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 kept with a minimal amount of fluid. Generally, an additive solution of citrate, dextrose, and adenine is mixed with the cells to keep them alive during storage. This process is sometimes done as automated apheresis, where the centrifuging and mixing take place at the donation site. Red blood cells are sometimes modified to address specific patient needs. The most common modification is leukoreduction, where the donor blood is filtered to remove white cells, although this is becoming increasingly universal throughout the blood supply (over 80% in the US, 100% in Europe). The blood may also be irradiated, which destroys