The standard human body plan includes two kidneys. Medical science reveals that this number is not absolute, with two distinct scenarios allowing a person to possess three or even four functioning or non-functioning renal units. These exceptions arise from a rare congenital anomaly during development or as a direct result of modern surgical procedures.
Understanding Supernumerary Kidneys
A supernumerary kidney (SNK) is an extremely rare congenital variation where a person is born with an extra renal organ. This anomaly is defined by the presence of a distinct mass that possesses its own separate blood supply and collecting system. Fewer than 100 instances of this condition have been documented, highlighting its exceptional rarity.
This unusual formation is thought to result from an abnormal division of the nephrogenic cord and the corresponding ureteral bud early in embryonic development. While most reported cases involve a third kidney, documented instances of bilateral supernumerary kidneys exist, resulting in a person having four renal units, two on each side of the body.
The accessory organ is often smaller than the native kidney and may not be fully functional, sometimes remaining rudimentary. If the extra kidney is functioning, its separate collecting system may have an abnormal connection, which can predispose it to medical issues.
Additional Kidneys Through Transplantation
Renal transplantation is the second pathway to possessing more than two kidneys. When a person experiences end-stage renal disease, a donor kidney is implanted in the lower abdomen to take over the function of the failing native organs. A recipient typically receives a single donor kidney.
The recipient’s own two diseased kidneys are usually left in their original anatomical positions near the spine. Surgeons avoid removing the native organs unless they are causing severe complications, such as uncontrolled high blood pressure or frequent infection. The standard practice means a transplant recipient typically lives with three kidneys: two native, non-functional ones and one newly transplanted, functional organ.
In rare circumstances, a patient may receive a second renal transplant if the first one fails, or two kidneys from a deceased donor may be implanted simultaneously. If a patient with two native, non-removed kidneys receives two transplanted organs, they would then possess a total of four renal units.
Diagnosing and Managing Extra Kidneys
Supernumerary kidneys are frequently discovered incidentally during imaging for an unrelated issue, as many individuals remain asymptomatic throughout their lives. Diagnosis relies on various imaging techniques, including abdominal ultrasound, computed tomography (CT) scans, and angiography.
If the supernumerary kidney is asymptomatic, management is typically conservative, involving regular monitoring. However, the abnormal anatomy of an SNK often makes it susceptible to hydronephrosis, stone formation, or recurrent infections. If complications arise or the organ is non-functional and symptomatic, surgical removal may be necessary.
For individuals with three or four kidneys following transplantation, the focus of care centers on the health of the transplanted organ. The native kidneys are generally left to atrophy and are monitored to ensure they do not become a source of infection or other complications that would warrant their removal. Therefore, clinical management always prioritizes the function and well-being of the organs responsible for the body’s filtration needs.