maintaining water and nitrogen balance

• What do kidneys do?

  Act as filters to 'clean the blood'

  1. Make urine by taking waste products and unwanted substances from blood

  2. Substances filtered out of blood as it passes through the kidney (filtration)

  3. Useful substances like glucose, some ions and the correct amount of water are absorbed back into blood (selective reabsorption)
• Selective Reabsorption


  Useful substances like glucose, some ions and the correct amount of water are absorbed back into blood
• What substances are removed from body in urine?

  Urea, ions and water
• What is urea?

  Proteins (amino acids) can't be stored in the body so excess amino acids are converted into fats and carbohydrates which can be stored

  Ammonia is a waste product from this process

  Ammonia is toxic so it's converted to urea in the liver
• What and why are ions lost?

  Ions eg sodium are taken in by food and absorb into blood

  If the balance of water and ions is incorrect, too much or too little water can absorb into cells via osmosis, this could damage the cells

  Some ions lost in sweat, but this is not regulated, the right balance of ions is maintained by kidneys
• Why is water filtered?


  Body has to constantly balance water intake and outtake

  We lose water via sweat and lungs when breathing out

  We cannot control how much we lose these ways, so the amount we consume is balanced out by being removed by the kidneys in urine
• What hormone controls concentration of urine?

  ADH (anti-diuretic hormone)

  More ADH = more water REABSORBED (or more ADH = less urine)

  This is released into the bloodstream from the pituitary gland

  Brain monitors water content of blood and instructs pituitary gland to release ADH into blood according to how much is needed
• Kidneys remove waste substances from blood


  If waste substances build up in blood, you lose your ability to control levels of ions and water, results in death eventually
• Treating kidney failure

  1. Dialysis

  2. Transplant
• Dialysis machines


  Filter blood

  1. The person's blood flows between a partially permeable membrane (permeable to water and ions, not proteins just like kidneys) surrounded by dialysis fluid.

  2. Dialysis fluid has the same concentration of dissolved ions and glucose as healthy blood. (Meaning useful dissolved ions and glucose won't be lost from blood during dialysis)

  3. Only waste substances eg urea, excess ions and water will diffuse across membrane

  4. Person is left with filtered blood

  
• Advantages of dialysis

  - Dialysis is a life-saving process

  - It gives a patient more time to find a donor kidney
• Disadvantages of dialysis

  Lasts 3-4 hours and is needed 3 times a week

  Increases risk of blood clots and infections

  Expensive process

  Unpleasant experience
• Cure for kidney failure


  Kidney transplant
• Advantages of kidney transplant

  - Freedom from time-consuming dialysis.

  - Feeling physically fitter.

  - Improved quality of life - able to travel.

  - Improved self image - no longer have a feeling of being chronically ill

  - Cheaper in the long run
• Disadvantages of kidney transplant

  - The kidney may be rejected by the body

  - There can be long waiting lists for kidney transplants

  - The patient often has to take immunosuppressant drugs. These can make a patient more prone to other infections

  - If donor is still alive, there is a risk for them
• Water leaves the body via the lungs during exhalation.
• Water, ions and urea are lost from the skin in sweat.
• There is no control over water, ion or urea loss by the lungs or skin.
• Excess water, ions and urea are removed via the kidneys in the urine.
• If body cells lose or gain too much water by osmosis they do not function efficiently.
• The digestion of proteins from the diet results in excess amino acids which need to be excreted safely. In the liver these amino acids are deaminated to form ammonia. Ammonia is toxic and so it is immediately converted to urea for safe excretion
• The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.
• The water level in the body is controlled by the hormone ADH which acts on the kidney tubules. ADH is released by the pituitary gland when the blood is too concentrated and it causes more water to be reabsorbed back into the blood from the kidney tubules. This is controlled by negative feedback.