The kidneys are the primary organs of the urinary system. The kidneys
are the organs that filter the blood, remove the wastes, and excrete
the wastes in the urine. They are the organs that perform the functions
of the urinary system. The other components are accessory structures
to eliminate the urine from the body.
The paired kidneys are located between the twelfth thoracic and
third lumbar vertebrae, one on each side of the vertebral column.
The right kidney usually is slightly lower than the left because
the liver displaces it downward. The kidneys protected by the lower
ribs, lie in shallow depressions against the posterior abdominal
wall and behind the parietal peritoneum. This means they are retroperitoneal.
Each kidney is held in place by connective tissue, called renal
fascia,
and is surrounded by a thick layer of adipose tissue, called perirenal
fat, which helps to protect it. A tough, fibrous, connective tissue
renal capsule closely envelopes each kidney and provides support
for the soft tissue that is inside.
In the adult, each kidney is approximately 3 cm thick, 6 cm wide,
and 12 cm long. It is roughly bean-shaped with an indentation, called
the hilum, on the medial side. The hilum leads to a large cavity,
called the renal sinus, within the kidney. The ureter and renal
vein leave the kidney, and the renal artery enters the kidney at
the hilum.
The outer, reddish region, next to the capsule,
is the renal cortex. This surrounds a darker reddish-brown
region called the renal medulla. The renal medulla consists
of a series of renal pyramids, which appear striated because
they
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contain straight tubular structures and blood vessels. The wide
bases of the pyramids are adjacent to the cortex and the pointed
ends, called renal papillae, are directed toward the center of the
kidney. Portions of the renal cortex extend into the spaces between
adjacent pyramids to form renal columns. The cortex and medulla
make up the parenchyma, or functional tissue, of the kidney.
The central region of the kidney contains the renal pelvis, which
is located in the renal sinus and is continuous with the ureter.
The renal pelvis is a large cavity that collects the urine as it
is produced. The periphery of the renal pelvis is interrupted by
cuplike projections called calyces. A minor calyx surrounds the
renal papillae of each pyramid and collects urine from that pyramid.
Several minor calyces converge to form a major calyx. From the major
calyces the urine flows into the renal pelvis and from there into
the ureter.
Each kidney contains over a million functional units, called nephrons,
in the parenchyma (cortex and medulla). A nephron has two parts:
a renal corpuscle and a renal tubule.The renal corpuscle consists
of a cluster of capillaries, called the glomerulus, surrounded by
a double-layered epithelial cup, called the glomerular capsule.
An afferent arteriole leads into the renal corpuscle and an efferent
arteriole leaves the renal corpuscle. Urine passes from the nephrons
into collecting ducts then into the minor calyces.
The juxtaglomerular apparatus, which monitors blood pressure and
secretes renin, is formed from modified cells in the afferent arteriole
and the ascending limb of the nephron loop.
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