There are two fluid-filled compartments in the eye. The fluid in the front of the lens is called aqueous humour; behind the lens is the vitreous humour. The more fluid fluid (!) is the front one, the aqueous humour. There is a constant secretion of the fluid into the front compartment of the eye, pumped in by cells found behind the iris (the coloured part of the eye) – they are tucked in the corners behind the iris, and are called the “ciliary epithelium”. The fluid then circulates around the iris, then heads back to the corners of the eyes (this time in front of the iris) to drain away through the “Canals of Schlemm” (great name). So, there is a slow flow from behind the iris to the front.
The water originally comes from your blood plasma (the watery part of the blood), which moves out into the spaces in between your cells, from where it can be pumped. The way in which cells pump fluid in the eye is quite interesting, and is something I get to teach to medical students – the same mechanisms is shared by the eye, parts of the kidney, parts of the lung, and parts of the lining of the brain. This pumping can be slowed by a drug called acetazolamide, which is use to treat glaucoma (a build up of pressure in the eye – stop the pumping and you stop the pressure) and also used to reduce the risk of mountain sickness (because in mountain sickness you get too much fluid pumped into the air spaces in your lungs and too much fluid pumped into your brain – the cerebrospinal fluid). The drug has a side effect though – it makes you pee a lot, because of its actions on the kidney!
Tears are fluid over the outside of the eye, and it comes from small glands called the lacrimal glands, found on the upper, outer corners of your eyes … However, I think I’ve been typing too much and I’ll soon send you to sleep!