Humans have two kidneys located in the rear part of the abdominal cavity, one on each side of the spine. The kidneys play a vital role in the urinary system and in regulating the chemical composition of the blood. Many people wonder why humans need two kidneys instead of just one. This article will examine the important functions of the kidneys and discuss the advantages of having two kidneys instead of one.
Functions of the Kidneys
The kidneys perform several critical regulatory roles in the body:
- Filtering the blood – The kidneys filter about 120-150 quarts of blood each day to remove waste products and excess fluid. The waste and extra fluid become urine.
- Regulating fluid balance – The kidneys maintain the body’s fluid balance by conserving water when you are dehydrated or making more urine when you have too much fluid.
- Regulating electrolytes – Electrolytes like sodium, potassium, and phosphate are critical for nerve impulse conduction, muscle contraction, and other functions. The kidneys keep electrolyte concentrations within the normal range.
- Regulating blood pressure – Kidneys release renin, an enzyme that regulates blood pressure through the renin-angiotensin system.
- Regulating blood pH – Kidneys excrete acids and regulate bicarbonate to maintain the body’s normal pH.
- Producing hormones – Kidneys produce hormones like calcitriol, erythropoietin, and renin that regulate different body functions.
- Producing an active form of vitamin D – Kidneys convert vitamin D into its active form to maintain calcium homeostasis.
- Removing waste and toxins – Kidneys filter metabolic waste products like urea and ammonium and drugs or toxins to be excreted.
As you can see, the kidneys play a central role in regulating the composition and volume of blood. They balance key electrolytes, fluid levels, blood pH, remove waste, and perform other vital functions.
Advantages of Having Two Kidneys
Humans have two kidneys for several important reasons:
Redundancy
Having two kidneys provides redundancy in case one kidney fails or is damaged. With two kidneys, the remaining kidney can take over the workload of both kidneys. However, with only one kidney, any damage can lead to kidney failure, which is potentially fatal.
Some key advantages of having a backup kidney include:
- Prevents kidney failure if one kidney is damaged or diseased
- Allows for continued function if one kidney needs to be donated
- Provides backup during kidney surgery on one kidney
- Reduces risk of needing dialysis or kidney transplant
According to research, people with a single kidney have a higher risk of developing chronic kidney disease. Having two kidneys reduces this risk substantially.
Increased Functional Capacity
Having two kidneys increases the total functional capacity of the kidneys. Two healthy kidneys can filter blood and perform other functions at double the rate of a single kidney.
Some key advantages include:
- Process higher volumes of blood per minute
- Produce more urine to get rid of excess fluid
- Excrete waste products and toxins more efficiently
- Have extra capacity to compensate if one kidney is not working at full capacity
This provides a buffer in case of increased demand on the kidneys, such as dehydration, blood pressure changes, or the use of certain prescription drugs. The two kidneys have extra functional reserve to meet these demands.
Prevention of Compensatory Hypertrophy
If a person has only one kidney, it usually undergoes compensatory hypertrophy. This means the single kidney gets bigger to compensate for the missing kidney and do the work of two kidneys.
However, compensatory hypertrophy has disadvantages:
- The kidney is more prone to damage and scarring
- Increased risk of high blood pressure and kidney disease
- Higher likelihood of failure later in life
Having two kidneys prevents compensatory hypertrophy and its associated risks. Each kidney can simply do its inherent job without taking on excess workload.
Kidney Transplants
When a person has irreversible kidney failure or damage, they may need a kidney transplant. In most cases, the healthiest option is to receive a kidney from a living donor.
