Page 131 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Surface Tension, Surfactant proteins and phospholipids (chiefly dipalmi-
toyl lecithin) secreted by alveolar type II cells.
Surface tension is the main factor that deter- Respiratory distress syndrome of the newborn, a
mines the compliance of the lung-chest system serious pulmonary gas exchange disorder, is
(! p. 116) and develops at gas-liquid interfaces caused by failure of the immature lung to pro-
or, in the case of the lungs, on the gas exchange duce sufficient quantities of surfactant. Lung
2
surface of the alveoli (ca. 100 m ). damage related to O 2 toxicity (! p. 136) is also
The effectiveness of these forces can be partly due to oxidative destruction of surfac-
demonstrated by filling an isolated and tant, leading to reduced compliance. This can
completely collapsed lung with (a) air or (b) ultimately result in alveolar collapse (atelecta-
liquid. In example (a), the lung exerts a much sis) and pulmonary edema.
higher resistance, especially at the beginning
of the filling phase. This represents the open- Dynamic Lung Function Tests
ing pressure, which raises the alveolar pres-
sure (PA) to about 2 kPa or 15 mmHg when the The maximum breathing capacity (MBC) is the
total lung capacity is reached (! p. 113 A). In greatest volume of gas that can be breathed
example (b), the resistance and therefore PA is (for 10 s) by voluntarily increasing the tidal
volume and respiratory rate (! B). The MBC
Respiration larger pressure requirement in example (a) is normally ranges from 120 to 170 L/min. This
only one-fourth as large. Accordingly, the
capacity can be useful for monitoring diseases
required to overcome surface tension.
affecting the respiratory muscles, e.g., my-
If a gas bubble with radius r is surrounded by
– 1
5 liquid, the surface tension γ (N ! m ) of the asthenia gravis.
The forced expiratory volume (FEV or Tif-
liquid raises the pressure inside the bubble
relative to the outside pressure (transmural feneau test) is the maximum volume of gas
pressure ∆P ! 0). According to Laplace’s law that can be expelled from the lungs. In clinical
(cf. p. 188): medicine, FEV in the first second (FEV 1) is
∆P " 2γ/r (Pa). [5.3] routinely measured. When its absolute value is
Since γ normally remains constant for the re- related to the forced vital capacity (FVC), the
– 1
spective liquid (e.g., plasma: 10 – 3 N ! m ), ∆P relative FEV 1 (normally ! 0.7) is obtained. (FVC
becomes larger and larger as r decreases. is the maximum volume of gas that can be ex-
Soap bubble model. If a flat soap bubble is pelled from the lungs as quickly and as force-
positioned on the opening of a cylinder, r will fully as possible from a position of full inspira-
be relatively large (! A1) and ∆P small. (Since tion; ! C). It is often slightly lower than the
two air-liquid interfaces have to be considered vital capacity VC (! p. 112). Maximum expira-
in this case, Eq. 5.3 yields ∆P = 4γ/r). For the tory flow, which is measured using a
bubble volume to expand, r must initially pneumotachygraph during FVC measurement,
decrease and ∆P must increase (! A2). Hence, is around 10 L/s.
a relatively high “opening pressure” is re- Dynamic lung function tests are useful for
quired. As the bubble further expands, r in- distinguishing restrictive lung disease (RLD)
creases again (! A3) and the pressure require- from obstructive lung disease (OLD). RLD is
ment/volume expansion ratio decreases. The characterized by a functional reduction of lung
alveoli work in a similar fashion. This model volume, as in pulmonary edema, pneumonia
demonstrates that, in the case of two alveoli and impaired lung inflation due to spinal cur-
connected with each other (! A4), the smaller vature, whereas OLD is characterized by physi-
one (∆P 2 high) would normally become even cal narrowing of the airways, as in asthma,
smaller while the larger one (∆P 1 low) be- bronchitis, emphysema, and vocal cord paraly-
comes larger due to pressure equalization. sis (! C2).
Surfactant (surface-active agent) lining the As with VC (! p. 112), empirical formulas
inner alveolar surface prevents this problem are also used to standardize FVC for age, height
by lowering γ in smaller alveoli more potently and sex.
118 than in larger alveoli. Surfactant is a mixture of
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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