Plasma is collected through a process called "plasmapheresis."  When you come in to donate, a needle is placed in your vein and your blood is pumped into a specialized spinning device that separates the plasma from the other whole blood components, such as red and white blood cells and platelets.  While the plasma is collected, the other blood components are filtered into a reservoir. 

Donating blood doesn’t pay, but donating plasma—the part of blood used to treat hemophiliacs and burn victims—does. See if you qualify.  And more free Personal Finance How-to Videos on Howcast.com.