Urea and uric acid are the products of the breakdown of proteins and nucleic acids, respectively. As a result, these metabolic byproducts should be extracted from the organism. Following the filtration, all the components of blood plasma, including urea and uric acid, are in the nephron tubules. Most of the nutrients are reabsorbed, whereas toxic and catabolic products (including urea and uric acid) are extracted from the organism with urine. However, urea is reabsorbed by passive diffusion to maintain the osmotic gradient in the renal medulla. As a result, only 15% of urea is extracted.