Kidneys produce about 180 liters of filtrate per day, but the actual urine production is about
1.2 to 1.5 liters per day. How this reduction takes place ?
Answers
Filtration is the bulk flow of water through a semipermeable membrane (filter), carrying with it those solutes which can pass through the filter. As mentioned above, the glomerular filter only excludes plasma proteins. Water moves by bulk flow through the filter as a consequence of pressure gradients. Immediately upstream and downstream from the glomerular capillaries, there are blood vessels which have smooth muscle in their walls, so that they can constrict or dilate, and so alter the resistance to the flow of blood. These vessels are, respectively, the afferent and efferent arterioles. They permit precise regulation of the hydrostatic pressure of the blood in the glomerular capillaries, which is maintained at a higher level than in capillaries in other parts of the body. This force drives plasma from the glomerular capillaries into the nephrons. However, two forces work in opposition to this movement. One is the osmotic pressureexerted by the plasma proteins, which increases as filtration proceeds and the proteins, because they are not filtered, get more concentrated. The other force opposing filtration is the hydrostatic pressure within the Bowman's capsule. The resultant is a net filtration pressure which diminishes as blood flows through the glomerlus. The amount of filtration that actually occurs is known as the glomerular filtration rate, or GFR. It is about 120 ml/min (180 l/day). This seems an enormous volume — and it is an enormous volume — but it is important to realize that it is only a small fraction of the total plasma delivered to the kidneys in the blood. In this respect, the kidneys are rather different from our everyday experiences of filters. For example, when we make filter coffee, we pour water over coffee in the filter, and essentially all the water goes through the filter, leaving a ‘sludge’ of coffee grounds in the filter. If all of the plasma delivered to the kidneys passed through the glomerular filters into the nephrons, the filters would be clogged with a ‘sludge’ of red cells, white cells, and plasma proteins. This is prevented because only 20% of the plasma arriving at the filter actually passes through. The remaining 80% continues into the efferent arterioles.
The volume of plasma in the whole of the circulating blood is only about 3 litres, yet we filter 180 litres per day of it. This apparently paradoxical situation is possible because, after filtration, almost all (99%) of the plasma is reabsorbed along the nephron, so can be filtered again and again (60 times a day!). The selectivity of the kidney — how it is able to conserve some substances and excrete others — is due to the transport processes (reabsorption and secretion) which occur along the nephron, modifying the composition of the glomerular filtrate.