*Blood Supply to the Kidneys.*—The method by which the kidneys do their work is suggested by the way in which the blood circulates through them. The renal artery entering each kidney divides into four branches and these send smaller divisions to all parts of the kidney. At the outer margin of the kidney, called the _cortex_, the blood is passed through _two sets of capillaries_. The first forms the clusters in the Malpighian capsules and receives the blood directly from the smallest arteries. The second forms a network around the uriniferous tubules and receives 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.