# Surface Tension - Lung Alveoli

1. Jun 2, 2014

### elemis

So, the way I understand this is as follows :

The alveoli (pretend they're bubbles) have diameters of the order of microns implying a massive pressure required to inflate them by the Young-Laplace equation.

$p_{in}-p_{out}=\frac{2\gamma}{r}$

However, the presence of pulmonary surfactant molecules (lets just pretend they're like detergents molecules in washing liquid) can effectively reduce the surface tension at the unexpanded alveoli and hence allow easy inflation.

Now this bit I don't understand :

As the alveoli expand the distance between the individual surfactant molecules on the alveoli increases and hence the surface tension rises again therefore decreasing the rate of expansion.

What is the mathematical connection between surface tension and separation between surfactant molecules ? How can I rationalise the statement in bold ?

2. Jun 2, 2014

### Staff: Mentor

The surfactant molecules separate the molecules of the alveoli, which are, apparently, highly attractive to one another. However, if the distance between the surfactant molecules increases (i.e., their concentration at the surface decreases), more molecules of alveloi are able to come into contact with one another, and this causes their attractive effect to increase. Just imagine if the concentration of the surfactant molecules was greatly reduced. It would be as if they were not even there.

Chet

3. Jun 2, 2014

### elemis

Hi Chet,

So to be clear, the alveoli expansion simply results in an drop in the effective concentration (activity) of the surfactant and hence since their surface excess decreases we note an increase in surface tension ?

4. Jun 2, 2014

### Staff: Mentor

That's my understanding of what the statement is saying.

Chet