A hearty "good on ya" to everyone who donates whole blood or blood products.
I did the apheresis process ("in which the blood of a donor or patient is passed through an apparatus that separates out one particular constituent and returns the remainder to the circulation") to donate platelets at my local Red Cross regularly for more than 15 years. Went every two weeks for a long while and made more than 200 donations to date. Don't know what that equates to in gallons because some were doubles and some weren't. 200 single pint donations would equate to 25 gallons.
I haven't done it for a few years because I was experienceing extreme cramping in my donor arm and, believe it or not, I've seemed to become more sensitive to the pain of the needle sticks. (You would think the area would become de-sensitized.) But I'm feeling guilty lately. I should go back.
When compared to whole blood donations, the positives (no pun intended) are 1) that you feel absolutely none of the weakness you typically feel following a whole blood transfusion and 2) you are helping many people (like cancer patients) that need platelet transfusions to stay alive.
The negatives are 1) they use two needles (double ouch!) and 2) it takes about two hours.
They usually insert one needle in the normal whole blood donation point (inside of one elbow) and completely immobilize that arm. They have options where to insert the return needle. The simplest is the inside of the other elbow, but that means your other arm is also immobilized. Don't know why, but that always made my nose itch.
So they can insert the return needle into a vein in your other wrist, on the back of your hand, or (I believe) even in one of your legs. I almost always had it in the wrist. They can wrap it up with Coban and you can move that arm enough to read, etc.
They can utilize any blood type. I am an AB negative which is the rarest blood type (1 out of 200 people) and they still called me regularly to donate.