Respiratory tract histology

The physiology of gaseous exchange (skip)

This occurs through diffusion across a membrane from areas containing a high conentration of the gas to areas containing a low concentration. The rate of diffusion is dependent upon several things (Fick's Laws)

Fick's laws suggest that the rate of diffusion in a given direction across and exchange surface:

Is directly proportional to the concentration gradient - the steeper the concentration gradient (larger partial pressure differential), the faster the rate of diffusion - this is dependent upon the other physiological processes in the body and is not modifiable by the design of the lungs.
Is directly proportional to the surface area- the greater the surface area of a membrane through which diffusion is taking place, the faster the rate of diffusion. The branching of the conducting air passages and the large amount of alveoli yields a total alveoliar surface area of ~50-100 m2
Is inversely proportional to the distance - the rate of diffusion decreases rapidly with distance. The alveolar membrane is only ~ 0.4 μm thick

In addition to these points, some gases are more soluble than others - these will diffuse more quickly. Again, the lung's design will not alter this. More details here, together with how the system responds to increased demands.

The problem of stickiness

The alveoli need to be kept moist to survive and to facilitate gaseous transport. To accomplish this a thin film of fluid lines each alveolus.
The alveoli tend to collapse with expiration and re-expand with inspiration. This creates a problem. When two wet surfaces touch, they become stuck together by surface tension in the layer of water between them - this effect become more powerful at smaller scales. There is a risk, therefore, that the alveoli may collapse and be impossible to reinflate.

Histology

Problem 1: short diffusion distance required

The alveoli are made up of two types of cell - the first (conventiently called the Type I pneumocyte) is a squamous cell.
These pneumocytes have a large surface area but are thin - so thin that the nucleus seems to bulge in cross section.

Problem 2: stickiness of surface tension

The type II pneumocyte is a plumper cell than the type I - it's function is not gaseous exchange but the secretion of the components of surfactant.
Surfactant is a product that is composed of lipds and phospholipids (90%) together with four proteins*.
Surfactant reduces surface tension increasing the ease with which the alveoli re-expand.
It's other functions include preventing too much fluid from entering the alveolus and ensuring that the different sized alveoli all expand and relax at comparable rates.

*Bronchiolar Clara cells can also secrete three of the four protein components (SP-A, SP-B and SP-D).

Pic 1 (top right) shows a schematic of an alveolus. Note the different shapes of the type I and II cells and also the alveolar macrophage that forms part of the body's immune system.

Pic 2 (2nd down) shows a high magnification view of two alveoli. You can see erythrocytes in the capillaries (black arrow); Type 1 pneumocytes (blue) and Type II pneumocytes (green)

Pic 3 (3rd down) shows a lower magnification view of several alveoli - note how the wall of each alveolus touches it's neighbours - you can see many many capillaries - these run between the walls of neighboring alveoli.

Type I pneumocytes are obvious by their large central nuclei while type II pneumocytes have a 'flattened' nuclei and a cytoplasm that spreads out to the side (like the side-view of a fried egg!).

Pic 4 (bottom right) shows a lower magnification view in which an arteriole (thick walled with central erythrocytes) can be seen.

The shape of the alveoli is an artifact of sample collection and preparation.