Properties, production and regulation of lung surfactant

This chapter is vaguely relevant to Section F3(vi) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "describe the properties, production and regulation of, surfactant and relate these to its role in influencing respiratory mechanics". The topic has come up just once in the written paper, in Question 6 from the second paper of 2012 which asked about the structure of surfactant as well as its effects on surface tension and lung mechanics. The college comment to this SAQ is remarkably detailed, and was very useful in the process of structuring this chapter.

In summary:

For surface tension and the law of Laplace, the most illuminating article is probably Prange (2003). Alveolar surfactant seems to be exceptionally popular, and there are several good reviews out there, but many are weakened by excessive detail; the best balance of brevity and thoroughness seems to be Goerke (1998)

Surface tension

Surface tension does not appear to have a formal definition, but this exerpt from the Oxford dictionary seems suitably definition-like:

"the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area."

This is a property of liquids, or specifically of liquid-gas interfaces. The molecules of liquids tend to stick together by strong intermolecular attractive forces (eg. the hydrogen bonds in water), and so the surface of a liquid naturally tends to collect the liquid into a roundish blob (or a sphere, where gravity is not an issue). At least when we are talking about liquids which have these strong intermolecular attractive forces (eg water, the surface tension of which is around 72 mN/m at 25°C). With increasing temperature, surface tension decreases; with increasing intermolecular forces it increases. A good example of extreme surface tension is liquid mercury, which has a surface tension of 487 mN/m. On the other hand, liquid helium is the all-time champion of low surface tension (0.37 mN/m).

The Law of Laplace

The college examiners, in their comment to Question 6 from the second paper of 2012 make reference to something called "La Place's law", likely a reference to  Laplace's law (after Pierre-Simon, marquis de Laplace). Its a very famous relationship, and in case one is ever in need of a dozen examples of where it becomes useful in medicine, one may be referred to the excellent article by  Jeffrey Basford (2002). For a version which fits an alveolocentric model of the universe, the Young-Laplace-Gauss equation can be represented as:

where

• γ  is the surface tension
• r is the radius of the spherical alveolus, and 
• P is the Laplace pressure, or the pressure difference between the inside and the outside of the curved fluid surface, i.e. the difference in pressure between the fluid layer and the gas inside the sphere. 

What is the significance of this? Well, the law itself is fairly straightforward. It basically says that the pressure inside an elastic sphere is inversely proportional to the radius, provided the surface tension remains stable. Obedient to this relationship, at any given surface tension value smaller spheres with smaller radii will have higher transmural pressure, i.e. they want to collapse more than larger spheres. The implications of this law for alveoli are:

• Small alveoli (i.e. collapsed and nearly collapsed) are more difficult to inflate then large alveoli; this contributes to the low compliance seen at small lung volumes
• Smaller alveoli will promote their own collapse by emptying into larger neighbouring alveoli
• Filtration of fluid across the pulmonary capillary wall depends on the hydrostatic pressure gradient, and alveolar surface tension adds to that gradient (i.e. it promotes the ultrafiltration of oedema fluid)

That all sounds very scientific, but how well does it represent the goings-on at the alveolar coalface? Probably to some minimal degree, seems to be the answer. Physiology textbooks have been pulling the wool over our eyes, according to Henry Prange (2003). Important points to note regarding the misapplication and limitation of Lapace's law would be:

• Alveoli are not spherical. They are polygons. Laplace law applies only to to the very small curved region in the fluid where these walls intersect.
• Alveoli do not collapse like deflating balloons, they fold like cardboard boxes. 
• Alveoli are not independent, but rather interconnected and suspended against each other with bands of elastic connective tissue, which serves to support them and prevent their collapse.

So, it is probably not 100% applicable to the reality of human lung mechanics. It is an example of medicine misappropriating the precise and streamlined reputations of physics and engineering, trying to press some crisp mathematical edges into the gooey mess of human biology.  To borrow Prange's words,

"We biologists often seem driven to seek the elegant and sophisticated mechanisms of physics in the far more complex structures of plants and animals. We are prone to fall into the trap of injecting physics into our work wherever we can, whether or not its use is justified or correct."

Effect of surface tension on lung compliance

When discussing the effect of water surface tension on lung compliance, a famous diagram is often pulled out which demonstrates what would happen of that air-liquid interface was not there. It is a compliance curve of a cat lung (Cat 27's lung, to be precise) which was filled with saline. 

This specific version of this diagram comes from T.E. Morgan (1971), but its origins are much older. These data were first published in 1929 by Kurt von Neergaard. At a basic level, this demonstrates that the surface tension of the alveoli makes the lung much more difficult to inflate, i.e. it is one of the most important determinants of lung compliance. With surface tension abolished (no surface, no tension!) the lung inflates apparently effortlessly, and notably also loses the poorly compliant early part of the compliance curve. We are all grateful to Dr Hopkinson, one of the readers, who has submitted this video of a PV look from a surfactant-lacking neonate, as it illustrates the same concept:

Effect of surfactant on the surface tension in the alveoli

The surface tension of water is about 70 mN/m at body temperature, whereas the surface tension of the alveolar surface fluid is obviously quite different. Clements (1957) used a Wilhelmy balance to test the surface tension of mushed lung extract ("mince of whole lungs in normal saline, filtered through loosely-packed cotton") and determined that its surface tension varied from 46 to 10 mN/m, with a substantial hysteresis. This was later confirmed by Schurch Goerke & Clements in 1976.  Lachmann et al (1980) demonstrated what this difference means for lung compliance by comparing rabbit lungs before and after a lavage which removed all the surfactant. Their graphics (below) demonstrate the substantial decrease in compliance (it was essentially halved).

