The kidneys lie in the paravertebral grooves at the level of the T12-L3 vertebrae. The medial margin of each is concave whereas the lateral margins are convex, giving the organ a bean-shaped appearance. In the adult, each kidney measures 10 to 12 cm in length, 5 to 7.5 cm in width, and 2.5 to 3.0 cm in thickness. In an adult man, each kidney weighs 125 to 170 g; in an adult woman, each kidney weighs 115 to155 g.
On the concave surface of the kidney is the hilum, through which the renal artery, vein, renal pelvis, a nerve plexus, and numerous lymphatics pass. On the convex surface, the kidney is surrounded by a fibrous capsule, which protects it, and a fatty capsule with a fibroareolar capsule called the renal fascia, which offers further protection and serves to anchor it in place.
The arterial supply begins with the renal artery, which is a direct branch of the aorta. On entering the hilum, this artery subdivides into branches supplying the five major segments of each kidney: the apical pole, the anterosuperior segment, the anteroinferior segment, the posterior segment, and the inferior pole. These arteries subsequently divide within each segment to become lobar arteries. In turn, these vessels give rise to arcuate arteries that diverge into the sharply branching interlobular arteries, which directly supply the glomerular tufts.
The cut surface of the kidney reveals a pale outer rim and a dark inner region corresponding to the cortex and medulla, respectively. The cortex is 1 cm thick and surrounds the base of each medullary pyramid. The medulla consists of between 8 and 18 cone-shaped areas called medullary pyramids; the apex of each area forms a papilla containing the ends of the collecting ducts. Urine empties from these ducts into the renal pelvis, flows into the ureters, and, subsequently, into the urinary bladder.
The kidneys maintain the constancy of the extracellular fluid by creating an ultrafiltrate of the plasma that is virtually free of cells and larger macromolecules, and then processing that filtrate, reclaiming what the body needs and letting the rest escape as urine. Every 24 hours, an adult's kidneys filter nearly 180 L of water (total body water is ∼ 25–60 L), and 25,000 mEq of sodium (total body Na+ is 1200–2800 mEq). Under normal circumstances, the kidneys regulate these two substances independent of each other, depending on the body's needs. Approximately 1% of the filtered water and 0.5% to 1% of the filtered Na+ are excreted.
Renal function begins with filtration at the glomerulus, a highly permeable capillary network stretched between two arterioles in series. The relative constriction or dilation of these vessels normally controls the glomerular filtration rate (GFR). Under normal circumstances, approximately 20% of the plasma water in the blood entering the glomeruli goes through the filter, carrying with it electrolytes, ...