A liquid carrying food and around the body of many-celled animals, and carries away waste products such as carbon dioxide. All the major groups of higher than the jellyfish have blood.
Protozoa and early multicellular animals have no blood and give and take substances with the environment by simple diffusion.
The blood of small worms and mollusks has no oxygen-binding substance. Later animals had molecules capable of transporting large amounts of oxygen. In many invertebrates these molecules are dissolved in the plasma.
Hemocyanin, a copper-containing protein chemically unlike hemoglobin, is found in some crabs and other lower animals. Hemocyanin is blue in color when bound to oxygen and colorless without it.
Some invertebrates have hemoglobin in solution in the plasma. In almost all vertebrates, including humans, hemoglobin is contained exclusively within the red cells (erythrocyte) of the blood.
The red cells of the lower vertebrates (e.g., birds) have a nucleus, whereas mammalian red cells have no nucleus.
The blood is pumped round the body by the heart. When emerged from the water onto the land, the way the blood circulated had to change. It no longer flowed through the gills, but now had to flow through the lungs, which were in a different part of the body.
In reptiles the blood coming from the lungs, which is rich in oxygen, is mixed up in the heart with blood coming from the body, which is low in oxygen. This blood is then pumped back to the body and the lungs. This means that the body never gets blood which is very rich in oxygen.
In mammals these two types of blood are separated in the two parts of the heart. This means that their bodies get blood which is rich in oxygen, so they are more efficient.
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