Why is a blood transfusion bag held above the patient, but a blood donation collection bag is held below the donor?

What decides which way blood flows in a tube

Blood (like any fluid) moves from higher pressure to lower pressure. So the key question is, "Where is the higher pressure: the bag or the vein?"

Why the transfusion bag is hung above the patient

A bag of donor blood is not being pumped by a heart, it is just a reservoir. By raising it a height $h$ above the vein, gravity creates extra driving pressure (hydrostatic head):

$$\Delta P = \rho g h$$

That added $\Delta P$ helps the blood flow into the patient’s vein despite:

• the patient’s venous pressure pushing the other way,
• resistance from the needle and tubing,
• any filter in the line.

Why the donation bag is placed below the donor’s arm

During donation, the donor’s own blood is still under pressure from the cardiovascular system. Venous pressure plus the needle opening is already enough to start flow outward. Putting the collection bag lower than the arm adds a helpful gravitational pressure drop along the tubing, so blood continues to drain smoothly into the bag, and it also reduces the chance of any backflow toward the vein.

A quick way to remember it

Transfusion: the bag must supply the pressure, so it goes higher. Donation: the body supplies the pressure, so the bag goes lower to help drainage.