water and nitrogen balance

• if water concentration of the blood increases:

  • cells in the body take up water by osmosis
  • because the concentration of water outside the cell is greater than the concentration of water inside the cell
  • cells expand as they take up water, and may eventually burst
• if water concentration of the blood decreases:

  • cells in the body lose water by osmosis
  • as the concentration of water inside the cell is greater than the concentration of water outside the cell (in the blood)
  • cells shrink as they lose water and plasmolyse. This can cause them to shrivel up
• 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 skin or lungs
• excess water, ions and urea are removed via the kidneys in urine
• excretion is the removal of toxic materials, the waste products of metabolism and excess materials such as amino acids
• 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
  • excess amino acids are converted into fats and carbohydrates for storage, because amino acids cannot be stored
  • this process happens in the liver, and is called 'deamination'
  • ammonia is the waste product of deamination, and ammonia is toxic so it is immediately converted into urea for safe excretion
  • urea is transported to the kidneys where it is filtered out the flood and excreted in urine
  • a small amount of urea is also lost in sweat
• how the kidneys work:

  • as blood passes through the kidneys, the tubules in the kidneys absorb any small molecules and filter them out the blood, including water, ions, glucose and urea
  • once all of these small molecules have been filtered out, selective reabsorption takes place where all the glucose is reabsorbed back into the bloodstream, some water and ions are reabsorbed and no urea is reabsorbed
  • the molecules that are not reabsorbed then become urine, ready to be excreted
• water regulation : too low -

  • the hypothalamus in the brain detects the water levels of the blood
  • if the water levels in the blood are too low, the hypothalamus sends signals to the pituitary gland to released ADH into the bloodstream
  • ADH travels to the kidneys, and it causes the kidney tubules to become more permeable to water
  • this causes the kidney tubules to reabsorb more water back into the bloodstream
  • and this causes the water levels to rise back to normal, and less urine is produced
• water regulation : too high -

  • the hypothalamus in the brain detects the water levels in the blood
  • if the water levels in the blood are too high, the hypothalamus stops sending signals to the pituitary gland telling it to released ADH
  • this means the pituitary gland will not release as much ADH into the bloodstream
  • because of this, less ADH will travel to the bloodstream, and so the tubules will reabsorb less water into the bloodstream in selective reabsorption
  • this causes the water level in the blood to drop back to normal, and more urine is produced
• the kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water
• the main consequences of kidney failure are:

  • a person won't be able to regulate ion and water levels
  • waste substances build up in the blood stream
  • and because of this a person would quickly become sick
• dialysis is not a long term solution, and is meant to keep someone alive until they can get a donor kidney. Dialysis isn't a long term solution because:

  • dialysis machines are extremely expensive to run
  • can lead to problems like infections and blood clots
  • time consuming as 3-4 hour sessions are needed 3 times a week
• kidney transplants are a better long term solution than dialysis as it is much cheaper than using a dialysis machine, means patients have more freedom and actually cures the problem
• kidney transplants come with drawbacks including that there are not enough donors, there is a risk of rejection (which can be reduced through tissue typing) and immunosuppressant drugs need to be taken for the rest of the recipients life which can have long term side effects and leave patients more vulnerable to infections