20 Facts about Kidneys

20 Facts about Kidneys

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In this article, we will describe 20 facts about kidneys.

Your kidneys are vital organs – just like your brain, heart, lungs and liver. Your kidneys clean your blood, help control your blood pressure, help make red blood cells and keep your bones healthy.

Where the kidneys are – and what they look like

1. You have two kidneys, each is bean-shaped, sitting on either side of your spine just below the rib cage. Each one is roughly the size of a palm — about 12 cm long (usually between 10–14 cm), 6 cm across and 3 cm thick, weighing around 150 grams.

Because the liver sits on the right side of the body, the right kidney is positioned slightly lower and is often a little smaller.

2. Although kidneys make up only about 0.5% of your body weight, they receive around 20% of the heart’s blood output – second only to the liver – showing just how crucial they are.

3. Most people are born with two kidneys, but roughly 1 in 1,000 people have only one. A single kidney can enlarge to compensate and is usually capable of maintaining normal kidney function unless disease develops.

Can you feel your kidneys?

4. No. Normally, kidneys are not something you can feel from the outside. If a kidney becomes large enough to be felt, it is usually because something is wrong.

In very slim individuals, the right kidney may occasionally be felt during a deep breath.

Kidney functions

5. The kidney performs seven major functions – only one of which involves removing waste.

The Main Job: Filtering the Blood

6. The kidneys act as the body’s filtration system, clearing out waste products and excess fluid. Each kidney contains about one million filtering units called nephrons.

A nephron has two main parts:

• Glomerulus – the filter
• Tubule – the pathway where the filtered fluid flows out.

If all the nephrons were laid end-to-end, they would stretch an astonishing 8 kilometres (about 5 miles).

How kidney function is measured

7. Kidney function is commonly assessed with a blood test called the glomerular filtration rate (GFR), which estimates how much blood the kidneys filter each minute.

• Normal GFR: 90-120 ml/min
• A GFR of 120 ml/min means the kidneys filter around 7 litres of blood every hour, or 180 litres each day.
• Creatinine, a waste product from muscle activity, is also used to estimate filtration. Normal levels are roughly 60-120 micromol/L, with lower values indicating better filtration (and function).

8. Even though about 180 litres of fluid are filtered daily, the kidneys return almost all of it to the bloodstream—producing about 2 litres of urine per day, depending on fluid intake.

What is Chronic Kidney Disease (CKD)?

9. CKD is a long-term condition in which the kidneys are damaged or do not work as effectively as they should.

• Around 10% of people have CKD
• Fortunately, only about 1 in 100 of these progress to end-stage renal failure (CKD stage 5), meaning about 1 in 1,000 of the population will need dialysis or a transplant.

10. Most people with early-stage CKD (stages 1-3) are older adults. For many of them, mild kidney impairment is partly age-related and may never progress to kidney failure.

11. CKD usually causes no symptoms until the more advanced stages (4-5) and can significantly increase the risk of heart disease, stroke, and death.

Gender and age trends

12. CKD affects more women than men, but men are more likely to reach ESRF.

• CKD increases with age: around 25% of people aged 65–74, and half of people over 75, have some degree of CKD
• The average age of starting dialysis is 63 years.

13. Keeping blood pressure well-controlled – ideally at or below 130/80 – is the single most effective way to protect the kidneys. Medicines such as ACE inhibitors, ARBs, and SGLT2 inhibitors are commonly used to safeguard kidney function.

14. Early detection of CKD is important. Although CKD cannot usually be reversed, treatment can slow or stop progression.

Healthy lifestyle habits –  regular physical activity, a balanced diet, maintaining a healthy weight, and routine kidney checks – also help.

How common is End-Stage Renal Failure (ESRF)?

15. ESRF is uncommon. In the UK,in 2023:

• The incidence was 158 new cases per million people per year = about 8,500 new patients annually.
  • Median age of onset was 63.2 years (White 64.9 years, Asian 61.6 years and Black 55.5 years).
  • 63.3% were male.
• Nearly 73,000 adults were on dialysis or with a kidney transplant); about 41,000 of these had functioning transplants.
• Of the 32,000 dialysis patients, about 80% were on haemodialysis (HD) and 20% on peritoneal dialysis (PD).

Data source: 27th annual UKKA report (2023)

Kidney Transplant Waiting Times

16. In the UK, the average wait for an adult kidney transplant from a deceased donor is about three years, though this varies by transplant centre (from 18 months to 5 years, yes, quite alot).

What is Acute Kidney Injury (AKI)?

17. AKI is different from CKD. It refers to a rapid drop in kidney function, occurring over hours or days.

Up to one-third of hospital patients experience some level of AKI. Most cases are pre-renal, meaning the kidneys are affected because they aren’t receiving enough blood flow – commonly due to dehydration or severe infection (sepsis).

Pre-renal AKI is usually reversible with appropriate treatment. A small number will need (usually short-term) dialysis. The kidneys usually recover and kidney function returns to baseline.

Urinary Tract Infections (UTIs) and Kidney Stones

18. Most urinary tract infections occur in the bladder (cystitis). When the kidneys become infected, it is called pyelonephritis.

19. Kidney stones are very common – about 1 in 10 people will experience one. Stones develop when certain substances in the urine crystallise and form solid lumps, often made of calcium oxalate.

When stones block the ureter, they can cause intense pain known as renal colic.

Kidney Cancer

20. Kidney cancer includes tumours of:

• The outer parts of the kidney (about 90%) – most of these are renal cell carcinomas (RCCs)
• The inner drainage system and the ureter (about 10%) – these are typically transitional cell carcinomas (TCCs).

Kidneys and CKD: in 3 Diagrams and One Video

Diagram 1: urinary tract, which the kidneys are part of 

Diagram 2: how the kidney works – filtration (in glomerulus) and reabsorption (tubule)

Diagram 3: how CKD is diagnosed – by GFR and classified into 5 stages

5 Stages of CKD

The diagnosis of CKD is largely based on a classification of its severity, based on GFR (glomerular filtration rate) – the higher the eGFR, the better.

Video: how do the kidneys work?

https://www.youtube.com/watch?v=hfKDA5kbL5o

Summary

We have described 20 facts about the kidneys. We hope you have found it helpful.

Other Resource

This is a good review article: Vaidya, 2022