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Hospital Británico
esquema aparato renal y urinario
  1. Urethra
  2. Urinary bladder
  3. Ureter
  4. Left kidney
  5. Right kidney
  6. Aorta artery
  7. Inferior vena cava
  8. Renal vein
  9. Renal artery


› What are the kidneys?
Kidneys are vital organs that perform a number of functions to clean and maintain the chemical balance in the blood (acidity). Kidneys produce the urine. Understanding how kidneys work may help you keep them healthy.

- Clearing waste products and excess fluid
- Balancing the levels of fluids and chemicals
- Producing certain hormones that regulate blood pressure and red blood cells
- Keeping bones healthy

› How many kidneys are there in the body?
In general, humans are all born with two kidneys.

› How do kidneys look?
Kidneys are bean-shaped and fist-sized organs (9 to 12 cm).

› Where are the kidneys?
Kidneys are located at either side of the mid-spine, just below the chest. See Kidney and urinary system.

› How do kidneys work?
Kidneys are a complex reprocessing system that can be likened to real computers. Our circulating blood volume is of a little more than 5 litres. Every day, around 190 litres of blood are purified to filter out around 1.9 litres of waste products and excess fluid.

› What are kidneys for?
Waste products and excess fluid are turned into urine, which passes down a tube called a ureter to the urinary bladder. The urinary bladder stores the urine until it is passed out when you go to the toilet.

Waste products in the blood, such as urea, creatinine and uric acid, are a result of the normal breakdown of active tissues and the food that we eat.

The body uses food as a source of energy and for self-repair. After the body takes what it needs from food, waste products are poured into the bloodstream. If the kidneys did not filter out waste products, these would accumulate in the blood and damage the body.

Urea is produced by the liver as a waste product of protein metabolism and is excreted through the kidneys. Urea is not toxic and increases in blood when there is kidney failure.

Creatinine is a waste product of muscle metabolism that is also excreted through the kidneys. Creatinine is not toxic and increases in blood when there is kidney failure.

Uric acid is a waste product of gene metabolism that is also excreted through the kidneys. Creatinine is toxic; creatinine levels may increase in blood with or without kidney failure. High creatinine levels may produce gout and renal damage, and are associated with high-pressure problems.

Filtration of blood products occurs in nephrons, the small units within the kidney. There are approximately one million nephrons in each kidney. Within each nephron, a capillary is connected to a small urine-collecting tube called a tubule. A complex exchange of chemical products occurs as the waste products and water leave the bloodstream and enter the urinary system.

Imagine that the kidney consisted of one million sieves connected to one million tubes. Sieves filter the blood, and the filtered fluid passes through the tubules, thereby producing urine.

Tubules initially receive the blood filtrate, i.e. a mixture of waste products and chemicals which are useful to the body. Tubules remove and assess the useful chemicals (water, sodium, potassium, calcium, phosphorus, magnesium, and so on) and send them back into the bloodstream and the rest of the body.

Waste products and excess salt and water are excreted. In this way, kidneys regulate the levels of these products. Life depends on a fine balance. Any imbalance will be harmful.

Other kidney functions include:
Acidity regulation (bases of life).
Blood pressure regulation, by managing the volume of water and salt (sodium).

Besides excreting waste products, kidneys also secrete three important hormones:
Erythropoietin (EPO), which stimulates the production of red blood cells in the bone marrow.
Renin, which regulates blood pressure.
Calcitriol, the active form of vitamin D, which helps keep calcium within the bones and the normal chemical balance of calcium and phosphorus.

› How can kidney function and damage be assessed?
In blood: urea and creatinine, haematocrit, ions and albumin.
In urine: total urine, 24-hour urine protein levels, 24-hour urine microalbumin levels. In blood and urine: creatinine clearance.
Imaging (shape): ultrasound scan, CT scan, and MRI.
Imaging (function): radioisotope scan.

› Why do kidneys fail?
Almost every disease damaging the kidneys attacks the nephrons, causing them to lose their filtration ability.

Nephron damage may occur in two ways:
- Rapidly, as a result of injury or drug poisoning (acute renal injury).
- More commonly, progressively without evident symptoms.

After some years or even decades, damage may present with symptoms. Most renal diseases attack both kidneys simultaneously.

Chronic kidney disease (CKD) is divided into 5 stages:
Stage 1: Kidney damage with a clearance > 90 mL/min.
Stage 2: Kidney damage with a clearance of 60-89 mL/min.
Stage 3: Kidney damage with a clearance of 30-59 mL/min.
Stage 4: Kidney damage with a clearance of 15-29 mL/min.
Stage 5: Kidney damage with a clearance < 15 mL/min or under haemodialysis.

› The two most common causes of kidney disease
Diabetes and high blood pressure (hypertension) are the two most common causes of kidney disease; very often, both can be prevented!
KIDNEY disease may also be produced by certain drugs, both over-the-counter and commonly used medicines

› What other diseases may produce kidney damage?
Glomerular nephritis: Kidney swelling of the glomeruli (sieves) may cause proteins to be excreted into the urine, damaging the kidneys (proteinuria).

Kidney cysts may also damage the kidneys, with the following variants:

Polycystic kidney disease:
These are congenital cysts. Polycystic kidney disease is a hereditary disease in 50% of cases, and cysts are identified at ultrasound at ages 10 to 30. They are usually asymptomatic until the age of 45-50 and produce progressive kidney damage. Cysts are filled with fluid. They may get infected or bleed.
There are also simple cysts. These are acquired (not inherited). Simple cysts appear with age and may grow progressively.
Simple cysts are controlled with ultrasound scan. They do not experience malignant transformation (do not turn into cancer). Chronic use of caffeine may increase the size of kidney cysts.

Prostate disorders may also damage the kidneys.

A low fluid intake stresses the kidneys and may compromise their function.

An excess of body fluid loss may produce kidney damage.

Iodine contrast materials may produce kidney damage.

› Causes of kidney damage
  • High blood pressure (hypertension)
  • Diabetes Polycystic kidney disease
  • Urinary tract infections
  • Kidney stones
  • Glomerular nephritis Urinary obstruction
  • Medications
› Signs and symptoms
  • Tiredness
  • Loss of appetite
  • Poor sleep
  • Cramps
  • Headache
  • Dizziness
  • Swollen eyes and legs
  • Absent-mindedness
  • Dry skin
  • Foamy urine
› Prevention
  • Regular medical care
  • Blood pressure control
  • Healthy diet
  • Diabetes control
  • Obesity prevention
  • Regular physical exercise
  • Moderate alcohol intake
  • No smoking
  • Avoidance of unnecessary pain-killing medications
  • Sleep apnoea
  • Avoidance of urinary tract infections
  • Treatment of prostate disorders.

If you would like to contact the Nephrology Service for other questions, please email:

Important: The contents of this section are for information only and should not be construed as a substitute for professional medical care by a qualified doctor or other health care professional. It is important that you keep your regular visits with your doctor.
Nephrology Service - British Hospital - Perdriel 74 1º Piso - CP1280 - Buenos Aires - Argentina - Phone: +54 11 4309-6400 -
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