I'm a nurse, so my physics background is about that of a lay person. Please explain on that level. Ordinarily an Intravenous (IV) fluid bag is one liter. An IV piggyback is usually 2500ml. usually the fluid in each is the same 0.45% NaCl. The piggyback may also get medication mixed in with it. such as a Gm of Rocephin. But the effect is the same without it.
The main one has a small gauge tube that is spiked into the port at the bottom of the bag that continues to the patient's arm, infused into the vein with, commonly a 18 Ga catheter. That tube will have a port, there the same gauge tube can be connected to the piggy back bag. The Main bag is on a pole, about 1.5 meters above the infusion site. the piggyback is placed just higher than the main bag. The accepted explanation for why the piggyback flows and runs out before the main one starts (intentional) is gravity. To say the higher one has higher gravity. I tend to disbelieve this explanation. Does anyone know what the real reason for this effect is? assume all settings are equal and there is no pump, and is gravity fed.