Force to hold contents of a syringe in 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!
 A: Basically, I would say it depends on the viscosity of the liquid in the syringe. The more viscose it is, the less it can propagate through a tiny space such as a needle because the wall adds friction and air's pressure can easily push it, preventing it to go through. It won't exchange with air because it doesn't allow much deformation. However, if you push enough, you can force it through.
If it's something else then air, than it's really the same thing, but you need to take into account both fluids' viscosity and pressure.
I hope this helps you!
A: If you place a syringe in a cup of water and raise the piston, then what little air is in the syringe will increase in volume, and thus the pressure of the air will decrease far below atmospheric pressure. Since atmospheric pressure is pushing on the water in the cup, it will be forced into the syringe because the force of the air pushing on the water from inside the syringe is less than the force of the atmosphere pushing the water. Water will enter the syringe until the additional weight of the water matches the force with which the atmosphere pushes on the water in the syringe from the outside. Thus, when a syringe has been filled, the liquid can't exit it without decreasing the internal pressure; the push of atmospheric pressure will keep the contents in it.
As for the second part of your question, I'm not entirely sure; gingras.ol's answer seems like a convincing explanation for that part of the question to me. As far as the plunger moving, the plunger is far from frictionless, so it would take a lot of pressure to move it, and blood pressure isn't that much greater than atmospheric pressure. 
A: 
My (basic) question is, if I fill a syringe full of liquid, what is
  stopping the liquid in it 'falling' out ...

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.

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 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.
