The kidneys are reddish brown, bean-shaped organs located bilateral to the vertebral column in the retroperitoneal space posterior to the abdominal cavity. They lie posterior to the parietal peritoneum, which covers their anterior surfaces. The kidneys are located between the levels of the twelfth thoracic vertebra and the third lumbar vertebra and are partially protected by the floating ribs. Each kidney is protected by three layers of connective tissue. A thin fibrous capsule tightly envelops each kidney, supporting the soft internal tissues. A thick layer of adipose tissue serves as a cushioning shock absorber, and a fibrous renal fascia attaches each kidney to the abdominal wall.
Gross Anatomy
Each kidney is convex laterally and concave medially with a medial indentation called the hilum. Blood vessels, lymphatic vessels, nerves, and the ureter enter or exit at the hilum. An adult kidney is about 12 cm long, 7 cm wide, and 2.5 cm thick.
The internal macroscopic anatomy of a kidney is best observed in frontal section. Two functional regions of the kidney are evident: the renal cortex and the renal medulla. The renal cortex is the relatively thin, superficial layer. Deep to the renal cortex is the renal medulla, which contains the coneshaped renal pyramids. The apex, or renal papilla, of each pyramid extends toward the renal pelvis, the most central structure of the kidney. Narrow portions of the renal cortex, the renal columns, extend into the renal medulla between the renal pyramids.
The work of the kidneys is performed by microscopic structures called nephrons (nef’-rons). Nephrons originate in the renal cortex, dip into the renal medulla, return to the renal cortex, and ultimately join a collecting duct. Nephrons produce urine, which flows into the collecting ducts of renal pyramids.
The renal papilla of each renal pyramid fits into a funnel-shaped minor calyx (ka ‘-lix), which receives urine from the collecting ducts. Two or three minor calyces (ka’-li-sez) converge to form a major calyx, and two or three major calyces merge to form the funnel-like renal pelvis. The renal pelvis is contiguous with the ureter. Thus, the pathway of urine from nephrons to ureter is as follows: nephrons ^ collecting ducts ^ minor calyces ^ major calyces ^ renal pelvis ^ ureter. Urine is carried by the ureter to the urinary bladder by peristalsis.
Microscopic Anatomy
Each kidney contains about 1 million nephrons, the functional units of the kidneys. A nephron consists of two major parts: a renal corpuscle and a renal tubule.
Renal corpuscles are located in the renal cortex of the kidneys. Each renal corpuscle is composed of a glomerulus (glo-mer’-u-lus, plural, glomeruli), a tuft of capillaries, which is enclosed in a double-walled glomerular (Bowman) capsule. The glomerular capsule is an expanded extension of a renal tubule.
A renal tubule leads away from the glomerular capsule and consists of three sequential segments. The first part of the renal tubule is the proximal convoluted tubule (PCT). It leads from the glomerular capsule to the nephron loop, the U-shaped second part of the tubule. The descending limb of the nephron loop descends into the renal medulla, and the ascending limb of the nephron loop ascends back into the renal cortex. The ascending limb of the nephron loop is continuous with the distal convoluted tubule (DCT), the third segment of the renal tubule, which unites with a collecting duct. Several nephrons unite with a single collecting duct. Collecting ducts begin in the renal cortex and extend the length of a renal pyramid to its papilla, where the collecting ducts merge before emptying into a minor calyx.
Types of Nephrons
There are two types of nephrons in the kidney: about 80% are cortical nephrons, and about 20% are juxtamedullary nephrons. The glomerular capsules of cortical nephrons are located superficially in the renal cortex. The nephron loops of these nephrons are located almost entirely in the renal cortex of the kidney. Cortical nephrons are important in adjusting the composition of the urine. In contrast, the glomerular capsules of juxtamedullary nephrons are located deep in the renal cortex near the renal medulla. The nephron loops of these nephrons penetrate deep into the medulla. Juxtamedullary nephrons play an important role in regulating water content of the blood plasma.
Renal Blood Supply
The kidneys receive a large volume of blood-1,200 ml per minute, which is about one-fourth of the total cardiac output. Each kidney receives blood via a renal artery, which branches from the abdominal aorta. Within each kidney, the renal artery branches to form three or four segmental arteries, which branch further to form several interlobar arteries that run along the renal columns between the renal pyramids to the renal cortex. These arteries branch to form smaller and smaller arteries and finally form arterioles.
In the renal cortex, afferent glomerular arterioles branch from the smallest arteries, and each afferent glomerular arteriole carries blood to a glomerulus. Blood leaves a glomerulus in an efferent glomerular arteriole. Note that a glomerulus is a capillary ball between two arterioles. The efferent glomerular arteriole usually leads to peritubular capillaries, which surround the cortical portion of the renal tubule. Sometimes the efferent glomerular arteriole leads to the vasa recta, which are vessels surrounding the nephron loops and collecting ducts within the renal medulla. Blood from the peritubular capillaries and vasa recta enters a venule, progressively larger veins that merge to form interlobar veins, which finally join to form the renal vein. A renal vein carries blood from each kidney to the inferior vena cava.
Juxtaglomerular Complex
The juxtaglomerular (juks-tah-glo-meb-u-lar) complex of each nephron is located where the ascending limb of the nephron loop contacts the afferent and efferent glomerular arterioles near the glomerulus. It consists of two groups of specialized cells: granular cells and the macula densa. Granular cells, or juxtaglomerular epithelioid cells, are large smooth muscle cells in the walls of the afferent and efferent glomerular arterioles near their attachment to the glomerulus. The macula densa consists of rather narrow, tightly packed cells composing the ascending limb where it contacts the afferent and efferent glomerular arterioles. The juxtaglomerular complex helps to regulate blood pressure.
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