What is clam juice to a clam? Is it blood, lymph, or some kind of filler fluid?

I am trying to figure out what clam juice is, biologically speaking. I know it comes from clams, obviously, but what is it to the clam itself while it is alive. The term juice to describe a bodily fluid of an animal is vague and unsettling. Close as I cold get from various sources suggests it is some kind of space fluid between the clam and the shell, like spinal fluid, but I am dubious.