Maintaining water and nitrogen balance

• Water leaves the body via the lungs when exhaling
• Excess water, ions and urea are removed via the kidneys in urine
• Water, ions and urea are lost from the skin in sweat
• Cytoplasm of body cells is mainly made of water - if cells lose or gain too much water by osmosis they do not function correctly
• Hypertonic 


  More concentrated solution than in cells
• Hypotonic 


  Less concentrated than solution in cells
• Isotonic 


  Same concentration as cells
• Hypertonic

  • Water leaves cell by osmosis
  • Cells shrink
  • Cell membrane wrinkles

  
• Hypotonic

  • Water enters the cell by osmosis
  • Volume increases
  • Pressure on cell membrane increases
  • Cell explodes / bursts

  
• Ions

  Include potassium, sodium, calcium, nitrogen
• Urea


  A waste product produced when proteins are broken down
• 1. Diet high in proteins
  2. Excess proteins are digested into amino acids that are absorbed into the blood
  3. Excess amino acids are deaminated in the liver to form ammonia
  4. Ammonia is toxic so it's converted to urea for safe excretion
• Kidney

  
• Kidneys filter the blood and form urine that is stored in the bladder.

  They are located in the lower back.

  Each kidney has an outer layer called the cortex and an inner layer called the medulla.

  There are millions of structures called kidney tubules [nephrons] and this is where the blood is filtered.
• 1. Filtration of glucose, urea, ions [salts] and water from the blood
  2. Selective reabsorption of all glucose, some ions [salts] and some water into the blood
  3. Excretion of all urea, excess ions [salts] and excess water into the urine
• The pituitary gland in the brain monitors the water levels in the blood.

  It releases a hormone called anti-diuretic hormone [ADH].
• Kidney tubules [nephrons] have some of the structures in both the cortex and the medulla.

  
• 1. Increased water content of the blood [more dilute]
  2. Pituitary gland stops releasing ADH into the blood
  3. Kidney tubules less permeable and reabsorb less water - less concentrated urine is produced
• 1. Decreased water content of the blood [more concentrated]
  2. Pituitary gland releases ADH into the blood
  3. Kidney tubules more permeable and reabsorb - more concentrated urine is produced
• Kidney failure:

  • Medical condition where the kidneys no longer work.
  • Kidneys re important in homeostasis.
  • Toxins build up in the blood and concentration of ions get out of balance if kidney fails.
  • Treatment are kidney dialysis and kidney transplants
• Kidney dialysis:

  • Treatments restores concentrations of dissolved substances in the blood to normal levels and has to be carried out at regular intervals
• In a dialysis machine:

  • Blood high in urea flows between partially permeable membranes in the opposite direction to the dialysis fluids [maintains the concentration gradient]
  • The dialysis fluid contains the same concentration of useful substances such as the blood - ensures that glucose and ions are not lost
  • Urea passes out from blood into dialysis fluid
• Kidney transplant:

  • A diseased kidney is replaced by a healthy donor kidney. This can be from a live donor or someone who has died.
• Organ rejection is a problem of kidney transplants as the antigens on the surface of the donor kidney are recognised by the immune system as foreign and can be attacked by the patient's antibodies.

  To reduce the chances of this happening two precautions are taken:

  • Immune-suppressant drugs are given
  • A donor kidney with a similar 'tissue type' is used
• Advantages of kidney dialysis:

  • Available to all kidney patients [no shortage]
  • No need for immune-suppressant drugs
• Disadvantages of kidney dialysis:

  • Patient must limit their salt and protein intake between dialysis sessions
  • Expensive for the NHS
  • Regular dialysis sessions [up to 8 hours] - impact on the patient's lifestyle
  • Risk of infection
• Advantage of kidney transplant:

  • Patients can lead a more normal life without having to watch what they eat and drink
  • Cheaper for the NHS overall
• Disadvantages of kidney transplants:

  • Must take immune-suppressant drugs which can increase the risk of infection
  • Shortage of organ donors
  • Transplanted kidney only lasts 8-9 years on average
  • Any operation carries risks