Kidney Function and Age - Physiology
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Kidney Function in the Old
As the animal begins to get old its glomerular filtration rate drops mainly due to two factors. One being a reduction in the number of nephrons and the other being vascular changes. Under normal conditions these kidneys function well but their reserve capacity is very limited. This is an important consideration when you consider that many drugs are excreted in the urine therefore if GFR is reduced their excretion will be also meaning an older animal will have higher plasma levels of the drug for longer.
Kidney Function in Newborns
During pregnancy the placenta takes care of the homeostatic function of the foetus. The kidneys just produce the amniotic fluid. This means that the foetus has a very low GFR compared to adults. When born animals such as calves and piglets have relatively substantially lower GFR's compared to their adult counterparts. This takes some time to be corrected. In humans it is only at 2 to 3 years of age that children have relatively the same GFR and adults. The increase is mainly thanks to an increase in blood perfusion and activation of nephrons, inactive during foetal life. As with older animals this is an important consideration when you consider that many drugs are excreted in the urine therefore if GFR is reduced their excretion will be also meaning an young animal will have higher plasma levels of the drug for longer.