Surface Tension

 SURFACE TENSION

Alveoli are lined by 2 different epithelial cells,

1.Type I pneumocyte:

• Type I pneumocytes are flat cells

• They cover around 95% of the entire surface area of alveoli 

• They provide an excellent space for gas exchange.

2. Type II pneumocyte:

• Type II pneumocytes are cuboidal cells 

• they produce surfactant, that decreases the surface tension

The surface of these cells is lined by a fluid such as water,

The water molecule, which is present in the deeper is attracted by all the directions & the force is equal and opposite, so the net force is zero. So they move in any direction in the fluid but the water molecule which is present at the air-water interface has a net downward force because they do not have to oppose upward force. So, all molecules which present at the air-fluid interface have unopposed downward attraction & when some molecule jumps in the deeper fluid than the number of water molecules at the surface becomes less so, those molecules which are present at the fluid surface have strong horizontal intermolecular attraction and the surface shrinks.

So, the Surface tension is the force imposed by water molecules at the air-liquid interface & acts to minimize the surface area of alveoli.

COMPLICATIONS OF INCREASED SURFACE TENSION

1. Tendency of Alveolar collapsing:  surface tension leads to a reduction in the surface area & it is making alveolar smaller that increases the collapsing tendency. Due to surface tension, there is collapsing pressure in alveoli.

1. The tendency of Edema:  if surface tension is increased too much then it pulls the water from the pulmonary capillary by simple diffusion in the alveoli. Alveoli is loaded with water that increases the thickness of the alveolar membrane, so thicker membranes can not participate in gas exchange.

1. Collapsing of smaller alveoli: Laplace’ Law states that pressure in a sphere being directly proportional to twice the surface tension and inversely proportional to the radius:

                        P=2T/r

P= collapsing pressureT

T surface tension

r= radius of alveoli

We have two spheres with different sizes. As the radius of the sphere decreases, the pressure increases as per Laplace's Law. Now, If the two spheres were connected with a tube, the smaller one should empty into the larger one based on the pressure difference. In other terms, smaller alveoli should empty into larger ones based on Laplace's law, causing the lungs to collapse.