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A layer of fluid lining the alveolus and containing surfactant that reduces the surface tension of the alveolar fluid

The alveolar epithelium composed of thin epithelial cells

An epithelial basement membrane A thin interstitial space between the alveolar epithelium and the capillary membrane

Fluid and surfactant layer

Epithelial Alveolar basement epithelium membrane

Fluid and surfactant layer

Red Blood Cell Ultrastructure

Red blood cell

Interstitial space

Capillary endothelium Capillary basement membrane

Figure 39-9

Red blood cell

Interstitial space

Capillary endothelium Capillary basement membrane

Figure 39-9

Ultrastructure of the alveolar respiratory membrane, shown in cross section.

5. A capillary basement membrane that in many places fuses with the alveolar epithelial basement membrane

6. The capillary endothelial membrane

Despite the large number of layers, the overall thickness of the respiratory membrane in some areas is as little as 0.2 micrometer, and it averages about 0.6 micrometer, except where there are cell nuclei. From histological studies, it has been estimated that the total surface area of the respiratory membrane is about 70 square meters in the normal adult human male. This is equivalent to the floor area of a 25-by-30-foot room. The total quantity of blood in the capillaries of the lungs at any given instant is 60 to 140 milliliters. Now imagine this small amount of blood spread over the entire surface of a 25-by-30-foot floor, and it is easy to understand the rapidity of the respiratory exchange of oxygen and carbon dioxide.

The average diameter of the pulmonary capillaries is only about 5 micrometers, which means that red blood cells must squeeze through them. The red blood cell membrane usually touches the capillary wall, so that oxygen and carbon dioxide need not pass through significant amounts of plasma as they diffuse between the alveolus and the red cell. This, too, increases the rapidity of diffusion.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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