Filtration is the mass movement of water and solutes from plasma to the renal tubule that occurs in the renal corpuscle. About 20% of the plasma volume passing through the glomerulus at any given time is filtered. This means that about 180 liters of fluid are filtered by the kidneys every day. Thus, the entire plasma volume (about 3 liters) is filtered 60 times a day! Filtration is primarily driven by hydraulic pressure (blood pressure) in the capillaries of the glomerulus.
Note that the kidneys filter much more fluid than the amount of urine that is actually excreted (about 1.5 liters per day). This is essential for the kidneys to rapidly remove waste and toxins from the plasma efficiently.
Reabsorption is the movement of water and solutes from the tubule back into the plasma. Reabsorption of water and specific solutes occurs to varying degrees over the entire length of the renal tubule. Bulk reabsorption, which is not under hormonal control, occurs largely in the proximal tubule. Over 70% the filtrate is reabsorbed here. In addition, many important solutes (glucose, amino acids, bicarbonate) are actively transported out of the proximal tubule such that their concentrations are normally extremely low in the remaining fluid. Further bulk reabsorption of sodium occurs in the loop of Henle.
Regulated reabsorption, in which hormones control the rate of transport of sodium and water depending on systemic conditions, takes place in the distal tubule and collecting duct.
Even after filtration has occured, the tubules continue to secrete additional substances into the tubular fluid. This enhances the kidney's ability to eliminate certain wastes and toxins. It is also essential to regulation of plasma potassium concentrations and pH. (See Fluid and electrolyte balance).
Excretion is what goes into the urine, the end result of the above three processes. Although the original concentration of a substance in the tubule fluid may initially be close to that of plasma, subsequent reabsorption and/or secretion can dramatically alter the final concentration in the urine.
The amount of a particular substance that is excreted is determined by the formula:
amount excreted = amount filtered - amount reabsorbed + amount secreted