eceives the blood which has
passed from the capillary clusters into a system of small veins (Fig. 90).
From the last set of capillaries the blood is passed into veins which
leave the kidneys where the artery branches enter, uniting there to form
the main renal veins.

*Work of the Kidneys.*--Why should the blood pass through two systems of
capillaries in the kidneys? 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. It was formerly believed that the kidneys obtained
their secretion by a process of filtration from the blood, but this belief
has been gradually modified. The prevailing view now is that the processes
of filtration and secretion are both carried on by the kidneys,--that the
capillary clusters in the Malpighian bodies serve as delicate filters for
the separation of water and salts, while the secreting cells of the
tubules separate substances by the process of secretion.

On account of the large volume of blood passing through the kidneys this
liquid is still a bright red color as it flows into the renal veins (Fig.
90). The kidney cells require oxygen, but the amount which they remove
from the blood is not sufficient to affect its color noticeably. The blood
in the renal veins, having given up most of its impurities and still
retaining its oxygen, is considered the purest blood in the body.

*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. The muscles also have
been suggested as a likely source of urea, for here the proteids are
broken down in largest quantities; but the muscles produce little if any
urea. Its production has been found to be the _work of the liver_. In the
muscular tissue, and in the other tissues as well, the proteids are
reduced to a lower order of compounds, such as the compounds of ammonia,
which pass into the blood and are then taken up by the liver. By the
action of the liver cells these are converted into urea and this is turned
back into the blood. From the blood the urea is se