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 (Lev. 3:10, 11; 9:19, 20), sin offerings (Lev. 4:8, 9; 8:14, 16; 9:10) and guilt offerings. (Lev. 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; Lev. 8:25, 27, 28) In this significance of choiceness, Moses spoke of Jehovah as feeding his people Israel with the “kidney fat of wheat” (“hearts of wheat,” NW, 1953 ed., ftn.).—Deut. 32:14.
The position of the kidneys deep in the body places them as among the most inaccessible organs. The Bible applies the term as relating to the inmost thoughts and deepest emotions. A wound in the kidneys would be a very deep wound, either literally or figuratively considered. (Job 16:13; Ps. 73:21; Lam. 3:13) Several times kidneys are mentioned in close connection with the heart, which is itself intimately associated with human emotions, such as affection, and motivation. (Jer. 11:20; 20:12) The kidneys are, in fact, affected by deep emotions, according to medical authorities, who say that sustained emotional strain can cause such diseases as diabetes insipidous (not “sugar diabetes”), in which the kidneys fail to function properly. So the Bible usage of the term is not based on imagination or tradition.
Jehovah knows the makeup of man in the most thorough and intimate manner, therefore He Is said to search out and to test out the “kidneys,” even as his Son also searches the “inmost thoughts [literally, “kidneys”] and hearts.” (Ps. 7:9; Rev. 2:23) Jehovah can “refine” the kidneys or “deepest emotions” of a person so that they become right before Him, and are made sensitive to that which is right or wrong.—Ps. 26:2; 16:7; Prov. 23:16; Jer. 12:2; compare NW, 1957, 1958 editions, footnotes.