Today I donated blood. Well, that's not really accurate. I actually donated only red blood cells. This was the first time I'd ever donated just red blood cells, as opposed to whole blood. It was an interesting experience.
The reason they only wanted to collect red blood cells is apparently because they have a shortage of them. They told me that when you donate whole blood, it gets split up into its separate parts into 4 different bags. This means that once you've extracted the red blood cells from the whole blood, you really don't have much blood. So, rather than donating whole blood and filtering out the red blood cells at the end, they simply collect only the red blood cells.
They do this by hooking you up to a machine that basically pumps out your blood, filters out the red blood cells, then replaces the missing cells with some saline solution and pumps it back in. I was expecting two needles in my arm. One to pump the blood out and one to pump the blood back in. It turned out it was all done through a single tube. It would just pump the blood out for a while, then it would stop pumping out and switch to pumping it back in. I was watching this tube gradually switching back and forth from red to clear. It was kind of neat to see.
I donate blood pretty regularly. I don't mind needles. I usually donate every two months, which is the normal minimum time between donations of whole blood. This time, however, they told me I had to wait four months before donating again. This is because when they filter out the red blood cells, they are taking out a larger volume of blood, so it takes the body longer to rebuild it's supply of blood.