Life is incompatible with a lack of kidney function (though hemodialysis can act as a substitute). But unlike the case with most other organs, we are born with an overabundant--or overengineered--kidney capacity. Indeed, a single kidney with only 75 percent of its functional capacity can sustain life very well.
This overengineering supplies us with 1.2 million of the basic functional filtering element, the microscopic nephron, in each kidney. Nephrons are tiny tubes that filter the blood plasma, adjust and then return optimized fluid to the body. Under most conditions, though totaling only a few pounds, the kidneys receive about 20 percent of all the blood pumped from the heart. Each day, about 120 liters of fluid and particles enter into the nephron to be filtered.
If only one kidney is present, that kidney can adjust to filter as much as two kidneys would normally. In such a situation, the nephrons compensate individually by increasing in size--a process known as hypertrophy--to handle the extra load. This happens with no adverse effects, even over years. In fact, if one functional kidney is missing from birth, the other kidney can grow to reach a size similar to the combined weight of two kidneys (about one pound).