Kidneys (Triple)

• Water in must equal water out over the course of a day

  • Too much water can cause cells to swell and even burst - water moves by osmosis out of cells
  • If this happens in the brain, it can cause the brain to be damaged
  • Too little water an lead to dehydration causing cells to shrivel and shrink meaning they stop functioning properly - water moves by osmosis into cells
• Water can be gained by

  • Product of respiration
  • Food
  • Drink
• Water can be lost via

  • Lungs when we exhale air - the body can't control this water loss
  • Sweat, losing water through the skin - sweat also contains ions such as sodium ions and the waste product urea - the body can't control how much water, ions or urea is lost by sweating as sweating is part of the body's temperature control system
  • Kidneys in urine - the body can control how much water is lost in urine - if the blood is too dilute, the kidneys remove the excess water and produce a greater volume of urine. Excess water, urea and excess ions are removed in urine
• How the kidneys produce urine:

  1. Blood (which contains urea) enters the kidney via the renal artery
  2. The kidney removes the excess urea as well as excess ions and excess water - these leave the kidney as urine and this is stored in the bladder
  3. Blood (now containing no urea) leaves the kidney through a vein

  
• • Each kidney contains millions of microscopic structures called nephrons. Millions of these means it creates a huge surface area
  • This is where filtration and selective reabsorption happens
  • Filtration - all small dissolved molecules fall through the filter into the tubule
  • Selective Reabsorption
  • The substances selectively reabsorbed are
  • All of the glucose originally filtered out by active transport
  • As much water and ions as the body needs to maintain the correct level in the blood plasma

  
• • Filtration and selective reabsorption happens in kidneys
  • Filtration - all small dissolved molecules fall through the filter into the tubule
  • Selective Reabsorption
  • The substances selectively reabsorbed are
  • All of the glucose originally filtered out by active transport
  • As much water and ions as the body needs to maintain the correct level in the blood plasma
  • The substances that aren't selectively reabsorbed are: urea (actively kept in the tubule), excess water and excess ions

  
• This shows the concentrations of glucose, ions and urea in the blood before and after it has passed through the kidneys. The concentration of glucose hasn't changed since the kidneys filter glucose out of the blood but then reabsorb it all back into the blood. The concentration of ions has decreased as the kidney filter ions out of the blood and reabsorb a certain amount of ions back into the blood depending on the body's needs. The concentration of urea in the blood falls to zero as it passes through the kidneys as the kidneys filter out all of the urea and don't reabsorb it

  
• 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 broken down to form ammonia - this process is called deamination. Ammonia is toxic and so the liver immediately convers it to urea for safe excretion.