Friday, April 24, 2009


Around the circumference of the alveolar ducts are numerous avlveoli and alveolar sacs. Aan alveolus (al-VĒ-ō-lus) is a cup shaped outpouching lined by simple squamos epithelium and support by a thin basement membrance; an alveolar sac consist of two or more alveoli that share a common opening . The walls of alveoli consist of two types of alveolar epithelial cells .

The more numerous type I alveolar cells are simple squamous epithelial cells that form a nearly continuous lining of the alveolar wall. Type II alveolar cells, also called septall cells, are fewer in number and are found between type I alveolar cells. Then thin type I alveolar cells are the main sites of gas exchange. Type II alveolar cells, rounded or cuboidal epithelial cells with free surfaces containing microvilli, secrete alveolar fluid, which keeps the surface between the cells and their air moist. Included in the alveolar fluid is surfactant (sur-FAK-tant), a complex mixture of phospholipids and lipoproteins. Surfactant lowers the surface tension of alveolar fluid, which reduces the tendency of alveoli to collapse (described later).

Asscciated with the alveolar wall are alveolar macrophages (dust cells), phagocytes that remove fine dust particles and other debris from the alveolar spaces. Also present are fibroblasts that produce reticular and elastic fibers. Underlying the layer of type I alveolar cells is an elastic basement membrane. On the outer surface of the alveoli, the lobule’s arteriole and venule disperse into a network of blood capillaries (see Figure 23.11a) that consist of a single layer of endothelial cells and basement membrane.

The exchange of O2 and CO2 between the air spaces in the lungs and the blood takes place by diffusion across the alveolar and capillary walls,which together form the respiratory membrane. Extending from the alveolar air space to blood plasma, the respiratory membrane consists of four layers :

  1. A layer of type I and II alveolar cells and associated alveolar macrophages that constitutes the alveolar wall.
  2. An epithelial basement membrane underlying the alveolar wall.
  3. A capillary basement membrane that is often fused to the epithelial basement membrane.
  4. The capillary endothelium

Despite having several layers, the respiratory membrane is very thin- only 0.5 µm thick, about one-sixteenth the diameter of a red blood cell – to allow rapid diffusion of gases. It has been estimated that the lungs contain 300 million alveoli, providing an immense surface area of 70m­­2(750ft2) – about the size of a racquetball court – for gas exchange.

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