I will be writing a post summarizing the topic of kidneys. Meanwhile, I see from the previous posts that people are having a hard time with the application of dibra Torah k'lashon bnei adam, “the Torah speaks in the language of men,” to the kidneys. Here is a quote from Rav Hirsch that should make it easier:

Jewish scholarship has never regarded the Bible as a textbook for physical or even abstract doctrines. In its view the main emphasis of the Bible is always on the ethical and social structure and development of life on earth; that is, on the observance of laws through which the momentous events of our nation’s history are converted from abstract truths into concrete convictions. That is why Jewish scholarship regards the Bible as speaking consistently in “human language;” the Bible does not describe things in terms of objective truths known only to God, but in terms of human understanding, which is, after all, the basis for human language and expression. (Collected Writings vol. 7 p. 57)