Surfactants 101 - The Physics Behind Physiology
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“Mayday, mayday, Physics Alert! Brace for impact.”

We carry millions and millions of minute airbags every passing second. These are the alveoli inside the lungs, which expand with the entry of air. 

This anatomical arrangement has the advantage of increased surface area for gaseous exchange. However, it brings about one problem, thanks to physics - surface tension. 

The Laplace Law:

Remember this guy - Laplace? Yes, the guy who made your life miserable by contributing too much to physics and mathematics.

He introduced what we call “Laplace’s law” regarding the distending pressure inside “elastic” spherical structures - in our case, an alveolus:

The difference in pressure between the inside and the outside of an elastic sphere is inversely proportional to its radius”.

In simple words - the smaller the elastic sphere (alveoli), the higher the pressure difference, and consequently, the higher the surface tension.

Now, thanks to nature, our alveoli are tiny. Thus, the magnitude of surface tension in the alveoli is enormous! So much so that, in theory, if there’s nothing to hold the walls of the alveoli apart, the airways would collapse in the absence of air!

Now, we know that this doesn’t happen every time we exhale. 

So what saves us from certain doom every time we breathe out?! The compounds that achieve this seemingly magical task are - surfactants.

What are surfactants?

Surface active agents are substances that can decrease the surface tension developed within the alveoli.

As seen before, as the surface tension decreases, so does the pressure difference. 

Image: The forces acting on the alveoli - with and without surfactants
Author: BQUB19-PDonatiu

Production of Surfactants

Surfactants are secreted by specialized type II alveolar epithelial cells.

Note: Clara cells also contribute to surfactant secretion. But, remember, Clara cells are specialized cells found in the small airways (bronchioles) - they are not present in alveoli.

So what are surfactants composed of?

When we talk about surfactants, we have to remember one word - dipalmitoylphosphatidylcholine (DPPC). Yes, we know it's a mouthful. So here’s another term for it - dipalmitoyl lecithin (DPL). DPPC forms a major chunk of the composition of surfactants.

The other significant components of surfactants include:

  • Phosphatidylglycerol
  • Neutral lipids
  • Proteins
  • Carbohydrate

Additionally, there are 4 surfactant proteins - A, B, C, and D (obviously!). The first and the last, meaning proteins A and D play a role in the innate immune system of the lungs, while proteins B and C are responsible for lowering the surface tension.

What happens when there is a lack of surfactants?

Whatever can happen - will happen.

Without surfactants, the surface tension within the alveoli increases, and so does the alveolar pressure difference. 

This ultimately leads to the collapse of alveoli, and by extension, the lungs. We in the medical fraternity call this phenomenon - atelectasis.

This is especially important in newborns. More specifically, in preterm births.
As we know, the lungs of preterm neonates are not mature enough to produce surfactants essential to aid the first breath of the newborn. 

If there is a deficiency of the surfactant in infants, it leads to respiratory distress. This is called Infant Respiratory Distress Syndrome (IRDS) or Hyaline Membrane Disease (HMD).

Such a condition may also be seen in adults. This is particularly true in an individual who has undergone cardiac surgery or any intervention that involves the disruption of the pulmonary circulation.

Exogenous surfactants to the rescue

For patients who cannot produce surfactants or for those who lack surfactants due to some unavoidable acute cause, the solution is the administration of exogenous surfactants.

These exogenous surfactants can be either natural or synthetic:

Natural exogenous surfactants are beractant and calfactant (derived from bovine sources), and poractant (from porcine sources).

Synthetic exogenous surfactants are the ones that are produced from scratch in labs. Examples include exosurf and lucinactant.

Although there are many studies in progress, surfactants are a domain of medical sciences that research has barely scratched the surface.

If you feel that the laws were excessive - think - there’s still so much that we don’t even know!!


  1. Ganong’s Review of Medical Physiology, 23rd Edition, Page no: 597 - 598
  2. Guyton and Hall textbook of Medical Physiology, 11th edition, page:528

Author’s footnote

Did you know that the detergent you use clean your clothes is fundamentally a surfactant? The whole process of removing dirt from clothes is based on the lowering of surface tension.

Recognize the image below?

Author: Devilal

Forget associating this image with Kung-Fu Panda 2 and see the physics behind it. You see, the water droplets prefer their own company and thus stick together as a single drop. Furthermore, notice that the leaf doesn’t get as wet as it should be even when the drop falls off it. This is an excellent example of surface tension in action.

This is precisely what happens in clothes. Unless you remove the surface tension, the water can’t spread and wet the clothes uniformly. By eliminating this surface tension, detergents allow the uniform distribution of water and, in doing so, clean clothes more efficiently.

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