“heart.” The Hebrew placed thinking and planning in the heart, and emotions in the organs of the abdomen, the bowels and kidneys. So this verse is saying that God tests our thoughts and emotions so that He can reward us justly.
“kidneys.” The Word of God points to the fact that our kidneys, bowels, and belly (or womb) are part of our mental/emotional life, not “just physical organs.” Our “gut,” including our intestines, bowels, kidneys and stomach contain as many nerve cells as our brain, and studies are now showing that our “gut” contributes significantly to our emotional life and health. [For more on the kidneys and our emotional life, see commentary on Rev. 2:23, “kidneys”].