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
Red blood cell form. When red blood cells sometimes occur as a stack, flat side next to flat side. This is known as "rouleaux formation", and it occurs more often if the levels of certain serum proteins are elevated, as for instance during inflammation. Red blood cells can be obtained from whole blood by centrifugation, which separates the cells from the blood plasma in a process known as blood fractionation. Packed red blood cells, which are made in this way from whole blood with the plasma removed, are used in transfusion medicine. During plasma donation, the red blood cells are pumped back into