Blood many invertebrates, these oxygen-carrying proteins are freely soluble in the blood; in vertebrates they are contained in specialized red blood cells, allowing for a higher concentration of respiratory pigments without increasing viscosity or damaging blood filtering organs like the kidneys. Giant tube worms have unusual hemoglobins that allow them to live in extraordinary environments. These hemoglobins also carry sulfides normally fatal in other animals. The coloring matter of blood (hemochrome) is largely due to the protein in the blood responsible for oxygen transport. Different groups of organisms use different proteins. Hemoglobin is the principal determinant of the color of blood
Packed red blood cells are used in these situations, since they are unlikely to cause a reaction. A potentially fatal reaction is possible if the recipient has pre-existing antibodies, and uncrossmatched blood is only used in dire circumstances. Since O negative blood is not common, other blood types may be used if the situation is desperate. Most frequently, whole blood is collected from a blood donation and is spun in a centrifuge. The red blood cells are denser and settle to the bottom, and the majority of the liquid blood plasma remains on the top. The plasma is separated and the red blood cells