By Abdulrasheed Haleemarh
University of Ibadan
▶ Lung Surfactants are surface active agents that help in reducing surface tension. The surfactants in the lung are called pulmonary surfactants. They are formed by Tubular myelin. It is produced by the type II alveolar epithelial cells, type 1 alveolar epithelial cell and the Clara cells.
▶ The two major functions of lung surfactants include
1. They help to prevent or to reduce surface tension produced by the air-fluid interface.
2. They prevent the lungs from collapsing.
▶Let’s see what is meant by surface tension when water comes in contact with air, water tends to contract due to strong attractions of water molecules with each other, thereby causing the water to contract.
Now, let’s look at the alveoli and see what happens when there is a fluid-filled interface in the inner structure of the alveoli. Here, water surface will attempt to contract. This will cause air to be forced out of the alveoli and so the alveoli tend to collapse because of the strong attractions between water molecules, this will cause an elastic contractile force on the whole lungs, which is referred to as surface tension elastic force.
What the lung surfactants try here is to prevent the collapse of the alveoli, so surfactants comes between the fluid and the air preventing the surface tension which might be caused by the fluid-air interface
▶Lung Surfactants are a complex mixture of :
I. lipids (phospholipids)
⭕The most important phospholipids being dipalmitoylphosphatidylcholine, for proteins is surfactants apoproteins, and ions being calcium ions.
▶The phospholipids help by reducing the surface tension, it helps by not completely dissolving in the fluid lining the alveoli, the hydrophilic part dissolves while the hydrophobic part spreads over the surface lining the alveoli
▶ The lung surfactants apoproteins are four namely SP-A, SP-A, SP-C, AND SP-C, also known as specific surfactants protein SP-A AND SP-D are hydrophilic while the other two are Hydrophobic.
▶Lung Surfactant apoproteins are very important in surfactants because, without them, surfactants may become inactive. Ions mostly present in surfactants are calcium and they play the key role in making active lung surfactants.
⭕The pressure(P) produced by surface tension(T) in an alveoli of known radius (R) is given by the law of Laplace which is : P =2T/R thus,the smaller R becomes, the greater the P so, if the alveoli are lined with a surfactants between the fluid and air, As the alveoli comes smaller, the surfactants film becomes thicker, so the surface tension lowering effect is higher, as the alveoli reduce in size surface tension becomes less, thereby preventing the alveoli from collapsing As the alveoli becomes bigger, the surfactants layer become thinner and the surface tension becomes higher, so as to prevent over distension of the alveoli. By convention, the surfactants help to stabilize the size of the alveoli
▶By preventing surface tension, it also helps to prevent fluid from being drawn from the alveolar capillaries into the alveoli lumen (inner surface) If surfactants are deficient, it causes surface tension in the alveoli to be great and also causes pulmonary oedema and in some cases viral and bacterial infection.
▶ In foetal life, the lungs are non-functional since the exchange of gas in the fetus is through the placenta. As soon as the fetus is born, with first few breaths, air enters the lungs and expand, unless surfactants are present in the lungs of the neonate, the pressure of the surface tension will be so great that it collapses the lungs. Surfactants deficiency causes healing membrane disease and in infants, it is called Respiratory distress of the newborn or infant Respiratory distress syndrome (IRDS), children born with this deficiency are unable to keep the alveoli expand. They become cyanosed and die soon after birth. The condition might be treated by applied continuous positive pressure breathing and deficiency of surfactants occurs in adults also are called adult Respiratory distress syndrome (ARDS). Glucocorticoids are the first line of treatment that induces surfactants production.