The main function of the kidneys is EXCRETION. They remove urea from the blood in a two stage process described in an earlier post, first by filtering the blood under high pressure in the glomerulus and then selectively reabsorbing the useful substances back into the blood as the filtrate passes along the nephron.
But the kidney has an equally important role in HOMEOSTASIS. It actually is the main effector organ for regulating a whole load of variables about the composition of the blood (e.g. blood pH and salt balance) but in this post I want to explain to you how the water balance of the body is regulated and the kidney’s role in this homeostatic system.
Why do you need to regulate the dilution (or water potential) of the blood?
If the blood becomes too dilute, then water will enter all the body cells by osmosis (from a dilute to a more concentrated solution). This net movement into cells would cause them to swell and eventually burst. Bad news all round…
If the blood becomes too concentrated, then water will leave the body cells by osmosis. Cells will shrivel up as they lose water into the blood and this will kill them. Bad news all round….
Remember: a hypertonic solution has a low water potential and is very concentrated. A hypotonic solution has a very high water potential and is very dilute.
The regulation of the water potential of the blood is a very important example of homeostasis in the human. It is often referred to as OSMOREGULATION.
The water potential (dilution) of the blood is measured continuously by a group of neurones in a region of the brain called the hypothalamus.
The hypothalamus is found right next to a very important hormone-secreting gland called the pituitary gland, marked as the red circular structure on the diagram above. When the hypothalamus detects that the blood’s water potential is dropping (i.e. it is getting too concentrated) this causes the posterior lobe of the pituitary gland to start secreting a hormone ADH into the bloodstream.
(You might remember that these brain structures appear elsewhere in the iGCSE specification. The hypothalamus also contains the temperature receptors that measure the temperature of the blood in thermoregulation; the pituitary gland plays a role in the menstrual cycle by producing FSH and LH)
Hormones such as ADH exert their effects elsewhere in the body. The main target tissue for ADH is the collecting duct walls in the kidney. ADH binds to receptors on these cells and makes the wall of the collecting duct much more permeable to water. This means as the urine passes down the collecting duct through the salty medulla of the kidney, lots of water can be reabsorbed into the blood by osmosis. This leaves a small volume of very concentrated urine and water loss is minimised.
ADH is secreted whenever the body is dehydrated. It might be because the person is losing plenty of water in sweating in which case it is vital that the kidney produces as small a volume of urine as is possible.
If you drink a litre of water, what effect will this have on the dilution of the blood: of course it makes the blood more dilute. This will be detected in the hypothalamus by osmoreceptors and they will cause the pituitary gland to stop secreting ADH into the bloodstream. If there is no ADH in the blood, the walls of the collecting duct remain totally impermeable to water. As the dilute urine passes down the collecting duct, no water can be reabsorbed into the blood by osmosis and so a large volume of dilute urine will be produced.
This is another beautiful example of negative feedback in homeostasis.
PMG tip: you can avoid getting confused in the exam about the effect of ADH if you can remember what it stands for. ADH is an acronym for anti-diuretic hormone (ADH). A diuretic is a drug that promotes urine production. They are banned drugs from WADA (World Anti-Doping Agency) since they can be taken as a masking drug to help flush out evidence of illegal drug taking. Shane Warne missed the 2003 cricket World Cup and served a ban for failing a drugs test due to diuretics in his sample.
So an anti-diuretic hormone will reduce urine production. This means it will be secreted when the body is dehydrated as the blood gets too concentrated.
Finally remember that it is not the whole nephron that is affected by ADH, just the collecting ducts and part of the distal convoluted tubules. Most water in the glomerular filtrate is absorbed in the nephron but the collecting duct has a role in “fine-tuning” the volume and dilution of urine.
This is a really important topic to master for an A* in your exam. Examiners seem to like asking questions on ADH and osmoregulation and often these questions are amongst the hardest marks to get in the exam, and so serve as a brilliant discriminator between A and A* candidates. Work hard to master this topic and with a little luck from the question-setters an A* grade is within your grasp……