Blood serum is yellow. The red color of human blood comes from hemochrome, a pigment that transports oxygen throughout the bloodstream. Blood's liquid nature, however, comes from the serum. According to About.com's Regina Bailey, plasma makes up 55 percent of human blood by volume.
The term "blood serum" refers to the liquid component of blood with the clotting factors removed. Serum contains water, proteins, glucose, hormones, electrolytes and carbon dioxide. It also transports platelets, red and white blood cells throughout the body. These cells are suspended within the plasma and are not part of it.
According to the American Red Cross, blood transfusions save thousands of lives daily. Some donors give "whole blood," which takes only moments but causes considerable depletion of the donor's red and white blood cells. The American Red Cross explains that this is why whole blood donors must wait at least eight weeks between donations.
Others donate plasma through a lengthy procedure called plasmapheresis. During plasmapheresis, a phlebotomist extracts whole blood, runs it through a centrifuge, pours off the plasma and returns the rest of the blood to the donor. Healthy donors often give plasma two to three times a week. According to BioLife Plasma Services, each plasmapheresis session lasts between 45 minutes to 1 hour.