lungs and liver
Formal or literary term for the lungs and liver as organs. Rare in everyday conversation; more common in medical or classical contexts.
depths of one's heart; innermost heart
Figurative, formal/literary expression for one's deepest feelings or true intentions. Often used in set phrases like 肺肝を披く (to open one's heart) or 肺肝を砕く (to rack one's brains).
In classical medicine, harmony between the lungs and liver was considered key to health.
He opened his heart to his friend and confided his troubles.
After racking his brains, he finally found a solution.