Surfactant is a complex substance that prevents the collapse of alveoli in the lungs. It is produced in fetal lungs and begins working as soon as the baby has reached full term and is delivered. The substance is composed of phospholipids and four surfactant proteins known as hydrophilic proteins SP-A and SP-D and hydrophobic proteins SP-B and SP-C. The two hydrophobic proteins help spread surfactant throughout the lungs.
Surfactant allows alveoli to stay open and compliant during both inspiration and expiration. During inspiration, alveoli may collapse if they do not contain surfactant. If they collapse, then gas exchange across the alveoli wall cannot occur. Surfactant allows the alveoli to stay open and gas exchange to occur. During expiration, both lungs have the tendency to collapse. If they do collapse, then it takes great effort to re-inflate them.
Surfactant prevents the lungs from collapsing by reducing surface tension throughout the lungs. Surface tension is the main force that is present within the alveoli of the lungs. Without surfactant, the surface tension present in the lungs causes the alveoli to stick together during expiration, which causes both lungs to collapse. Surfactant forms a thin film that covers that alveoli and breaks the surface tension. This reduces the alveoli’s tendency to collapse and reduces the risks of the lungs collapsing. Overall, surfactant allows humans to inspire and expire easier.