This is an excellent question, especially because kidney disease and kidney transplants are so common (approximately 10,000 to 15,000 Americans receive kidney transplants each year). Most humans are born with two kidneys as the functional components of what is called the renal system, which also includes two ureters, a bladder and a urethra. The kidneys have many functions, including regulating blood pressure, producing red blood cells, activating vitamin D and producing some glucose. Most evidently, however, the kidneys filter body fluids via the bloodstream to regulate and optimize their amount, composition, pH and osmotic pressure. Excess water, electrolytes, nitrogen and other wastes get excreted as urine. These functions maintain and optimize the "milieu interieur" (internal environment) of the body--the fluids in which our cells live.
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).
The kidneys filter this large amount of fluid on a daily basis because nephrons are fairly indiscriminant filters, removing all contents from the blood except for larger proteins and cells. The nephrons, however, are extremely accomplished in processing the filtrate and substances critical to survival--such as water, glucose, amino acids and electrolytes, which are actively reabsorbed into the blood. The water and waste (including urea and creatinine, acids, bases, toxins and drug metabolites) that remain in the nephrons become urine.
In addition to being able to support life with only one kidney, the renal system has other safeguards. Although nephrons stop functioning at a rate of 1 percent per year after 40 years of age, the remaining nephrons tend to enlarge and fully compensate for this demise. Evidence strongly suggests that living kidney donors are highly unlikely to develop significant long-term detrimental effects to their health, as illustrated by donors whose renal function has been assessed for up to 30 years following donation. The main problems with donors are rare instances of complications having to do with the surgery, not the lack of the kidney.