What Is the Respiratory Surface in the Lungs?

The average human has about 300 million alveoli, tiny air sacs, in the lungs. The walls of these alveoli comprise the respiratory surface membrane. If stretched out, the thin membrane would cover a tennis court.

The respiratory surface membrane allows air exchange while still providing a protective barrier between the organism and its environment.

A versatile structure, it also regulates fluid in the lungs and helps to block viruses and other respiratory invaders from entering the bloodstream.

Efficient Gas Exchange

• The large, thin surface area allows fast gas exchange during exercise.

  This delicate membrane's main function is to absorb oxygen from the air and eliminate carbon dioxide from the blood. Thin blood capillaries throughout the membrane provide close contact and rapid gas exchange between the alveolar air and the blood. Dissolved gases move across the respiratory surface both ways, from air to blood and from blood to air. Its large overall surface area optimizes swift gas exchange to keep up with respiratory demands such as during exercise.

Structure

• Termed the "alveolar capillary membrane" or the "blood-air barrier," the respiratory surface is made up of three parts: the alveolar epithelium, the capillary endothelium and the basement membrane.

Alveolar Epithelium

• The alveolar epithelium helps clear foreign substances from the lungs.

  The alveolar epithelium is made up of type I pneumocytes, type II pneumocytes and alveolar macrophages.

  Type I pneumocytes are thin cells that form the main structure of the alveolar wall. Joined tightly against each other, they make up 90 to 95 percent of the alveolar surface area. They enable gas diffusion through close contact with capillary cells.

  Type II pneumocytes make up 1 to 5 percent of the alveolar surface area. They secrete surfactant, which lowers the surface tension of the moisture inside the alveolar sacs and allows the alveoli to remain open and pliant. These cells are also involved in ion transport and fluid removal from the alveolar spaces.

  Alveolar macrophages are immune cells that function as the front line of defence against respiratory intruders. They ingest and kill invading microbes, initiate immune responses and clear debris such as dust, dead cells or smoke particles. They also help moderate inflammation in lung tissue.

Capillary Endothelium

• In addition to enabling gas exchange, the capillary endothelium acts as a filter, a reservoir and a buffer.

  The capillary endothelium is a tortuous network of thin blood vessels that stretches out smoothly in one-cell-thick layers between the alveoli. The capillary endothelium has an enormous surface area that is comparable to the total area of the respiratory surface. The capillary endothelium allows gas and fluid exchange both in and out of the bloodstream. When the organism is at rest, many of these cells will not be in use. This system also serves important functions in filtering blood clots and waste, moderating the action of chemicals, storing immune cells during infections and providing nutrients to lung cells.

Basement Membrane

• The basement membrane provides stability while not interfering with gas exchange.

  The basement membrane largely provides structural stability. It consists of two parts, the alveolar basement membrane (ABM) and the capillary basement membrane (CBM).

  The ABM is a dense, loose structure with fine filaments for cell attachments and holes where Type II cells can penetrate through.

  The CBM is lacy in appearance and fuses with the ABM in some locations into a single, thin structure.

  Where these structures join together into thinner areas, the barrier to gas exchange becomes minimal.