Dark Banquet - Bill Schutt [40]
Unlike large complex creatures (like us), microscopic organisms don’t have blood or a wildly intricate circulatory system (and they don’t have organs or organ systems either). For the majority of species on this planet, things are generally much simpler.
Picture how easy it would be for a single-celled amoeba to exchange gases with its environment. With only one cell to supply, oxygen and other incoming substances are obtained directly from the environment. Although some of this transport requires energy expenditure, in many instances, incoming and outgoing material and gases simply follow concentration gradients across the organism’s thin cell membrane. Now imagine a creature shaped like a ball—composed of millions of cells. How would the cells toward the center of the ball get their nutrients and oxygen or rid themselves of waste? Although you might be able to think up several ways that this might occur—the solution that evolved for many creatures here on earth consists of a muscular pump (the heart), a remarkably intricate vascular transportation system, and a unique and versatile tissue carried within it: blood.
Before we get too carried away with just how cool our circulatory systems are, you should know that there are some relatively large organisms that get by just fine without elaborate circulatory systems. Insects, for example have low-pressure systems that are termed open circulatory systems because they don’t form a completely closed loop between the body and heart. Hemolymph (the arthropod equivalent of blood) is circulated through the body by a series of heartlike dorsal pumps and by movement of the insect’s body. The hemolymph moves through vessels that eventually lead to open sinuses called hemocoels. Here, the surrounding internal organs are literally bathed in the nutrient-bearing fluid, which eventually percolates back and reenters the hearts through tiny valves called ostia.
The key here is that unlike animals like vertebrates, insect circulatory systems aren’t involved in transporting gases like O2 and CO2 or exchanging those gases with body tissues.*58 A mosquito’s oxygen requirements are met through a series of openings (spiracles) along both sides of its thorax and abdomen. Air passes from the environment into the spiracles (which can also be closed to prevent water loss), and then through a complex of tubes called tracheae. The tracheae get smaller and smaller, finally branching into microscopic vessels called tracheoles, through which the air finally arrives to supply the tissues and cells.
This system works just fine for small creatures, but there are limitations. For example, tracheal respiration is probably a key reason why mosquitoes (and other insects like bed bugs) aren’t a whole lot bigger in size than they are. Larger animals are composed of too many cells to be efficiently supplied by this type of respiratory system.
Some of you might be saying, “Wait a minute, what about those pictures of ancient dragonflies with three-foot wing spans? How did they get enough oxygen?”
The answer is that there is evidence from the Carboniferous period (290–360 million years ago) that widespread forests and lush plant life resulted in a higher percentage of atmospheric O2 than exists in today’s atmosphere. This extra O2 was apparently enough to support larger species of insects that employed the same tracheal system as their smaller modern-day cousins. Still, during the Carboniferous or even during the age of dinosaurs (when some creatures reached gigantic proportions), there is absolutely