U.S. scientists are getting a better understanding of human tears.

A team from Ohio State University say they've identified a new class of lipids (a type of fat) that makes up part of the tear film.

These new lipids, including a specific kind called oleamide, are contained in a layer of tear film called meibum, which is the outermost of the three distinct layers in tear film. There is also a middle, watery layer and an inner layer of mucus.

The meibum, which includes fatty, oily substances, is spread over the surface of the eyes each time we blink. The meibum keeps the watery middle layer in place, ensuring the eyes stay moist.

Identifying the lipids contained in the meibum may help researchers learn more about the causes of eye problems such as dry eye disease, which affects about 12 million to 14 million Americans, said study author Kelly Nichols, an assistant professor of optometry at Ohio State.

"The lack of certain compounds in the tear film may result in a number of different eye-related disorders, including dry eye. The amount of oleamide and related lipids in tear film may be related to these disorders," Nichols said in a prepared statement.

The study is published in the current issue of Investigative Ophthalmology and Visual Science.

It was already known that oleamide plays a role in the brain, where one of its tasks is to help induce sleep, and in the central nervous system. This is the first study to identify oleamide and related lipids in tear film.

"The findings could give us more insight into the role of lipid activity in humans and may also indicate a new function for oleamide and related lipids in cellular signaling in the eye and in the maintenance of tear film," Nichols said.

The function of oleamide and related lipids in tear film isn't fully understood.

"Oleamide appears to predominate in tear film. It's there for a reason, but we're not sure yet what that reason is," Nichols said.