The
urine leaving the kidney differs considerably in composition from the plasma
entering it . The study of renal function must account for these differences;
e.g., the absence of protein and glucose from the urine, a change in the pH of
urine as compared with that of plasma, and the high levels of ammonia and
creatinine in the urine, while sodium and calcium remain at similar low levels
in both urine and plasma.
A large volume of ultrafiltrate (i.e., a liquid from which the blood cells and
the blood proteins have been filtered out) is produced by the glomerulus into
the capsule. As this liquid traverses the proximal convoluted tubule, most of
its water and salts are reabsorbed, some of the solutes completely and others
partially; i.e., there is a separation of substances that must be retained from
those due for rejection. Subsequently the loop of Henle, distal convoluted
tubule, and collecting ducts are mainly concerned with the fine control of water
and electrolyte balance.