Pulmonary Surfactant

What Is Pulmonary Surfactant?

Pulmonary surfactant is a slippery, soap-like mixture of lipids and proteins that coats the wet inner surface of the alveoli and reduces the pull of surface tension. Without it, the moist walls of each tiny air sac would tend to stick together and collapse every time you breathed out.

Pulmonary surfactant definition

The word surfactant is short for surface-active agent: any substance that gathers at the boundary between a liquid and a gas, or between two liquids, and lowers the tension there. Soaps and detergents are everyday surfactants. Pulmonary surfactant is the natural surfactant of the lungs, produced by the body itself and found only in the air spaces.

Explained simply

Water molecules hold on to each other tightly, which is why a drop of water pulls itself into a bead. Inside an alveolus, that same pull tries to shrink the air sac like a deflating balloon. Surfactant squeezes in between the water molecules and weakens their grip, so the air sacs stay open with very little effort.

Where Is Surfactant Found in the Lungs?

Surfactant is found as a very thin film at the air-liquid surface of the alveoli, floating on the watery lining that covers the alveolar wall, and small amounts reach the smallest bronchioles. It is made by type II pneumocytes, the rounded cells tucked into the corners of each alveolus.

The film forms the topmost layer of the alveolar wall, just above the respiratory membrane that oxygen must cross to reach the blood. Beneath the film is a thin liquid layer called the hypophase, which holds a reserve of surfactant waiting to move up to the surface. Type I pneumocytes, the flat cells that form most of the alveolar wall, lie under that liquid.

Surfactant works alongside other residents of the alveolus. Alveolar macrophages patrol the surface and swallow old surfactant along with dust and germs, while the alveolar ducts and sacs share the same lining.

What Is Pulmonary Surfactant Made Of?

Pulmonary surfactant is made of about 90% lipids and about 10% proteins. The lipids do most of the work of lowering surface tension, and the proteins help the lipids spread, recycle and defend the lung.

Lipids

The most important lipid is dipalmitoylphosphatidylcholine, usually shortened to DPPC. It is a phospholipid, a molecule with a water-loving head and two straight, water-avoiding fatty acid tails. At the surface, DPPC molecules line up side by side with their heads in the liquid and their tails pointing into the air, and when the alveolus shrinks they pack so tightly that surface tension falls close to zero. Other phospholipids, such as phosphatidylglycerol, plus a little cholesterol keep the film fluid enough to spread.

Surfactant proteins

Four surfactant proteins, named SP-A, SP-B, SP-C and SP-D, make up much of the protein part. SP-B and SP-C are small and water-repelling; they help the lipids move from the liquid to the surface and spread into a smooth film. SP-A and SP-D are larger and water-loving; they bind bacteria, viruses and fungi and help immune cells clear them. The table below lists the main components and their roles.

How Is Surfactant Made, Released and Recycled?

Surfactant is built inside type II pneumocytes, stored in layered granules called lamellar bodies, released into the alveolus, spread as a film on the surface, and then largely taken back by the same cells to be reused. This loop runs constantly throughout life.

1. Production: type II cells assemble the lipids and proteins inside their endoplasmic reticulum and Golgi apparatus.
2. Storage: the mixture is packed into lamellar bodies, which look like stacks of tightly rolled sheets under the electron microscope.
3. Release: lamellar bodies fuse with the cell surface and empty into the alveolar fluid. Stretching of the alveoli during deeper breaths is one of the strongest triggers.
4. Spreading: in the liquid the material unfolds into a lattice called tubular myelin, which feeds lipids up into the surface film.
5. Recycling and clearance: used surfactant is taken back into type II cells and much of it is reused, while alveolar macrophages break down the rest.

The cells also respond to hormones. Cortisol and thyroid hormones help type II cells mature before birth, which is part of how the lungs prepare for their first breath, as described on our development of the respiratory system page.

What Is the Function of Pulmonary Surfactant?

The main function of pulmonary surfactant is to lower the surface tension of the fluid lining the alveoli, which keeps the alveoli open at the end of exhalation, makes the lungs easier to inflate and helps keep fluid out of the air spaces. It also has a smaller but real role in defending the lung against infection.

Keeping the alveoli open

The pressure needed to keep a bubble open follows the law of Laplace: pressure equals two times the surface tension divided by the radius (P = 2T/r). A small alveolus therefore needs more pressure than a large one, and without help the small ones would empty into the large ones and collapse. Surfactant solves this because its effect gets stronger as an alveolus shrinks: the molecules crowd together and surface tension drops, so small and large alveoli stay stable side by side.

Making breathing easier

By cutting surface tension, surfactant increases lung compliance, the ease with which the lungs stretch. The breathing muscles then need far less effort to pull air in during inhalation, a link explained on our mechanics of breathing page.

Keeping the alveoli dry and defended

High surface tension would also suck fluid out of the capillaries into the air sacs, so surfactant helps the lungs stay dry enough for gas exchange. SP-A and SP-D add a defense role by tagging microbes for removal, a quiet partner to the cilia and mucus that clean the larger airways.

Why Is Surfactant Important in the Lungs at Birth?

Surfactant is especially important at birth, because a newborn must open its fluid-filled lungs with its very first breaths and then keep them open with every breath after. Without enough surfactant, each breath would be almost as hard as the first.

Type II cells begin making surfactant late in the second trimester of pregnancy, and the amount usually becomes sufficient at around 34 to 36 weeks. A baby born well before this point may not have enough, and its alveoli tend to collapse at the end of each breath. Doctors can estimate lung maturity before birth by measuring surfactant lipids in the amniotic fluid.

Surfactant matters for adults too. It keeps millions of small alveoli inflated through each night of quiet breathing and protects the huge alveolar surface that the respiratory system depends on.

Common Conditions That Affect Pulmonary Surfactant

Surfactant problems arise when too little is made, when it is damaged or washed out by injury, when it is built incorrectly because of a gene change, or when it is not cleared and builds up. Our respiratory diseases section covers lung conditions more broadly, and anyone with breathing symptoms should see a doctor.

• Neonatal respiratory distress syndrome: a lung condition of premature babies, caused mainly by not enough surfactant. It was once called hyaline membrane disease.
• Acute respiratory distress syndrome (ARDS): in adults and children, severe lung injury floods the alveoli with protein-rich fluid that blocks and breaks down surfactant.
• Genetic surfactant disorders: rare changes in the genes for SP-B, SP-C or the lipid transporter ABCA3 can cause lung disease from birth or later in life.
• Pulmonary alveolar proteinosis: a rare condition in which surfactant is not cleared properly by macrophages and slowly fills the air sacs.
• Pneumonia: infection and inflammation can disturb the surfactant film in the affected alveoli.

In all of these, the shared result is stiffer lungs and collapsed or flooded alveoli, which lower the oxygen level of the blood, a state called hypoxemia.