Puzzling over the acient Hebrew inner sense

Spurred on by something, I began simply praying through the Psalms, from beginning to end, last week; and it has been good. Some things are strange, though, and by far the strangest thing in them (though it's hardly the first time I've been struck by this strangeness since diving into the deep end of Catholic Tradition) ... an intermittent expression,

Enflame [our] heart and [our] reins

The heart I understand. I can feel my heart doing all sorts of weird things, reflecting bits of mood that aren't in the top of my head, from affection to choler. Dread sits somewhere else (but, again, is reflected in the heart in a way I notice quicker) But... those "reins"... I'm pretty sure that word is "kidneys". But I can't recall my kidneys ever telling me anything except (sometimes) to wake up. Is... is there more? Certainly, knowing when to wake up is handy, but... and "viscera misericordiae" is... again, frankly, weird, but does certainly express depth and thoroughness anyways.

To be sure, there are ancient traditions situating courage in the Liver, of all things, which again puzzles me (I've never felt my liver do anything at all, and I think we prefer it that way), except that brandy is known to act on both.

Can any of you good folks enlighten me?