In addition to reabsorbing materials
from plasma filtrate, the nephron can
secrete materials across the tubular
epithelium and into the filtrate. In this
process, the reverse of tubular reabsorption,
carrier proteins in the tubular
epithelial cells selectively transport
substances from blood in capillaries
outside the tubule to the filtrate inside
the tubule. Tubular secretion enables
the kidney to build up the urine concentrations
of materials to be excreted,
such as hydrogen and potassium ions,
drugs, and various foreign organic
materials. The distal convoluted tubule
is the site of most tubular secretion.
In the kidneys of bony marine
fishes, reptiles, and birds, tubular
secretion is a much more highly developed
process than it is in mammalian
kidneys. Marine bony fishes actively
secrete large amounts of magnesium
and sulfate, seawater salts that are byproducts
of their mode of osmotic regulation.
Reptiles and birds excrete uric
acid instead of urea as their major
nitrogenous waste. The material is
actively secreted by the tubular epithelium.
Since uric acid is nearly insoluble,
it forms crystals in the urine and
requires little water for excretion. Thus
excretion of uric acid is an important
adaptation for water conservation.