Structures of the Urinary System
KIDNEYS NEPHRON
cortex ureters
medulla BLADDER
renal pelvis URETHRA
Functions of the Urinary System
The primary functions of the urinary system’s
organs and structures are to filter and excrete
wastes from the BLOODand to maintain the body’s
fluid and electrolyte balances. The KIDNEYSmake,
and the bladder contains and then excretes, urine,
a watery fluid that carries dissolved and sus-
pended wastes from the body. As well, the KIDNEYS
produce two essential hormones: RENIN, which
helps regulate BLOOD PRESSURE, and ERYTHROPOIETIN
(EPO), which stimulates the BONE MARROW to
produce erythrocytes (red blood cells). The kid-
neys also convert vitamin D from its inactive
dietary form to its active form as the HORMONEcal-
citriol, which is necessary for proper calcium
absorption.
The kidneys: cleansing the blood The paired
KIDNEYS reside in the upper posterior abdomen,
behind the peritoneum along the spine and
within the protection of the rib cage. The kidneys
are slightly offset from one another in the symme-
try of their positioning, with the right kidney
being about an inch lower than the left to accom-
modate the LIVER. Each kidney is about the size of
a man’s fist, shaped like the bean that bears its
name. The dark reddish brown kidneys curve
toward each other, turning their backs to the
body’s sides. Though the kidneys have a combined
weight of about 10 ounces, they hold 20 percent
of the body’s blood supply. The renal arteries
branch directly from the abdominal AORTA, ensur-
ing that the kidneys are among the first of the
organs to receive blood with each contraction of
the HEART. The renal veins channel blood return-
ing to the circulation from the kidneys directly to
the inferior VENA CAVA.
The kidney’s structure features two general
divisions. The outer layer is the renal cortex,
intensely vascular tissue where the filtration of
blood takes place. The inner layer is the renal
medulla, where URINE collects. The workhorse
structure of the kidney is the microscopic NEPHRON,
which is made up of two elements: the renal cor-
puscle, contained in the renal cortex, and the
tubule, contained in the renal medulla. The renal
corpuscle consists of a containment capsule
(called Bowman’s capsule) that encloses the
GLOMERULUS, a tightly coiled capillary network that
receives blood from the body for filtration. The
space between the inner wall of Bowman’s cap-
sule and the walls of the glomerulus collects the
molecules of water, electrolytes, and metabolic
wastes that pass from the blood, forming a mix-
ture called the filtrate. The tubule is a loosely
coiled structure that wraps around the renal cor-
puscle. It reabsorbs electrolytes and water the
body needs from the filtrate and sends the
remainder on with the wastes to become urine.
Each tubule is about 1.25 inches in total length;
were a kidney’s tubules removed and stretchedTHE URINARY SYSTEM
The urinary system cleanses metabolic wastes and toxins from the blood. Physician specialists who treat conditions of
the urinary system are urologists (surgeons) and nephrologists (internists). This section, “The Urinary System,” pres-
ents a discussion of the organs and structures of the urinary system, an overview of urinary and renal health and dis-
orders, and entries about the health conditions that involve the urinary system.
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