Urine formation and the adjustment of blood composition involve three major processes: glomerular filtration by the glomeruli, tubular reabsorption and tubular secretion in the renal tubules. In addition, the collecting ducts work in concert with the nephrons to make concentrated or dilute urine.
How do the kidneys "clean" the blood? Conceptually, it's really very simple. The kidneys "dump" (by glomerular filtration) (1) cell- and protein-free blood into a separate "container" (the renal tubules and collecting ducts). From this container, the kidneys reclaimed (by tubular reabsorption) (2) everything that the body need to keep. This is almost everything-all of the glucose and amino acid, and some 99% of the water, salt and other components. Anything that is not reabsorbed becomes urine. In addition, some things are selectively added to the container (by tubular secretion) (3)Fine-tuning the bodys chemical balance,
The volume of blood proceed by the kidneys each day is enormous. Of the approximately 1200 ml of blood that passes through the glomeruli each minute, some 650 ml is plasma, and about one-fifth of this (120-125 ml) is forced into the renal tubules. This is equivalent to filtering out your entire plasma volume more than 60 times each day. Considering the magnitude of their task, it is not surprising that the kidneys (which account for only 1% of body weight) consume 20-25% of all oxygen used by the body at rest.
Filtrate and urine are quite different. Filtrate contains everything found in blood plasma except proteins. Urine contains mostly metabolic wastes and unneeded substances. The kidneys process about 180 L of blood-derived fluid daily. Of this amount, less than 1% (1,5L) typically leaves the body as urine, the rest returns to the circulation.
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