Donated blood cannot be manufactured and does not keep for long. Managing the supply is therefore a scheduling problem with an unusually unforgiving deadline.
Components have different shelf lives
Whole blood is separated soon after collection into red cells, plasma and platelets, each stored differently and each usable for a different span of time.
Red cells keep for weeks under refrigeration. Plasma can be frozen and held far longer. Platelets are the constraint, lasting only a handful of days at room temperature.
Because platelets expire fastest and cannot be frozen, they must be collected continuously rather than gathered in advance of expected need.
Demand is not evenly distributed
Hospitals draw on the supply for scheduled surgery, cancer treatment, childbirth complications and trauma. Only some of that is predictable in advance.
Scheduled procedures can be planned around, but trauma arrives without notice and can consume a large volume in a single case.
Collection organisations therefore hold a buffer sized for the unpredictable share, knowing that some of the buffer will expire unused.
Blood types complicate the arithmetic
Compatibility rules mean the inventory is not one pool but several. A surplus of one type does not help a shortage of another.
Some types can be given to a wider range of recipients, which makes them disproportionately valuable in emergencies where there is no time to test.
The distribution of types in the donor population does not perfectly match the distribution of need, so certain types are chronically tighter than others.
Supply follows the calendar
Collection depends on donor availability, which falls during holidays, severe weather and periods when workplace and campus drives are not running.
Demand does not fall in the same weeks, and in some cases rises. The mismatch produces recurring seasonal shortages that are largely predictable in timing.
Organisations respond by encouraging donation ahead of known gaps, though the short shelf life limits how far in advance that can usefully be done.
Screening removes part of the pool
Every donation is tested, and donors are screened by questionnaire before collection. Both steps exist to protect recipients and both reduce usable volume.
Eligibility criteria are set by health authorities and revised as evidence changes, which alters the size of the donor base over time.
Anyone considering donating should check current eligibility with the collecting organisation, since criteria differ by country and are updated periodically.