Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 584-A   December 6, 1975
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

Not all blood is red. We are so accustomed to the idea that blood is red that some of us are surprised to learn that it comes in other colors. It is true that all animals with backbones have red blood -- mammals, birds, turtles, snakes, frogs, salamanders and fish. However, among lower animals we find blood that is colorless, blue or green -- as well as a few with red blood.

If an animal is large enough to be seen with the naked eye, it is almost certain to have some sort of a circulatory system. Usually, this is a plumbing system with pipes, valves and pumps which brings nourishment and oxygen to the living cells of the animal's body and carries away waste products. The fluid flowing in this plumbing system is blood. In man and the higher animals this blood stream is completely enclosed in the heart, arteries, veins and their smaller branches. That is called a closed circulatory system.

Many lower animals, in contrast, have an open circulatory system in which blood fills the body spaces and bathes the living tissues directly. A few lower forms of animal life, although quite large, have no blood at all. The sponge, for example, instead of blood, circulates water through a network of small canals. The living parts of a jellyfish are in a thin outer layer in close contact with the water while its inner jelly is practically lifeless.

An insect has a simple tubular heart which pumps the blood forward into the head. From there it flows around the internal organs, through the legs and other appendages, then back into a central pool, or blood sinus, in which the heart lies. In many naked caterpillars with a transparent streak down the back, this pulsating heart can be seen just beneath the surface, and, under a microscope, the blood corpuscles as they rush into the heart each time the valves along its sides open. In insects which squirm or wriggle, the blood and the internal organs surge back and forth with every movement.

Insects are exceptional. Their blood furnishes little or no oxygen to the living tissues. Instead, they have many-branched ducts opening to the outside, called a tracheal system, which carry air directly to every part of the body.

The blood of all higher animals, from man down to fish, is made up largely of red blood cells whose principal work is to carry oxygen from the lungs or gills throughout the body. These red blood cells contain hemoglobin -- a protein with an atom of iron in each of its large molecules -- which is able to pick up or release large amounts of oxygen readily. Such blood is bright red when it leaves the lungs or gills and is dark purplish red when it returns after giving up its oxygen. Some lower animals such as earthworms, pond snails, land snails, water fleas and midge larvae also have red blood. In these the hemoglobin is not in blood corpuscles but is dissolved in the blood liquids.

Many other lower animals carry oxygen in their blood stream by means of a dissolved substance called hemocyanin which is blue. At least it is blue when it is oxygenated, but colorless after the oxygen is released. Hemocyanin is much like hemoglobin except that the iron atom in the protein molecule is replaced by one of copper. The lobster, crab, crayfish, scorpion, octopus, squid, clam and mussel all have this blue blood. A few marine worms have green blood.

The blood stream of all kinds of animals has a variety of free-floating or creeping cells much like the white blood cells in our own blood. Among other tasks, these devour bacteria, foreign substances and bits of dead tissue.

Blue-blooded aristocrats bleed red.

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