Introductory Human Physiology by Emma Jakoi & Jennifer Carbrey

Author:Emma Jakoi & Jennifer Carbrey [Jakoi, Emma]
Language: eng
Format: epub
ISBN: 9781312925199
Publisher: Lulu.com
Published: 2015-02-17T05:00:00+00:00

ELASTIC RECOIL AND COMPLIANCE

LUNG COMPLIANCE is defined as the stretchability of the lung for any 1-cm change in pressure across the lung. CL = ∆VL/ (PA – Pip)

The greater the compliance, the easier it is to expand the lungs at any given change in trans-pulmonary pressure. Compliance is the inverse of elastic recoil or stiffness.

Figure 74. Pressure-volume relationship during inflation of isolated lungs. In the restrictive lung disease, fibrosis, the lung shows decreased compliance and reduced volume (vital capacity). The emphysema lung shows increased compliance and increased volume (vital capacity). Image copyrighted by Rick Melges, Duke Univ.

The most common way to obtain a compliance curve is to have an individual inspire to total lung capacity and then exhale slowly in small increments. When airflow is temporarily stopped, volume and transpulmonary pressure are recorded. A pressure-volume curve is constructed (Figure 74). The slope of the pressure - volume curve at any given point is lung compliance at that point.

Note that the pressure-volume curve is not linear (Figure 74). At high lung volumes, the lungs are almost maximally stretched and a large change in pressure produces only a small change in volume. Therefore, compliance is usually measured in the mid-range of the pressure - volume curve during tidal volume breathing. A normal value for lung compliance at this point is 0.2 liter/cm H2O.

Lung compliance is determined in part by the elastic tissue of the lung. A lung with high compliance is easy to stretch. The disease emphysema destroys this elastic tissue and thus increases lung compliance. A lung with low compliance is stiff and hard to stretch and so is hard to fill with air. This is seen in the disease fibrosis.

Compliance of the lung is determined also by the surface tension generated at the air-water interfaces within the alveoli. The alveoli are air filled sacs lined with water. The attractive force between the water molecules (known as surface tension) resists stretching. The surface tension of pure water is so great that were the alveoli lined by water alone, lung expansion would require exhausting muscular effort and the lungs would tend to collapse. The detergent-like substance, surfactant, markedly reduces this surface tension and thereby increases lung compliance.

Lung surfactant is synthesized by the alveolar type II cell and secreted into the alveolar space by stretching the type II cells during breathing. The major component in surfactant is a phospholipid which is inserted perpendicularly into the gas-liquid interface so that its non-polar, hydrophobic fatty acids are pointed toward the gas and its polar end is in the liquid. The phospholipids form a monolayer that generates a film pressure opposing the surface tension. When this film is compressed (as the volume of the lungs is reduced) the film pressure rises and surface tension falls even further. This property stabilizes the lungs.

The lungs of many premature babies are unable to produce adequate amounts of functional surfactant. Approximately 50% of babies born before the 31st week of gestation will suffer from Respiratory Distress Syndrome. Because of the

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