Trying to answer the question; what is lung surfactant?,its composition and action, will help us to understand why we do not spend much energy during breathing, especially during exhalation. Are you ready? ok lets go.
Lung surfactants are lipoproteins ( a conjugate of a protein and a lipid molecule) that is found on the surface of lungs, which induces surface tension and permits efficient penetration and exchange of oxygen and carbon dioxide gases. Remember; when breathing, we inhale large amounts of oxygen in combination with other gases, like nitrogen and exhale carbon dioxide in combination with other gases. These surfactants are made from animal lung extract.
Natural lung surfactants are produced by the aveolar cells in the lungs and line, mainly the aveoli and small bronchioles. They prevent the aveoli to collapse after taking a long strain during inhalation. okay, take a breath, then exhale, did you notice the stain created when you inhaled?
These natural surfactants are first produced by the fetus, so as to prepare them for proper breathing after the gestation period (about 37 week). Hence, in case of a pre-term labor, artificial lung surfactants are administered to the premature infant intratracheally to prevent the occurrence of respiratory distress syndrome.