Filtration in the glomeruli
Because the afferent blood vessel (the one entering the glomerulus) is larger than the efferent one, the blood in the glomerulus is under rather high pressure (70-80 mmHg, compared with 15-18 mmHg in normal blood capillaries).
About 1,200 ml of blood passes through the kidneys during the course of one minute. The blood which enters the glomerulus is separated from the space in the Bowman’s capsule by the following two layers of cells: the inner wall of the Bowman’s capsule, and the walls of the capillaries themselves. These two thin cell layers (membranes) act like very fine filters, with a combined surface area of about 1,5 m².
Not all the contents of the blood plasma can be filtered. The pores in the ‘membrane’ are extremely fine, so much so that only substances with a molecular mass of 68,000 or less can pass through them. Plasma proteins such as fibrinogen, albumin, etc. are therefore not filtered. Even so, about 180 liters of fluid are filtered every 24 hours under normal circumstances. This is more than 20 times the volume of blood.
There are waste products present in the filtrate, such as urea, uric acid, creatine, and others. Useful products are also present there, such as amino acids, glucose, mineral salts, and water.
About 1.5 liters of urine is excreted per day on average. This means that about 90% of the 180 liters of fluid that gets filtered every day must be re-absorbed into the system.
While the filtrate is passing along the renal tubules, some of the substances in it are selectively reabsorbed by the epithelial cells which line these tubules. These substances then pass into the surrounding blood capillaries. The substances which will be reabsorbed depend on the current needs of the body.
All of the foodstuffs (glucose, amino acids, etc.) will be reabsorbed under normal conditions, but the reabsorption of such substances as water and mineral salts (eg. salts of magnesium, sodium, and calcium) will depend on the concentration of these salts in the blood that surrounds the tubules.
Those substances which the body cannot use (urea, uric acid, and creatine) will either not be absorbed in all, or absorbed to only a very limited extent.
The mechanism by which these substances are reabsorbed is different in the different sections of the renal tubule. Foodstuffs, for instance, will be reabsorbed in an active manner, and energy is required for this. Some of the other substances, on the other hand, will leave the tubules in a passive fashion, by means of the processes and osmosis and diffusion.
In the first convoluted tubule, water is absorbed by means of osmosis, but in the second of the convoluted tubules as well as in the collecting tubule, the process of water absorption is under the control of a hormone from the posterior of the brain, namely vasopressin or ADH (anti-diuretic hormone).
Secretion by the renal tubules
Some substances, such as calcium salts, creatine, and hydrogen ions, are removed from the surrounding capillaries by the epithelial cells of the tubules, and then are added to the contents of the tubule. This is practically the exact opposite process to that of active re-absorption, and it seems to occur in the same way, but in the opposite direction. This process also seems to keep the composition of the blood plasma more or less constant.
Synthesis of certain substances
It would appear that the epithelial cells of the renal tubules can manufacture certain substances in an active way, aided by enzymes. These substances are then added to the filtrate. This is the way ammonia is synthesized in different ways from other substances.
This ammonia then diffuses into the renal tubule, there to combine with hydrogen ions, forming ammonium radicles which can be excreted. Hippuric acid is synthesized in a similar way from the more harmful benzoic acid and is then excreted.
At this stage, the liquid present in the renal tubules can be called urine. This flows into the collecting tubules, eventually dripping down into the calyces from the ducts of Bellini, and from there passing to the pelvis, to be transported down the ureter to the bladder.