KIDNEYS (כְּלָיֹ֖ות; LXX νεφροί; in KJV also called reins) are paired organs, kidney bean in shape, approximately 4 x 2 x 1 inches in size, located in the loin directly in back of the abdominal cavity on either side of the vertebral spine. They are well protected from behind by the thick muscles of the back. Their function is primarily that of excretion of waste products from the blood. To accomplish this, blood flows through the capillaries of the glomerulus, of which there are approximately 1,000,000 in each kidney, each one being surrounded by a so-called Bowman’s capsule. In doing so, filtration from the blood which courses through these capillaries takes place. From each Bowman’s capsule this filtrate fluid is carried off into a corresponding tubule from which the lining cells selectively re-absorb some of the substances which the body needs to conserve including water, and selectively excrete other substances of which the body needs to be relieved. The end product which is emptied out of these tubules is urine which is collected into the funnel-shaped pelvis of the kidney whence it passes down the ureter to the bladder for elimination from the body.
Apparently the ancients were misinformed as to the function of the kidneys. Many of the functions, which we now know are performed by the brain including thinking and emotional reactions, were considered by them to be performed by the kidneys. This was not without some degree of justification. When the kidneys become severely diseased, uremia often precipitously puts in its appearance because of inadequate kidney function. While symptoms of uremia are exceedingly variable, one of the most startling symptoms is sudden lapse into coma or unconsciousness. Because this state is not necessarily irreversible, it can be easily seen that with return of consciousness associated with return of kidney function as manifested by reappearance of urine excretion, the ancients had some justification in attributing thinking and emotional reactions to the kidneys instead of recognizing them as brain functions. Thus one finds these functions attributed to the kidneys (reins) (Ps 7:9 KJV; 16:7; 26:2; Rev 2:23).
R. Dunglison, A Dictionary of Medical Science (1868), 835; H. W. Smith, “Excretion (Kidney),” EBr (1963) VIII, 949, 950; S. E. Bradley, “Kidney, Diseases of,” EBr (1963) XIII, 371, 372; P. J. Harman, “Urinary System,” EBr (1963) XXII, 897, 898.