*Urea* is the most abundant solid constituent of the urine and is the chief waste product arising from the oxidation of nitrogenous substances in the body. Although secreted by the cells lining the uriniferous tubules, it is not formed in the kidneys. The secreting cells simply separate it from the blood where it already exists.
This is because the separation of waste is done in part by the Malpighian capsules and in part by the uriniferous tubules. Water and salts are removed chiefly at the capsules, while the remaining solid constituents of the urine pass through the secreting cells that line the tubules.
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 papillæ, are crowded with minute openings, the mouths of the uriniferous tubules, which form the substance of the kidney.
How do the waste materials get from the cells to the organs of excretion? Show by a drawing the connections of the kidneys with the large blood vessels and the bladder. Name parts of drawing. In what do the uriniferous tubes have their beginning? In what do they terminate? With what are they lined? Why should the blood pass through two sets of capillaries in the kidneys?
Small artery entering capsule and forming cluster of capillaries within. e. Small vein leaving capsule and branching into c, a second set of capillaries, h. Beginning of uriniferous tubule. The uriniferous tubules are lined with secreting cells. These differ greatly at different places, but they all rest upon a basement membrane and are well supplied with capillaries. Branch from renal artery. 2.
Bright’s disease of the kidneys affects the uriniferous tubes and interferes with their work. What impurity is then left in the blood? Trace water and salts from the Malpighian capsules to the bladder, naming parts through which they pass. Trace carbon dioxide from the cells to the outside atmosphere.
Branch from renal vein. 3. Small artery branches, one of which enters a Malpighian capsule . 6. Small vein leaving the capsule and branching into the capillaries which surround the uriniferous tubules. 4. Small veins which receive blood from the second set of capillaries. 8. Tubule showing lining of secreting cells.