Why Must Insects and Spiders Breathe Air?
Among most animals, gas exchange systems are constructed to bring together two convective flows—air and blood in the case of the lung—at a diffusion exchange surface that is very thin. In the human lung, for example, about a micrometer separates the alveolar air from the blood, and the surface area of the entire exchange surface in the lungs is about 80 m2. This design is known as a coupled convection-diffusion exchanger. The coupling of convection and diffusion is found in many configurations among many gas exchange organs, and its ubiquity suggests that the limitations on diffusion exchange are widespread.
The gas exchangers of terrestrial spiders and insects, in contrast, operate by diffusion alone, with minimal or no requirement for convection. Despite this limitation, insects can grow to large body sizes and engage in vigorous physical activities such as flight. They are able to do so because their respiratory organs are designed to use air, not blood, as a medium for distribution of oxygen.
Insects exchange gases through small breathing holes, called spiracles, piercing the animals' flanks at regular intervals. The spiracles open up into large air-filled tubes, the tracheae, which ramify into a network of tubes reminiscent of the bronchi and bronchioles of the lung. The tracheae branch and penetrate throughout the body of the animal, ending in blind-ended tubes, also filled with air, called tracheoles. The spiracles, tracheae, and tracheoles together form the gas exchange organs of terrestrial arthropods, the tracheal system.
Spiders' gas exchange organs are similar in principle to insects', although they differ in some details of their construction. Spiders and other arachnids have a voluminous gas exchange organ known as a book lung, which consists of a series of chitinous sheets, known as lamellae, bounded on one side by air and on the other by the spider's body fluids, or hemolymph. In most spiders, the book lungs have elaborated into a tracheal system that is constructed and works very much like the tracheal systems of insects.
Gas exchange in the tracheal system takes place solely by diffusion between the air in the tracheole and the cell the tracheole serves, along the length of the tracheoles, and subsequently between the tracheolar air and outside air. The importance of using air as a distribution medium may be understood by examining Fick's law:
(See equation 5.4 for a review of Fick's law.) Generally, diffusion coefficients for oxygen diffusing through air are about 10,000 times larger than those for oxygen diffusing through water. By using air as the medium for distributing gases through the body, insects and arachnids can exploit these very high diffusion coefficients in a way vertebrates have not. Vertebrates, of course, distribute gases via the blood, and the very low diffusion rates for gases in blood have forced vertebrates into adopting convection-driven gas exchange systems.
It is important to remember at this stage that convection systems do offer significant advantages over strictly diffusion-based systems. For one thing, they allow animals possessing them to attain much larger body sizes. Vertebrates, with their complicated convective exchange systems, have attained body sizes much larger than any insect, living or extinct, of which we are aware. Also, convection systems offer a greater degree of control over gas exchange. Diffusion-based gas exchangers can alter exchange rates only by altering the partial pressure gradients that drive gases. Because insects exchange gases with the very large sources and sinks of the atmosphere, these partial pressure gradients can change only through variation of the gas partial pressures in the insect. An insect that increases its oxygen consumption rate, for example, does so by becoming hypoxic, which can have undesirable consequences for the insect's metabolism. In a convection exchanger, gas exchange rates can be altered by changing convection rates rather than partial pressure gradients. When we exercise, for example, the higher exchange rates for oxygen and carbon dioxide are accommodated mostly by increased ventilation of the lungs and increased heart rate. The blood concentrations of oxygen and carbon dioxide, in contrast to those in insects, remain relatively steady. Indeed, many highly active insects have developed ventilatory schemes to take advantage of the benefits afforded by convection exchange.
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