Key Points

The cortex of the kidney is the outer, richly vascular layer; the medulla is the inner, less vascular layer.

The ureter exits the kidney in the same region as the renal artery and renal vein and conveys urine to the bladder.

The glomerular capillaries and the peritubular capillaries form the microcirculation of the kidney. Fluid is filtered from the high-pressure glomerular capillaries, whereas solutes and fluid are absorbed from the interstitial space by the low-pressure peritubular capillaries. The afferent arterioles and efferent arterioles control pressure in the capillaries. Each of the nephrons (there are about 1 million in each kidney) is divided into functional units, beginning with Bowman's space in the glomer-ulus, where the initial filtrate collects and enters the proximal tubule.

The ascending loop of Henle in the nephron returns from the medulla and extends to contact the arterioles in the glomerulus of its origin. This specialized region of contact is called the juxtaglomerular apparatus and is involved in regulating secretion of the hormone renin.

Superficial nephrons are so named because their glomeruli lie in the outer region of the cortex and they have short loops of Henle, whereas juxtamedullary nephrons arise from glomeruli in the minor cortex and have long loops of Henle.

The kidney is essential for the conversion of vitamin D3 to its most active form, 1,25-dihydroxycholecalciferol, and for the production of erythropoietin, an endocrine hormone that stimulates the production of red blood cells. The kidney also releases angiotensin II and the prostaglandins PGE2 and PGI2, which play important rolls in the maintenance of extracellular fluid volume and blood pressure.

Essential Medical Physiology, Third Edition

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