This is the reason why the glomerular filtration decreases. Mathematics is important in renal physiology in another way in tubular flow through the rest of the nephron. The important thing to consider here, is not so much direct calculation, but rather concentrations and moving of substances in and out of the tubule. The filtered fluid from the glomerular filtration contains nutrients, electrolytes, and other substances that the body cannot afford to lose in the urine. It is important that the necessary materials that are filtered are reabsorbed and returned to the blood.
Tubular reabsorption is a highly selective process.
All constituents except plasma proteins are at the same concentration in the glomerular filtrate as the plasma. In general terms, the tubules have a high reabsorptive capacity for substances needed by the body and little or no reabsorptive capacity for substances of little or no value. That which is of little or no value is referred to as wastes and are eliminated in the urine. There have been many studies that calculated the percentages of the substances that get reabsorbed by the body. The flow of salt and water flows through the tubule in specific ways in order to concentrate the wastes that need to be removed from the urine.