Your eyes are wet all day and yet they do not drip. That balance is the work of a small plumbing system with a supply end, a spreading mechanism and a drain, all of it running quietly behind your eyelids.
When people notice the system at all, it is because something has gone wrong at one end or the other. Eyes that feel like sandpaper mean supply trouble. Eyes that stream down your cheek on a cold morning usually mean the drain is struggling.
Here is how the whole loop is put together.
Where tears are made
The main tear factory is the lacrimal gland, an almond-shaped structure tucked up under the bone at the outer top corner of each eye socket. You cannot see it or feel it. A small lobe of it reaches down behind the upper lid, and the larger part sits deeper in the orbit.
The gland sends its output through several tiny ducts that open under the upper lid. From there, blinking does the distribution. Each blink sweeps fluid across the front of the eye the way a wiper clears a windshield, and pushes the used fluid toward the inner corner.
Nerves control the tap. Signals from the surface of the eye, and from other parts of the nervous system, tell the gland to produce more or less. That is why a gust of wind, a chopped onion or a strong emotion all end with the same result even though the trigger is completely different.
Three layers, three different sources
What sits on your eye is not plain water. It is a thin film with three components, and no single gland makes all of it.
- An oily outer layer from the row of small glands along the lid margins. It slows evaporation and keeps the film from spilling over the lid edge.
- A watery middle layer, the bulk of the film, from the lacrimal gland and smaller accessory glands. It carries oxygen, salts, antibodies and enzymes that discourage bacteria.
- A sticky inner layer of mucus, produced by cells in the clear membrane over the white of the eye. It lets the watery layer grip a surface that would otherwise repel it.
All three have to be present and in reasonable proportion. A film that is short on oil evaporates too quickly. A film short on mucus beads up and slides off. Either way, the surface goes patchy, and patchy is what your brain registers as blur, grit and burning.
Three kinds of tears
Doctors usually separate tears into three types. Basal tears are the steady background supply, present every waking minute. Reflex tears are the flood that arrives when something irritates the eye, and they come fast and in volume to flush the irritant away.
Emotional tears are the outlier. They are produced by the same gland but triggered through different pathways, and their chemistry is not identical to the other two. That difference is real, though what it means is still debated.
The drain, and why crying makes your nose run
Look closely at the inner corner of your eyelid, near the nose, and you can see a tiny opening on both the upper and lower lid. These are the puncta, the entrance to the drainage system.
From each punctum, a short channel runs inward. The two channels merge into a small sac beside the bridge of the nose, and from that sac a tube called the nasolacrimal duct travels down and empties into the nasal cavity behind the nostril. Blinking acts as a pump, drawing fluid in and pushing it along.
So the answer to the runny nose is simple: your tears were never meant to go down your face. They were meant to go down your nose. When production outruns the capacity of that drain, the excess spills over the lid instead, and what does get through arrives in your nose as a drip.
When production falls short
The gland can under-produce for many reasons. Age reduces output. So do certain autoimmune conditions, some common medications, hormonal changes and long stretches of concentrated screen work, where the blink rate drops and the film thins between blinks.
Confusingly, one of the most familiar signs of a dry eye is a watery eye. A surface that has dried out sends an irritation signal, the gland answers with a burst of reflex tears, and you get flooding on top of dryness. Treating that as an excess of tears rather than a shortage is a common wrong turn.
When drainage fails
Blockage can happen anywhere along the route. Puncta can narrow or seal over. The duct can be obstructed by inflammation, scarring, or in newborns a membrane that has not yet opened. When fluid stalls in the sac, it can become infected, which shows up as a tender, red swelling beside the nose.
Constant tearing is worth having looked at rather than living with, partly for comfort and partly because the causes range from trivial to worth treating promptly. An eye doctor can test whether the drain is open and where the holdup is.
Common questions
Can you run out of tears?
Not permanently. Heavy crying can outpace production for a while and leave the eyes sore and gritty, but the gland refills.
Why does one eye water more than the other?
Drainage is rarely identical on both sides, and a partial narrowing on one side is common. Persistent one-sided tearing is worth mentioning at an exam.
Do tears have any role in healing?
Yes. Along with antibodies and antibacterial enzymes, tears carry growth factors that help the surface of the eye repair small injuries.
Most tear problems are comfort problems, but they are also the kind that a proper look at the lids, film and drainage can sort out quickly. If your eyes are constantly dry, constantly wet, or somehow both, it is worth booking time with an eye doctor in your plan's network to find out which end of the system is at fault.