 You would have noticed that many of our thermodynamic systems can be modded or can be approximated or abstracted as a cylinder piston arrangement. And because of that you will notice that a large number of illustrations and figures in thermodynamics books will be that of a cylinder piston arrangement. So if you see this arrangement you will find the cylinder inside the cylinder there will be a piston and there will be a mechanism a piston rod or a connecting rod which will move the piston up and down. The cylinder will have a cover known as the cylinder head and typically let us assume that the cover will have two valves and two poles in which the valves are located. So let us say this is the inlet port and this is the exhaust port. This will be a typical cylinder piston arrangement. This is the inlet port with inlet valve and this is the exhaust port and this is the exhaust valve. Our system will be whatever is the fluid which is enclosed inside this cylinder piston arrangement. If both valves are closed then we have a closed thermodynamic system and our illustrations of the wheel, the water in the bottle and a few others are illustrations of closed system. Whereas the syringe the food pump which we saw in the previous set of illustrations are open system because if at least one valve or one passage is open then we have an open system. Remember that the cylinder piston arrangement is an illustration for an approximation or an abstraction of thermodynamic system. And as we proceed we will find we will be able to approximate not only closed systems and open systems by using this scheme but even in real life you will notice that not only a syringe but a similar arrangement of that of a gun or a cannon is that of a cylinder piston arrangement. Not only that you consider our lungs to be an open thermodynamic system. We breathe in air and we breathe out whatever comes out of it. And there is a diaphragm which moves up and down so that something similar to a cylinder piston arrangement. So as I breathe in and as I breathe out that diaphragm is moving up and down so there is some sort of a natural cylinder piston arrangement even within us.