255.  Structure of the Kidneys.  The pelvis is surrounded by reddish cones, about twelve in number, projecting into it, called the pyramids of Malpighi.  The apices of these cones, known as the papillae, are crowded with minute openings, the mouths of the uriniferous tubules, which form the substance of the kidney.  These lie parallel in the medullary or central structure, but On reaching the cortical or outer layer, they wind about and interlace, ending, at last, in dilated closed sacs called Malpighian capsules.

[Illustration:  Fig. 108.—­Vertical Section of the Kidney.

  A, pyramids of Malpighi;
  B, apices, or papillae, of the pyramids, surrounded by subdivisions of
     the pelvis 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 is the type.