Kidneys

• how kidneys maintain a normal blood volume in health:

  • when blood volume increases you would initially bulk up your blood volume and increase your blood pressure
  • This increased blood pressure means that the blood perfusing the kidneys is also under a higher than normal pressure
• how kidneys maintain a normal blood volume in health:

  • one of the jobs of the kidneys is filtration
  • So when blood entering the kidneys is under high pressure, the filtration pressure is raised too
  • In effect, more water is filtered out of your blood to end up sitting in your bladder
  • Water in your bladder (urine), represents fluid lost from the system and hence blood volume starts to become normalised and as a consequence blood pressure is brought down towards normal and any fine tuning can be achieved via neuroendocrine response
• how kidneys maintain a normal blood volume in health:

  • when your blood volume is low, your blood pressure is low, renal filtration pressures are low, less water is lost as urine, more water is conserved in blood so blood volume and BP start to rise and normalise
  • Full normalisation can be aided by the addition of renin (angiotensin II) secretion and baroreceptor mediated (reduced vasomotor centre suppression) increased sympathetic activity to arterioles (vasoconstriction), veins (venous return) and the heart (inotropy & chronotropy), plus stimulation of the thirst centre