Concise Bible Dictionary:

Human Kidneys
The kidneys, used symbolically for the inward thoughts and feelings (Psa. 7:99Oh let the wickedness of the wicked come to an end; but establish the just: for the righteous God trieth the hearts and reins. (Psalm 7:9); Psa. 16:77I will bless the Lord, who hath given me counsel: my reins also instruct me in the night seasons. (Psalm 16:7); Psa. 26:22Examine me, O Lord, and prove me; try my reins and my heart. (Psalm 26:2); Psa. 139:1313For thou hast possessed my reins: thou hast covered me in my mother's womb. (Psalm 139:13); Prov. 23:1616Yea, my reins shall rejoice, when thy lips speak right things. (Proverbs 23:16); Jer. 12:22Thou hast planted them, yea, they have taken root: they grow, yea, they bring forth fruit: thou art near in their mouth, and far from their reins. (Jeremiah 12:2); Jer. 17:1010I the Lord search the heart, I try the reins, even to give every man according to his ways, and according to the fruit of his doings. (Jeremiah 17:10); Jer. 20:1212But, O Lord of hosts, that triest the righteous, and seest the reins and the heart, let me see thy vengeance on them: for unto thee have I opened my cause. (Jeremiah 20:12); Lam. 3:1313He hath caused the arrows of his quiver to enter into my reins. (Lamentations 3:13); Rev. 2:2323And I will kill her children with death; and all the churches shall know that I am he which searcheth the reins and hearts: and I will give unto every one of you according to your works. (Revelation 2:23)). The word translated “reins” in Isaiah 11:55And righteousness shall be the girdle of his loins, and faithfulness the girdle of his reins. (Isaiah 11:5) is elsewhere translated “loins.”