Touching Lives - November 2005
The kidney - the body’s housekeeper
Research Training Fellow, Sally Johnson LINK, is working on a rare but serious kidney disease which damages the lining of the thousands of tiny blood vessels.This interferes with the ‘filtering’ action of the kidneys.
What the kidneys actually do is much more complicated than filtering, like in a coffee machine where the volume of liquid going into the filter is much the same as that coming out. The kidney has a much more complex job.
For example, eating protein like meat, nuts, eggs and milk, all of which contain nitrogen, produces a waste substance called urea, which circulates in the bloodstream. But the blood also carries a host of useful substances like glucose. ^The kidneys must sort the toxins in the blood from the useful properties.^
The quantity of water in the body also has to be controlled by the kidney, irrespective of whether you have had a liquid lunch of five pints of beer, or are rather short of fluid because you have sweated a lot.
The kidneys really are astonishing, bearing in mind that they are each only about the size of a small fist and weigh 150g. The blood pressure is used to force the watery part of the blood through thin membranes, at the rate of about 170 litres per day. This is about 4 times the amount of water in the body of an adult.
Altogether there are about one million nephrons [resembling microscopic funnels — Ed.] working in parallel, each of which handles the whole process from blood to urine. The filtered fluid passes down a narrow tube, where most of the water is reabsorbed together with those substances the body wants to keep, like the glucose. A bit further down, salts, present in excess, are also pumped into the urine. ^The result of the process is a solution in which most substances are more concentrated than they are in the blood.^ For instance the urea in urine could be 20 times more concentrated.
Just as an aside, birds, which tend to be fairly short of water and in any case do not want to carry the extra weight about, get rid of their nitrogen waste as uric acid; a white powder, slightly lubricated, which when landing on you from a height is apparently…lucky!