Pulmonary Surfactant - A Brief Overview

Introduction

Pulmonary surfactants are present in the alveolar cells type II. They are required to increase the surface tension of the lungs by increasing compliance and alveolar size. They are formed of different proteins and lipids like dipalmitoylphosphatidylcholine and surface proteins-A (SP-A). The pulmonary surfactant, its functioning, and its composition are further discussed below.

What Is a Pulmonary Surfactant?

Pulmonary surfactants are complexes formed by phospholipids and protein conjunctions that are surface active and present on the alveolar cells of type II. These conjunctions of phospholipids and proteins have a hydrophilic (a tendency to mix with water) and hydrophobic (water-resistant) end. It has its hydrophilic heads in the water and hydrophobic tails interacting in the air to adsorb air-water interfaces of alveoli; along with these ends, the body contains a main component of surfactant which is a lipid named DPPC (dipalmitoylphosphatidylcholine) that reduces the surface tension of the alveoli.

What Are the Functions of Pulmonary Surfactants?

The functions of pulmonary surfactant are:

• Compliance: It is the ability to expand the lungs and thorax (the region in the body between the stomach region and the neck). As surfactant decreases the alveolar surface tension, it increases compliance (the action of complying with a command). The normal surface tension of the lungs is 25 dyn/cm (dyne per centimeter). Pulmonary surfactant decreases the surface tension of the lungs by increasing compliance by making lung inflation easier and decreasing the work process of breathing. It also decreases the pressure difference. But these lung compliances reduce the chance of diseases.

• Alveoli Size Increase: On increasing the size of alveoli, the surfactant easily spreads on liquid. And thus, the surface tension also increases with a slow expansion rate. And all this helps the alveoli present in the lungs to expand more easily and fast with a slow rate of expansion and high rise of surface tension. So, the surfactant can decrease the surface tension more easily when the alveoli are small in size as these surfactants are concentrated.

• Fluid Accumulation Is Prevented With Dryness of the Airway Maintained Simultaneously: The surface tension provides the fluids to move from the capillaries to the alveoli. The surfactant here decreases the fluid accumulated and provides the airways with dryness by decreasing the surface tension.

• Providing Innate Immunity: The protein present in the surfactant SP-A and SP-D provides immunity. These proteins easily bind with sugars present in the pathogens and thus go for the phagocytosis (the method by which a cell engulfs a big particle using its plasma membrane) process. These also contribute to surfactant degradation by improving lung function and decreasing the chances of inflammation and infection.

What Is the Composition of the Surfactant?

Pulmonary surfactant is made up of phosphatidylcholine (a type of phospholipid), surfactant proteins, and cholesterol. Among these, phosphatidylcholine contains dipalmitoylphosphatidylcholine, and palmitoyl-myristoyl phosphatidylcholine, which form nearly 75 percent of the pulmonary surfactant. Other anionic phosphatidylglycerol and lipids having cholesterol are about 10 percent. Some of the surfactant proteins like SP-A, SP-B, SP-C, and SP-D are also found around 5 to 10 percent.

A. 40 Percent Dipalmitoylphosphatidylcholine (DPPC):

It comprises phospholipids (which work as a barrier to protect the cell) and has a sixteen-carbon chain, a phosphate, and an amine group. These together form one of the strongest pulmonary surfactant formations. It has a capacity that is higher than phospholipids, but it has a slow adsorption rate. It can get transitioned to a liquid form from a gel at 41.5 degrees Celsius which is particularly more than the body temperatures of humans.

B. 40 Percent Phospholipids:`

They have acyl-saturated chains. Anionic phosphatidylglycerol is nearly 10 percent of the surfactant, with an unsaturated fatty acid chain. They have cholesterol and neutral lipids to accompany them. The components of such lipids are drained into the blood, where type II alveolar cells assemble and pack these to secrete these organelles named lamellar bodies.

C. 10 Percent Surfactant Proteins:

Surfactant proteins contribute to around 10 percent of pulmonary surfactants. Apolipoproteins, surfactant proteins, and plasma proteins form these surfactant proteins. The surfactant protein present is SP-A, SP-B, SP-C, and SP-D. Apolipoproteins are the protein part of plasma lipoprotein rings formed by connecting lipids and proteins formed by the secretary pathways in alveolar type II cells. And there they undergo translation and reside in lamellar bodies at the end. The presence of these proteins SP-A and SP-D shows innate immunity to kill bacteria and viruses through the phagocytosis process. SP-B and SP-C are the hydrophobic proteins that increase the surfactant's capability to spread or expand. They both are required in the functioning of the lungs. The main function of these proteins is to lower the critical temperature of dipalmitoylphosphatidylcholine, which increases the adsorption and spreading. As the adsorption velocity increases, it increases the gas exchanges in the lungs.

D. 10 Percent Lipids:

Lipids are also present in surfactants. They can cover nearly 10 percent of the composition. Lipids like cholesterol are possibly seen in pulmonary surfactants.

How Are the Production and Degradation of Surfactant Carried?

Surfactants are formed in the alveolar cells in the alveolar sac stages of lung development when the baby is in the womb. A full term baby is estimated to have nearly 100 mg/kg (milligrams per kilogram) of alveolar surfactants.

Alveolar surfactants have a half-life of nearly 5 to 10 hours. After which, they are taken up by the macrophages and the lamellar structures in the cytoplasm of the cell, usually by the type II pneumocytes. The 90 percent of DPPC-dipalmitoylphosphatidylcholine of the surfactant is taken back from alveolar spaces with the help of SP-A (stimulating receptor mediator) or endocytosis. And the remaining 10 percent is digested by the macrophages.

What Are the Diseases Related to the Pulmonary Surfactant?

The diseases that are related to pulmonary surfactants are:

• Infant Respiratory Distress Syndrome - It is a condition when the baby's lungs are not fully mature and developed to provide enough oxygen as the baby does not have surfactant. It causes breathing problems in the lungs. It is mostly seen in premature or preterm babies.

• Congenital Surfactant Deficiency - It occurs when there are abnormalities in surfactant composition or even in the function of the surfactant. It can lead to lung disorders as there is difficulty in breathing. It can be present from birth, so it is considered a congenital disorder.

• Pulmonary Alveolar Proteinosis - It is one of the lung diseases in which the lung surfactant accumulates within the alveoli. This pulmonary alveolar proteinosis leads to clogged alveoli of the lung and low blood oxygen levels. It can be considered a congenital disorder also.

• Surfactant Metabolism Dysfunctioning - It is a lung condition in which the pulmonary surfactants are not adequate enough for respiration. And the alveoli can collapse after expiration because of surface tension interfaces.

Conclusion:

Pulmonary surfactants are pulmonary complexes present over the alveoli cells that help to increase the compliance and surface tension of the lung. These also provide innate immunity and increase the alveolar size for easier passage of airways. They are composed of dipalmitoylphosphatidylcholine, cholesterol, and surface proteins like SP-A and SP-D. If these surfactants are absent, they can cause syndromes like infant respiratory distress syndrome. But they have an important role as they reduce surface tension to provide a better airway.