Antonio Manaligod/Dose

Blood banks don’t want you to know this.

Welcome to the Glad You Asked series, a shame-free zone where we tackle topics you’re too embarrassed to ask even your BFF about. Don’t worry, we gotchu

After September 11, the Red Cross received over 475,000 units of blood. (One unit = about 1 pint.) There was just one problem: Only 258 units were required to treat the 139 hospitalized survivors.

Blood donation is a surprisingly sensitive subject. Blood banks are loath to disclose how much blood gets thrown away every year, fearing that these figures will dissuade potential donors from volunteering their time and bodily fluids.

The Red Cross supplies about 40% of America’s blood stock. The organization conducts over 145,000 blood drives a year, soliciting donations from volunteers.

The blood donated in the aftermath of 9/11 didn’t completely go to waste: The Red Cross used much of the excess to treat other patients around the country. Some of the blood, however, still needed to be destroyed.

Which begs the question: Where does unused blood go?

Remind me how blood works, again?

Adults carry around 10 pints of blood in their bodies, but blood banks only accept one pint per volunteer during donation. Most volunteers donate “whole blood,” which is then broken down into three main components: Red cells, plasma and platelets.

The red cells are used to transport oxygen throughout the body?—?patients suffering from anemia or iron deficiencies frequently receive red cell transfusions. Blood banks receive more red cell transfusions than any other blood component, which means that these donations are the most frequently discarded.

Platelet donations are available only at select Red Cross donation centers and are a vital part of cancer and organ transplant treatments. Platelets assist in blood clotting and the process of donating platelets can take up to two-and-a-half hours. Platelet donors are in demand: It takes six whole blood donations to achieve the same number of platelets that one platelet donation can obtain.

Plasma carries nutrients, hormones and proteins throughout the body. A plasma donation occurs concurrently with a platelet donation, but excess cells are filtered out and returned to the donor.

Where does unused blood go?

The main challenge in blood donation is shelf life: Blood can last for 42 days when stored in a refrigerator at a temperature of 6 degrees Celsius; platelets can last for five days, provided they are stored in platelet agitators, which control for temperature; plasma is frozen and can last for an entire year. Blood components that expire are utilized as research material or treated as medical waste and incinerated.

Like plasma, blood can be frozen and stored for up to ten years, but experts agree that this is a less than ideal way of preserving blood. Generally, only very rare blood types are frozen, and only in special circumstances.

The Red Cross website writes that “Blood cannot be manufactured?—?it can only come from generous donors.” This isn’t entirely true: Scientists are producing artificial blood, but the product isn’t nearly as functional as human blood. Artificial blood can carry oxygen to the body and has a much longer shelf life than its natural counterpart, but it cannot replace the functions completed by plasma or platelets.

How your blood type affects your donation

It’s understandable that blood donors would take issue with the idea that their generous contributions are going to waste. However, blood inventory fluctuates, so the best way to prevent a blood surplus is to ask a local Red Cross chapter or to look for areas that advertise a specific blood shortage.

Healthy donors are eligible to donate every 56 days, and blood banks request that donors space their contributions out throughout the year, rather than waiting to donate on the heels of a natural disaster.

Understanding your blood type is also key in preventing blood redundancies: People with Type O blood are universal donors and are always needed to donate red blood cells. Those with Type AB blood should donate plasma, as this blood group is a universal plasma donor. By donating the parts of your blood that are most sought after, blood banks can more efficiently utilize the product and prevent excess and waste.