Some advantages of using a living donor kidney include:
- Kidneys from living donors tend to last longer – on average 8-12 years longer than a deceased donor kidney
- Shorter wait times – no need to stay on the national transplant waiting list for years
- Plan the surgery in advance for optimal timing
- Living donor kidneys start working right away
For recipients getting a kidney transplant, there are two options:
One Kidney Transplant
The diseased kidneys are left in place and a donor kidney is implanted. Advantages:
- Leaving the old kidneys in place preserves nerve connections and blood flow
- Avoid risks associated with removing both old kidneys
- If the transplanted kidney fails, the remaining original kidney can keep working
Disadvantages:
- Higher risk of rejection of the new kidney
- Harder to monitor function of the transplanted kidney
- Old kidneys may become infected
Two Kidney Transplant
Both original kidneys are removed and two healthy donor kidneys are implanted. Advantages:
- Lowers risk of rejection since no diseased kidneys remain
- Easier to monitor function of the new kidneys
- Prevents complications from old infected kidneys
Disadvantages:
- Increased risks from surgically removing both kidneys
- No backup kidney if one transplant kidney fails
Doctors determine the best approach based on the recipient’s specific condition. But in general, having two kidneys, whether original or transplanted, provides health benefits over having just one kidney.
Kidney Cancer
Kidney cancer develops when healthy kidney cells change and grow out of control, forming a tumor. The most common type is renal cell carcinoma. Some key facts:
- Kidney cancer is among the 10 most common cancers in both men and women
- Over 70,000 new cases every year in the United States
- Usually develops in adults in their 60s and 70s
- Often diagnosed at an early stage when surgical removal is the treatment
- 5-year survival rate if diagnosed and treated early is over 90%
Some risk factors for kidney cancer include smoking, obesity, workplace exposure to chemicals, and high blood pressure.
If kidney cancer is diagnosed when still small and confined to the kidney, surgery to remove the tumor is usually the treatment. A radical nephrectomy involves removing the entire affected kidney. A partial nephrectomy removes just the tumor while sparing healthy parts of the kidney.
Some advantages if surgery is done on just one kidney:
- Still have another healthy kidney performing normal functions
- Avoid risks of having both kidneys removed
- Prevents kidney failure from losing all kidney function
- Reduces likelihood of needing long-term dialysis or kidney transplant
So thanks to having two kidneys, a person can usually maintain adequate kidney function after surgery to treat localized kidney cancer.
Kidney Infections
Kidney infections, also called pyelonephritis, are a common type of urinary tract infection (UTI). Bacteria enter the urinary tract and travel up to infect one or both kidneys.
Kidney infections often develop as complications of lower UTIs like cystitis (bladder infection). Symptoms include:
- Fever and chills
- Nausea and vomiting
- Pain in the back, sides, or groin
- Frequent and painful urination
- Cloudy, bloody, or foul-smelling urine
Without treatment, a kidney infection can permanently damage kidneys and cause kidney failure. Having two kidneys gives some safety benefits:
- If one kidney is infected, the other can still function normally
- Prevents total kidney shutdown from spread of infection to both kidneys
- Remaining healthy kidney can take over work of infected kidney
So while a kidney infection can certainly be serious, having a second kidney provides a margin of safety and reduces the overall risk to the person’s life.
Here is a table summarizing the pros and cons of having two kidneys versus one kidney:
| Benefits of Two Kidneys | Risks of One Kidney |
|---|---|
| Redundancy if one kidney fails | Kidney failure if kidney is damaged |
| Increased overall function | Loss of adequate function with damage |
| Prevents compensatory hypertrophy | Hypertrophy and associated issues |
| Backup kidney during transplant | No backup if transplant fails |
| Preserve function if cancer in one | Higher risk of kidney failure |
| Protects from infection spreading | Total shutdown if both infected |
Conclusion
In summary, humans evolved to have two kidneys due to the vital life-sustaining roles that healthy kidneys perform. Having two kidneys provides critical redundancy in case of kidney failure, damage, cancer, infections, or other threats to kidney health. Two kidneys increase overall functional capacity compared to a single kidney. And they prevent the compensatory hypertrophy and subsequent health risks that come from relying on just one kidney.
So while it is possible for someone to live a normal lifespan with one kidney, having two kidneys provides significant advantages for overall health and kidney function. Our two kidney design provides important safeguards against kidney disease, damage, and failure – safeguards that are essential for maintaining filtration, fluid balance, blood pressure control, and the many other functions performed by these vital organs.