is known as cups or calices;
C, pelvis of the kidney;
D, upper end of ureter.
]

256. Function of the Kidneys. The Malpighian capsules are really the
beginning of the tubules, for here the work of excretion begins. The thin
wall of the capillaries within each capsule separates the blood from the
cavity of the tubule. The blood-pressure on the delicate capillary walls
causes the exudation of the watery portions of the blood through the cell
walls into the capsule. The epithelial cell membrane allows the water of
the blood with certain salts in solution to pass, but rejects the albumen.
From the capsules, the excretion passes through the tubules into the
pelvis, and on through the ureters to the bladder. But the delicate
epithelial walls of the tubules through which it passes permit the inflow
of urea and other waste products from the surrounding capillaries. By this
twofold process are separated from the blood the fluid portions of the
renal secretion with soluble salts, and the urea with other waste
material.

257. How the Action of the Kidneys may be Modified. The action of the
kidneys is subject to very marked and sudden modifications, especially
those operating through the nervous system. Thus whatever raises the
blood-pressure in the capillaries of the capsules, will increase the
quantity of fluid filtering through them. That is, the watery portion of
the secretion will be increased without necessarily adding to its solids.
So anything which lowers the blood-pressure will diminish the watery
portion of the secretion, that is, the secretion will be scanty, but
concentrated.

The Renal Secretion.--The function of the kidneys is to secrete a
fluid commonly known as the urine. The average quantity passed in 24 hours
by an adult varies from 40 to 60 fluid ounces. Normal urine consists of
about 96 per cent of water and 4 per cent of solids. The latter consist
chiefly of certain nitrogenous substances known as urea and uric acid, a
considerable quantity of mineral salts, and some coloring matter. Urea,
the most important and most abundant constituent of urine, contains the
four elements, but nitrogen forms one-half its weight. While, therefore,
the lungs expel carbon dioxid chiefly, the kidneys expel nitrogen. Both of
these substances express the result of oxidations going on in the body.
The urea and uric acids represent the final result of the breaking down in
the body of nitrogenous substances, of which albumen