1. Anatomy of gas transport and exchange in insects and vertebrates.

• Insects transfer gases by a system of air passageways that include tracheal tubes that open to the outside surface of the animal through spiracles, and then branch into tracheoles, taking advantage of the fact that oxygen and carbon dioxide diffuse by random movement about 10,000 times faster in air than dissolved in water, thus making up for the simple circulatory systems of insects.
  • Insects are small. But, imagine a scaled up, huge insect, with the same proportions. Surface to volume considerations show that there wouldn't be enough surface area on the tracheal system to support the relatively larger volume. Diffusion times increase proportionally to the square of distance (notice that surface area also increases as square of a linear dimension but volume increases as the cube of a linear dimension), thus making the diffusion system to be impractical for huge (ficticious) insects to use for moving gases through the tracheal system.
  • Not only do vertebrates have a finely divided air passage ways in the lung, instead of the less elaborate tracheal system of insects, but larger mammals have more surface area for absorption inside the lungs than do smaller mammals. (Eckert, Fig. 13-20a)

• The human gas transport system (Eckert, Fig. 13-1) consists of active pumping of air into the lungs (Eckert, Fig. 13-28), diffusion over a short distance in the alveoli, across the cells and membranes of the lung and into the circulatory system.

• Mammals have active pumping of air in and out of the lungs through movement of diaphram and ribs (Eckert, Fig. 13-30a) (Eckert, Fig. 13-30b).

2. The human lung.

• A bronchus divides into lobules and acini that are further divided into bronchioles and finally alveoli (Diagram of a bronchus, lobule, and alveolus) .

• Notice the Pores of Kohn that connect adjacent alveoli and provide ventilation in case of a blockage of a small bronchiole. (Detail of a alvolus) (Eckert, Fig. 13-22a).

• Now, let's look at a section of a human lung - (Histology of the lung).

• In the bronchi, we find ciliated cells and Goblet cells, that secrete mucus. As the bronchi divide into bronchioles we see fewer and fewer Goblet cells. Goblet cells are replaced by Clara cells, that secrete a protein surfactant.

• Blood flows through alveolar capillaries that are separated from the alveolar space by as little as one cell!

• In class we will calculate the approximate time for gas to diffuse from the center of an alveolus to the center of a alveolar capillary and compare this to the transit time for blood flow around an alveolus. Don't miss this exciting calculation, based upon the observation that diffusion times are proportional to the square of the distance that randomly diffusing molecules travel. (We'll work off this diagram - Study it to understand how it relates to the micrographs and diagrams shown previously in this lecture.

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2006, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.