the lung tissue are determined mainly by elastin and collagen fibers. In deflated lungs, these fibers are in an elastically contracted and kinked state. when the lungs expand, the fibers become stretched and unkinked, thereby elongating and exerting even more elastic force.
* When the lungs are filled with air, there is an interface between the alveolar fluid and the air in the alveoli. * In the case of the saline solution–filled lungs, there is no air-fluid interface, the surface tension effect is not present and only tissue elastic forces are operative in the saline solution–filled lung.
The pressures required to expand air-filled lungs are about three times as great as those required to expand saline solution–filled lungs. Conclusion that the tissue elastic forces tending to cause collapse of the air-filled lung represent only about one third of the total lung elasticity. fluid-air surface tension forces in the alveoli represent about two thirds.-The fluid-air surface tension elastic forces of thelungs also increase when the surfactant is not present in the alveolar fluid.
Surfactant, Surface Tension
Principle of Surface Tension. When water forms a surface with air, the water molecules on the surface of the water have an especially strong attraction for one another. As a result, the water surface is always attempting to contract. This is what holds raindrops together: that is, there is a tight contractile membrane of water molecules around the entire surface of the raindrop.If reverse these principles, on the inner surfaces of the alveoli, the water surface is also attempting to contract. This results in an attempt to force the air out of the alveoli through the bronchi and, in doing so, causes the alveoli to try to collapse. The net effect is to cause an elastic contractile force of the entire lungs, which is called the surface tension elastic force.
Surfactant is a complex mixture of several phospholipids, proteins, and ions. greatly reduces the surface tension of water. It is secreted by special surfactant-secreting epithelial cells called type II alveolar epithelial cells, which constitute about 10 % of the surface area of the alveoli. .
Surfactant normally begin to be secreted into the alveoli between the sixth and seventh months of gestation. Therefore, many premature babies have little or no surfactant in the alveoli when they are born, and their lungs have an extreme tendency to collapse, more than that in a normal adult person. This causes the condition called respiratory distress syndrome of the newborn. It is fatal if not treated .
pressure in occluded alveoli If the air passages leading from the alveoli of the lungs are blocked, the surface tension in the alveoli tends to collapse the alveoli. This creates positive pressure in the alveoli, attempting to push the air out. The amount of pressure generated in this way in an alveolus can be calculated from the following formula:
If an average-sized alveolus with a radius of about 100 micrometers and lined with normal surfactant, this calculates to be about 4 centimeters of water pressure(3 mm Hg). If the alveoli were lined with pure water without any surfactant, the pressure would calculate to be about 18 centimeters of water pressure, 4.5 times as great.
Alveolar radius and Pressure caused by surface tension The pressure generated as a result of surface tension in the alveoli is inversely affected by the radius of the alveolus, (the smaller the alveolus, the greater the alveolar pressure caused by the surface tension). Thus, when the alveoli have half the normal radius (50 instead of 100 micrometers), the pressures are doubled. This is especially significant in small premature babies, many of whom have alveoli with radii less than one quarter that of an adult person.
Effect of the thoracic cage on lung expansibility The thoracic cage has its own elastic and viscous characteristics, similar to those of the lungs; even if the lungs were not present in the thorax, muscular effort would still be required to expand the thoracic cage.
Compliance of the thorax and the lungs together The compliance of the entire pulmonary system (the lungs and thoracic cage together) is measured while expanding the lungs of a totally relaxed or paralyzed person. To do this, air is forced into the lungs a little at a time while recording lung pressures and volumes. To inflate this total pulmonary system, almost twice as much pressure is needed as to inflate the same lungs after removal from the chest cage.
The compliance of the combined lung-thorax system is one half that of the lungs alone—110 ml of volume per cm of water pressure for the combined system, compared with 200 ml/cm for the lungs alone.
“Work” of Breathingduring normal quiet breathing, all respiratory muscle contraction occurs during inspiration; expiration is almost entirely a passive process caused by elastic recoil of the lungs and chest cage. Thus, under resting conditions, the respiratorymuscles normally perform “work” to cause inspiration but not to cause expiration.
The work of inspiration can be divided into three fractions: (1) that required to expand the lungs against the lung and chest elastic forces, called compliance work or elastic work; (2) that required to overcome the viscosity of the lung and chest wall structures, called tissue resistance work; and (3) that required to overcome airway resistance to movement of air into the lungs, called airway resistance work.
Energy Required for Respiration. During normal quiet respiration,only 3 to 5 per cent of the total energy expendedby the body is required for pulmonary ventilation. But during heavy exercise, the amount of energy required can increase as much as 50-fold, especially if the person has any degree of increased airway resistance or decreased pulmonary compliance. Therefore, one of the major limitations on the intensity of exercise that can be performed is the person’s ability to provide enough muscle energy for the respiratory process alone.