Fish and amphibians get rid of wastes in the
form of ammonia, bypassing its toxicity because of the ample amounts of water
available to dilute it. Birds have access to little water, so their urine is
expelled with feces in the form of uric acid, a white paste. Mammals convert
nitrogenous waste into urea, which is diluted and expelled as urine, a mildly
diluted waste product. That is because mammals have some access to water, but
not as much as fish in their respective environments.
The pressure from blood is what forces water
and small solutes out of the glomerulus, through the epithelial cells and into
the nephron. Without the blood pressure, solutes could never filter through
the Bowman's capsule, and would accumulate in the blood, rising to dangerous
levels. The leaky capillaries of the glomerulus and podocytes of the Bowman's
capsule are the filters of the nephron, allowing waer and small solutes to pass
through and blocking blood cells and large molecules from entry into the nephron.
The podocytes make up the inner layer of the Bowman's capsule and allow solutes
such as salts, glucose, vitamins, water and urea to enter and become the filtrate.
That filtrate, after a little trip through the Loop of Henle, ultimately becomes
urine...Interesting eh?