As a result of the non-selectivity of the kidney's filtration of small molecules, see concept 44.4 (page 986) as a result of the non-selectivity of the kidney's filtration of small molecules, see concept 44.4 (page 986) the kidneys have little control over body fluid composition useful substances must be selectively reabsorbed the proportions of all the substances in the blood are the same as in the urine many useful substances are lost in the urine urine is always much less concentrated than blood