Blood flows from general circulation through renal arteries to the glomerular capillaries. From there, it goes to the peritubular capillaries , which surround the nephrons. Blood leaves the renal veins and returns to general circulation. The glomerular capillaries, and the proximal tubules and distal tubules of the nephrons filter blood to remove waste. Nephron function is comprised of glomerular filtration , blood flow , and tubular transport .
Tubules remove serum phosphate, urate, urinary amino acids, etc.
Creatinine is a waste product of creatinine phosphate and is normally excreted at a constant rate. It normally is excreted at about 1 milligram per decaliter of urine. If slowly progressing illness, may go lower. Urea is excreted at an uneven rate depending on several factors such as amount of urine. If problems are found with the GFR, the diagnosis will involve some form of glomerular disease or nephrosis.