These substances include different end products of cellular
metabolism, such as urea and uric acid; they also
include excesses of ions and water from the food that
might have accumulated in the extracellular fluid.
The kidneys perform their function by first filtering
large quantities of plasma through the glomeruli into the
tubules and then reabsorbing into the blood those substances
needed by the body, such as glucose, amino acids,
appropriate amounts of water, and many of the ions. Most
of the other substances that are not needed by the body,
especially the metabolic end products such as urea, are
reabsorbed poorly and pass through the renal tubules into
the urine.