Its primary structural role is to define the free edge of the distal nail plate.
The lunula is most visible on the thumb, however it is not visible on every person.
The lunula and the nail itself are good places to find warning signs of diseases such as liver disease, kidney disease, lung disease and heart problems. Any color differences or unusual lines may indicate some type of disease or insufficiency. The vertical lines on fingernails however are normal as we age.

Lunular anomalies include: changes in form, structure or color.
Lunular dysmorphologic features can be characterized by macrolunula, microlunula, anolunula, or a nonconvex lunula.
Lunular dyschromias (color anomalies) can be spotted or characterized by longitudinal colored bands that traverse the lunula. Alterations in the morphologic features or color of the lunula can be an indication of a cutaneous or a systemic disorder.
A deep red lunula and pale nails are associated with congestive heart failure.