I know this will most likely be a relatively simply answer but I am currently doing some questions regarding forces acting on the piston of a syringe.
My (basic) question is, if I fill a syringe full of liquid, what is stopping the liquid in it 'falling' out (and exchanging with air)? Is it that the atmospheric pressure is acting on the plunger and the mouth of it, creating an equilibrium condition? Or that the liquid and air can't escape/interchange due to some force?
Secondly if I connect this syringe to a body of liquid, e.g. the human blood stream, what stops blood from interchanging with the liquid in the syringe. If the blood is at a higher pressure than the syringe liquid, then it would flow in? or vice versa? without moving the plunger?
Any help would be greatly appreciated!
Best Answer
For a typical syringe that uses either a Luerlock or catheter connection, the diameter is relatively small and the capillary (surface tension) forces at the liquid/air interface are sufficiently strong to prevent shearing of the interface and intrusion of an air bubble when the syringe is pointed down.
The liquid column in the barrel, although having weight and resulting in a downward pressure at the bottom of the barrel is opposed by vacuum pressure at the upper part near the plunger. It's this net pressure that challenges only that area where the connection channel extends. So the downward force of the liquid in that channel is relatively small; smaller than the surface tension forces at the interface.
If you are connected to a vein, venous pressure is usually not high enough to create enough force to move the plunger back and allow blood to enter the syringe. But if connected to an artery, and the syringe barrel diameter is large enough (say a 50 mL syringe for example) there may be enough pressure to move the plunger back. This is why it's better to use a smaller diameter syringe and longer barrel for venous connections.