Charles H. Spurgeon on Psalms 73:21

Reins. Before all the other intestines there are the kidneys (twylb, nefroi), placed on both sides of the lumbar vertebrae on the hinder wall of the abdomen, of which the Scripture makes such frequent mention, and in the most psychically significant manner. It brings the most tender and the most inward experience of a manifold kind into association with them. When man is suffering most deeply within, he is pricked in his kidneys ("reins"). When fretting affliction overcomes him, his kidneys are cloven asunder (Job 16:13; compare La 3:13); when he rejoices profoundly, they exult (Pr 23:16); when he feels himself very penetratingly warned, they chasten him (Ps 16:7); when he very earnestly longs, they are consumed away with his body (Job 19:27). As the omniscient and all penetrating knower of the most secret hidden things of man, God is frequently called (from Ps 7:10 to the Apocalypse) the Trier of the hearts and reins; and of the ungodly it is said, that God is far from their reins (Jer 12:2), that is, that he, being withdrawn back into himself, allows not himself to be perceived by them. Franz Delitzsch.