The urinary system & The Kidneys


  • The urinary system is designed to remove waste products such as urea, as well as excess ions and water from our blood.
  • The kidneys contain many nephrons which remove any waste, before reabsorbing any substances the body needs.
  • Waste is stored in the bladder before being removed as urine.
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The Kidneys

The kidneys are part of the urinary system, together with the ureter, uretha and bladder. Humans have two kidneys.

The renal arteries take blood with waste products to the kidneys to be filtered. Renal veins then return the filtered blood to be circulated around the body.

Blood vessels take the blood though the kidneys where the waste products are removed into convoluted tubules.

These tubules join together to form the ureter, which transports urine to the bladder where it is stored.

Urine is then passed from the bladder to the urethra to be released.

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Kidney Failure

Since the kidney is responsible for the removal of waste from the blood, any damage (either from accidents or disease) can lead to a build-up of poisonous waste products in the body.

We can survive without one kidney very well, but total kidney failure would be fatal if not treated.

Treatment can take the form of dialysis on a kidney machine or a kidney transplant.

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Kidney Dialysis

A dialysis machine can keep a patient alive until a transplant becomes available. The patient’s blood is passed through the machine and cleaned of any waste products the kidneys would have otherwise removed. However, dialysis machines have several disadvantages:

  • They are expensive.

  • The patient must have his or her blood connected to the machine for several hours each week.

  • Patients must follow a very strict diet to avoid complications.

  • They only work for a limited time for a patient.

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Kidney Transplant

A kidney transplant can save the patient's life, and after a transplant the patient can live a relatively normal life. This is clearly a better option than a machine, but kidney transplants also have several disadvantages:

  • Any major surgery carries some risk.

  • The kidney may be rejected by the body of the patient and so drugs (immuno-suppressant drugs) are used constantly to help prevent rejection.

  • A precise match of tissue type is needed. About half the donated kidneys come from family members (this is known as a ‘living donor’).

  • There is a severe shortage of donors.

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The Nephron - Part 1

Urine is produced in microscopic structures in the kidney called nephrons. There are approximately 1 million nephrons in each kidney.

The Bowman's capsule collects the filtrate and it enters the tubules. All glucose is reabsorbed immediately into the blood capillaries.

As the rest of the filtrate travels through the tubules, water and salts needed by the body are reabsorbed into the blood capillaries. The loop of Henlé helps maintain the correct water balance in the body by filtering out salts.

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The Nephrone - Part 2

The waste, consisting of excess water, excess salts and urea, is urine. The collecting duct collects the urine, which is then transported in the ureter to the bladder. The bladder stores urine until the body is ready to expel it through the urethra.

This process can be summarised in three important steps:

  1. Filtration - where lots of water, ions, urea and sugar are squeezed from the blood into the tubules.

  2. Selective reabsorption – the useful substances (ions and sugars) are reabsorbed back into the blood from the tubules. The amount of water in the blood is regulated here to maintain it at a constant rate. This is known as ‘osmoregulation’.

  3. Excretion of waste - urea and excess water and ions travel to the bladder as urine, to be released from the body.

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