The kidneys are situated opposite to the last dorsal and upper two or three lumbar vertebrae ; they lie in the hypochondriac and epigastric regions, and the right kidney is also slightly in the right lumbar and umbilical region, being rather lower than the left, probably because of the size of the right lobe of the liver. They are each about 4 inches long, 2.5 inches wide, 1.5- inches thick, and weigh 4.5 ounces. The left is slightly longer and narrower than the right.

On the concave side there is a longitudinal depression, called the hilum, through which bloodvessels, nerves, lymphatics and ducts pass in and out.

All the tissues and organs and the circulatory blood contain urea, which is the product of the nitrogenous waste of the body ; the kidneys take up this urea from the blood ; dissolved in water it, with other nitrogenous extracts and salt, forms the urine, which passes through the ureters to the bladder, and from thence is ejected from the body by the urethra. The renal artery supplies blood to the kidneys direct from the aorta. The blood returned by the renal vein is still bright red, being purified in the kidney.

The kidneys are embedded in loose areolar tissue, which contains much fat.

Relations.—Right kidney lies behind hepatic flexure and right lobe of liver ; left kidney lies behind large end of stomach and spleen.

The suprarenal capsules are two small bodies which surmount the kidneys like helmets ; their use is not very well understood.