Most people are born with two kidneys, yet we only need one kidney to live a healthy life. This seems puzzling at first glance. Why would we evolve to have an extra kidney if it isn’t necessary? As it turns out, having two kidneys provides important benefits and backup protection.
Quick Answers
– We have two kidneys for redundancy in case one fails
– Two kidneys allow more efficient waste filtration and regulation of body processes
– The kidneys are located in a vulnerable position, so having a backup protects against injury
– Having two kidneys may provide developmental advantages in the womb
Kidney Functions
To understand why we have two kidneys, it helps to review the important functions they provide:
- Filtering waste – Kidneys filter about 200 quarts of blood daily to remove waste and extra fluid, which is excreted as urine. This regulates body composition and prevents waste buildup.
- Regulating minerals – Kidneys maintain normal levels of minerals like sodium, potassium, and phosphorus in the blood.
- Regulating blood pressure – Kidneys release hormones to direct blood vessel dilation and constriction to maintain healthy blood pressure.
- Making hormones – Kidneys produce hormones that control red blood cell production and calcium absorption.
- Balancing fluids – Kidneys regulate overall fluid balance to prevent dehydration or over-hydration.
These vital processes are necessary for survival. So why did we evolve with two kidneys instead of just one? As it turns out, having two kidneys provides important backup protection and efficiency benefits.
Kidney Redundancy for Backup
One major reason we have two kidneys is for redundancy. If one kidney fails due to disease, injury or abnormalities, the second kidney can take over the workload to maintain normal function.
Many people can live a normal healthy life with just one functioning kidney, a situation called mononephrosis. The remaining kidney simply works harder, enlarges and takes on the workload of both. However, having a second kidney provides a critical backup plan if one kidney becomes damaged or diseased.
Common Kidney Conditions
There are various kidney conditions that may impair function:
- Kidney infection – An untreated urinary tract infection can spread to the kidneys.
- Kidney stones – Mineral deposits that cause blockage and damage.
- Glomerulonephritis – Inflammation that damages glomeruli and impairs filtering.
- Polycystic kidney disease – Genetic disorder where cysts interfere with kidney function.
- Hypertension – High blood pressure can overwork and damage nephrons.
With any of these conditions, the unaffected kidney can compensate to maintain normal filtering, hormone production, and fluid/mineral balance. Having two kidneys provides a safety net.
Kidney Injury Risk
The kidneys are also located in a vulnerable position, so having a second kidney protects against physical trauma. The kidneys sit on either side of the spine at the level of the lower ribs in the back of the abdomen. This allows direct trauma to the kidney area during sports, car accidents, or falls. Damaged blood vessels around the kidney can also impair function.
With two kidneys, if one kidney suffers trauma or damaged blood flow, the other kidney can maintain normal function. Having just one kidney would make any injury much more devastating.
Increased Filtration Capacity
Another advantage of two kidneys is increased total filtration capacity. Each kidney contains over 1 million nephrons – the functional units that filter blood. Having twice as many nephrons allows more efficient waste processing and filtering. This provides:
- Greater ability to concentrate urine and conserve water if dehydrated
- Improved removal of metabolic waste products
- More precise regulation of body fluid and mineral composition
With greater combined filtration power, the kidneys can swiftly adjust output and maintain optimal internal balance even with varying environmental conditions like dehydration, exercise, or high protein intake.
Developmental Advantages
There may also be developmental advantages during fetal growth to having two kidneys:
- Symmetric kidney location aids proper positioning in the womb.
- Having two kidneys provides more overall development space compared to one enlarged kidney.
- Presence of two kidneys stimulates optimal ureteric branching.
By providing symmetry and spaciousness, two kidneys support normal fetal positioning and optimal development of urinary structures. After birth, the benefits of backup protection and increased filtration capacity make two kidneys advantageous.
When Do We Need Two Kidneys?
For most people under normal circumstances, one healthy kidney is sufficient for basic functioning. However, there are some situations where having two kidneys provides a distinct benefit:
- Kidney disease – Having a second kidney maintains function if one diseased kidney fails.
- Kidney donation – Can safely donate one kidney while remaining healthy with the other.
- Chronic renal failure – May delay needing dialysis since both kidneys function until late stage disease.
- High blood pressure – The combined workload and hormonal output of two kidneys can better regulate blood pressure.
- Diabetes – Reduced risk of developing chronic kidney disease, a common complication.
Individuals at higher risk for kidney problems benefit most from having two kidneys. But even in healthy people, the backup security of an extra kidney can save a life in case of unforeseen trauma or health issues.
Do Other Animals Have Two Kidneys?
Humans aren’t the only species with two kidneys. In fact, having two kidneys is the normal pattern across vertebrates including mammals, birds and reptiles. Some examples:
- Dogs, cats, pigs – Two kidneys is standard for most household mammals.
- Sheep, cows, horses – Livestock species also have two kidneys.
- Whales, dolphins, seals – Marine mammals have two kidneys adapted to freshwater conservation.
- Hawks, eagles, ostriches – Birds have smaller two kidneys relative to their mass.
- Turtles, snakes, lizards – Reptiles have two elongated kidneys near their tail.
This reflects the evolutionary advantage of having a backup kidney, which provides vital protective redundancy. The pattern of two kidneys occurs across a wide range of land and marine vertebrates.
Kidney Numbers in Other Species
Some interesting exceptions where species have different kidney numbers:
- Fish – Range from 1 to hundreds of clustered kidneys.
- Frogs – Start with two kidneys as tadpoles which fuse into one as adults.
- Sharks – Some larger sharks have up to 7 kidneys.
- Kangaroos – Have more concentrated urine and get by with just one large kidney.
- Rodents – Mice have just one kidney on the left side.
These examples reflect species adaptations to different environmental conditions. But in general, having two kidneys is the predominant pattern in vertebrates, mirroring the human form.
Conclusion
The answer to “why do we have two kidneys” has to do with redundancy, increased filtration power and optimal fetal development. While we can live with one kidney alone, having two kidneys provides major advantages:
- Backup security if one kidney is damaged
- Greater waste filtering capacity
- Support for proper fetal positioning and growth
- Overall resilience against disease, toxins and physical harm
Our two kidneys complement each other to sustain healthy function even if one kidney is compromised. So while we only need one, having two kidneys makes us better equipped to handle life’s unexpected challenges.
