Twin organs located in the lower back. Their function is to filter impurities from the blood. Regarding the Hebrew word kela·yohthʹ (kidneys), J. N. Oswalt wrote: “When used figuratively, the term refers to the innermost aspects of personality.” (Theological Wordbook of the Old Testament, edited by R. Laird Harris, 1980, Vol. 1, p. 440; compare Ps 7:9, ftn.) The same is true of the Greek word ne·phroiʹ (kidneys).​—Re 2:23, ftn.

As with all the organs of the body, the kidneys were directly designed by Jehovah God the Creator. (Ps 139:13) In sacrificial animals, the fat around the kidneys was considered especially choice and was specifically mentioned as something that was to be made to smoke on the altar along with the kidneys in communion sacrifices (Le 3:10, 11; 9:19, 20), sin offerings (Le 4:8, 9; 8:14, 16; 9:10), and guilt offerings (Le 7:1, 4). In the installation of the priesthood, the kidneys of the ram of installation were first waved and then burned on the altar. (Ex 29:22, 24, 25; Le 8:25, 27, 28) In this significance of choiceness, Moses spoke of Jehovah as feeding his people Israel with “the kidney fat of wheat.”​—De 32:14.

Because of their position deep in the body, the kidneys are among the most inaccessible organs. With good reason, therefore, the Bible uses the term to represent the deepest thoughts and emotions of one’s personality. A wound in the kidneys would be a very deep wound, either literally or figuratively considered. (Job 16:13; Ps 73:21; La 3:13) At times kidneys are mentioned in close connection with the heart, which is used figuratively for the total inner person.​—Jer 11:20; 17:10; 20:12; see HEART.

Our Creator knows the makeup of man in the most thorough and intimate way. Therefore, Jehovah is said to test out “heart and kidneys,” even as his Son also searches “the kidneys and hearts.” (Ps 7:9; Re 2:23) Just as one refines silver, Jehovah can “refine” the kidneys and heart of a person so he may become right before God, being made more sensitive to Jehovah’s ways.​—Ps 26:2; 66:10.

At Psalm 16:7 David wrote: “I shall bless Jehovah, who has given me advice. Really, during the nights my kidneys have corrected me.” God’s advice reached the innermost recesses of David’s being as a worshiper of the true God. Because it had settled so deeply within, the “advice” was identified with the “kidneys,” and thus it could be said that the kidneys corrected David.

God’s Word, although it might be on the lips of the wicked ones, does not reach down to the innermost aspects of their personality. Thus, regarding the wicked ones, Jeremiah 12:2 says: “They keep going ahead; they have also produced fruit. You are near in their mouth, but far away from their kidneys.” This parallels the scripture at Isaiah 29:13, quoted by Jesus at Matthew 15:7, 8, which says the same thing about the heart of the wicked, thus showing that at times “heart” and “kidneys” are used in a corresponding way.