The basic unit of the filtering mechanism is the Nephron. There are approximately one million of these individual units in each kidney. Each consists of a small network of blood vessels clumped up in a bunch called the glomerulus. This is surrounded by an outer layer of cells called Bowman’s capsule, which connects to a longer, twisted tube called the tubule. In turn, this merges into a larger pipe called a collecting tubule, which finally empties into the large internal opening of the kidney called the pelvis. In turn, this is connected to a much larger tube, called the urethra, leading to the bladder.
Arterial blood coming from the body reaches the glomerulus. Here, by a simple filtering mechanism, much of its fluid content, salts and metabolic products diffuse into the space of the Bowman’s capsule. The filtered fluid then flows back into the tubules. As this happens a great deal of the contents are reabsorbed by fine blood capillaries that run in close contact to the tubules.
In fact, a great majority of the fluids and salts are absorbed by the bloodstream in this manner. However, the nature and amount of fluid and substances reabsorbed depend on the body’s needs at the given moment. In this way, an up-to-the-moment selection of what is required will be accepted. The remainder will be rejected, to be expelled as urine. This is an intricate but very effective mechanism, and serves the body well, keeping its requirements in a fine state of balance at all times.
A few figures will give some idea of the magnitude of this complex system. The filtering surface of the one million glomeruli in each kidney equals about 1.5 square meters. The length of each tubule is between 3.5 and 5.0 cm (1 and 2 inches). This gives a total tubular length of about 72 km (45 miles). The figures start to become mind-boggling.
The rate at which blood is pumped through the kidneys is about 1.3 liters a minute, which is equivalent to one-quarter to one-third of the cardiac output. Every minute the giomeruli can filter 120 ml of blood, or 180 liters in 24 hours. This equals 60 times the total plasma volume of the entire body. Of this, only about 1.5 litres are produced as urine, which means that 178.5 litres of fluid are reabsorbed by the tubule system each 24 hours.