So, what is lung surfactant, and where does it get these miraculous properties, and how does it compare to, say, dishwashing detergent?

Physical and chemical properties of lung surfactant

It is much better than dishwashing detergent, which generally only decreases the surface tension of water down to about 25-30 mN/m. For those with the need to know absolutely everything about this substance, an excellent monograph by Notter & Wang (1997) would be more than enough to agitate their passions. Its 118-page length and the complexity of its contents notwithstanding, in a crime against good typography practice Reviews in Chemical Engineering for some reason published it in a bold font, making it even more difficult to read. No matter: the most important points will be summarised here.

• It is a complex soup of phospholipid
• Of the dry mass, 
  • 85-90% is phospholipid
  • 8-10% is protein - mainly SP proteins A B and C, all small (~4-5 kDa )
  • 2-5% is neutral lipid, eg. cholesterol
• Phospholipid components are:
  • Dipalmitoyl phosphatidylcholine (DPPC, or lecithin) is about 2/3rds of the total phospholipid content, and does most of the surfactant work.
  • The rest are random phospholipids (a heterogeneous group), including phosphatidylglycerol, phosphatidylinositol, phosphatidylethanolamine, sphingomyelin and phosphatidylserine.
  • These latter elements are essential as they contribute to the stability of the mixture. For example, DPPC the star player surfactant is not really soluble in water, and tends to be a solid up to a temperature of 41.6 °C, unless there is a decent amount of phosphatidylglycerol in the mixture.

Generally speaking, at the gas-liquid interface, phospholipids tend to form monolayers which are oriented with the hydrophilic heads buried in the water and the hydrophobic tail chains pointing out into the air. The aim of the game is to pack as much of this stuff at the air-liquid interface as possible; the water molecules' attraction to each other is the main reason for the surface tension, and so if you eliminate all the surface water molecules by filling the surface with phospholipid, theoretically the surface tension should be minimised. That is the theory behind the choice of dipalmitoyl phosphatidylcholine, which really isn't found anywhere else except the lungs. As a surfactant agent, this one is a good choice because its disaturated form can be "packed to a very high density at the air–water interface" (Pérez-Gil et al, 2008). You can't do that with many of the other phospholipids because they have "kinked" unsaturated chains which don't line up neatly. Thus, as the alveoli shrink in expiration and their internal surface area decreases, the DPPC molecules bunch together and form an impenetrable waterless carpet at the interface, essentially a solid (albeit a very thin one). Theoretically, this solidification of surfactant should decrease surface tension to zero (Morley & Bangham, 1981). In other words, lung surfactant might under certain circumstances beat the surface tension record of creepy-as-hell superfluid liquid helium, which tends to sneak out of its beaker and generally behaves like a swarm of bosons. 

Production and regulation of lung surfactant

Surfactant is produced and recycled almost exclusively by Type II alveolar cells. Some of the best sources for this topic are Van Golde et al (1988) and  Chander & Fisher (1990). In summary:

• Preformed surfactant is stored in the lamellar bodies inside the Type 2 cells; these are bell-shaped structures which, upon sectioning, reveal a concentric internal structure like an onion. These bodies have a relatively acidic internal pH and also contain some surprisingly lysosomal enzymes, eg. hydrolases.
• Lamellar bodies appear in Type 2 cells at around the 20th week of gestation
• Each cell has about 150 lamellar bodies, and about 15 of these are exocytosed every hour
• Release of surfactant is enhanced by
  • Catecholamines (a β-adrenergic receptor is involved)
  • Cholinergic drugs
  • Vasopressin
  • Adenosine
• Release of surfactant is inhibited by
  • High concentrations of surfactant proteins
  • Lectins (eg. plant lectin), which are homologous with surfactant proteins
  • Inflammatory mediators
• Lung surfactant substance has a half-life of about 5-10 hours, i.e. each hour 10-30% of it needs to be removed somehow. This happens by:
  • Resorption into Type 2 cells (this accounts for 95% of the removal)
  • Transport up the airways and elimination as exhaled aerosol
  • Degradation in alveolar macrophages
  • Extracellular enzymic degradation in the alveoli
  • Clearance via lymph or blood

The CICM syllabus asks for production and regulation of surfactant, implying that there may be some scenario where one might experience an unregulated surfactant excess. That is in fact the case if one has alveolar proteinosis. Apart from that example, Whitsett et al (2010) describe a range of conditions where the production of surfactant is altered with profound effects.