Human Anatomy,
First Edition
McKinley & O'Loughlin
Chapter 27 Lecture Outline:
Urinary System
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General Structure and Functions
of the Urinary System
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The kidneys filter waste products from the bloodstream and
convert the filtrate into urine.
The ureters, urinary bladder, and urethra are collectively known
as the urinary tract because they transport the urine out of the
body.
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Functions of the Urinary System
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Removing waste products from the bloodstream.
Storage of urine.
 the urinary bladder is an expandable, muscular sac that can
store as much as 1 liter of urine
Excretion of urine.
Blood volume regulation.
 the kidneys control the volume of interstitial fluid and blood
under the direction of certain hormones
Regulation of erythrocyte production.
 as the kidneys filter the blood, they are also indirectly
measuring the oxygen level in the blood
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Blood Supply to the Kidney
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Blood plasma is filtered across the glomerulus into
the glomerular space.
Once the blood plasma is filtered, blood leaves the
glomerulus and enters an efferent arteriole.
Note that the efferent arteriole is still carrying
oxygenated blood because a gas and nutrient
exchange with the kidney tissues has not yet
occurred.
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Blood Supply to the Kidney
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The efferent arterioles branch into one of two types
of capillary networks:
 peritubular capillaries or
 vasa recta
 these capillary networks are responsible for the
actual exchange of gases and nutrients
Peritubular capillaries primarily reside in the cortex of
the kidney.
Vasa recta surround the thin tubes that project into
the medulla.
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Nephrons
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The functional filtration unit in the kidney.
Consists of the following components:
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a renal corpuscle, a proximal convoluted tubule, a nephron
loop, and a distal convoluted tubule and are collectively
known as the renal tubule
Together, both kidneys house approximately 2.5
million nephrons.
These microscopic structures measure less than 5
centimeters in total length.
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Proximal Convoluted Tubule
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The proximal convoluted tubule originates at the
tubular pole of the renal corpuscle.
The cells of the proximal convoluted tubule actively
reabsorb almost all nutrients (glucose and amino
acids), electrolytes, and any plasma proteins from the
tubular fluid.
Approximately 60% to 65% of the water in the
tubular fluid is reabsorbed by osmosis.
The solutes and water are returned to the vascular
system via the peritubular capillaries.
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Nephron Loop
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The nephron loop (loop of Henle) originates at a
sharp bend in the proximal convoluted tubule and
projects internally toward and into the medulla.
Each loop has two limbs.
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descending limb extending from the cortex toward and into
the medulla
ascending limb returns back to the renal cortex
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Distal Convoluted Tubule
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The distal convoluted tubule originates in the renal
cortex at the end of the thick ascending limb of the
nephron loop adjacent to the afferent arteriole.
The primary function of the distal convoluted tubule
is secretion.
Its cells secrete ions such as potassium (K+) and
acid (H+) into the tubular fluid.
Reabsorption of water also occurs here under the
influence of two hormones, aldosterone and
antidiuretic hormone.
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Collecting Tubules and Collecting
Ducts
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If an individual is well hydrated, the collecting ducts
simply transport the tubular fluid into the papillary
duct and then into the minor calyx.
However, if an individual is dehydrated, water
conservation must occur, and more-concentrated
urine is produced.
ADH may act on the collecting duct epithelium,
making it more able to absorb water from the tubular
fluid.
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Innervation of the Kidney
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Each kidney is innervated by a mass of autonomic
nervous system fibers collectively called the renal
plexus.
The renal plexus accompanies each renal artery and
enters the kidney through the hilum into the renal
sinus.
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Urinary Tract
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The ureters are long, fibromuscular tubes that
conduct urine from the kidneys to the urinary
bladder.
Each tube averages 25 centimeters in length and is
retroperitoneal.
The ureters originate at the renal pelvis as it exits the
hilum of the kidney, and then extend inferiorly to
enter the posterolateral wall of the base of the
urinary bladder.
The wall of the ureter is composed of three
concentric tunics.
From innermost to outermost, these tunics are the
mucosa, muscularis, and adventitia.
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Urinary Tract – Urinary Bladder
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The urinary bladder is an expandable, muscular container that
serves as a reservoir for urine. It is positioned immediately
superior and posterior to the pubic symphysis.
In females, the urinary bladder is in contact with the uterus
posterosuperiorly and with the vagina posteroinferiorly.
In males, it is in contact with the rectum posterosuperiorly and
is immediately superior to the prostate gland.
The urinary bladder is a retroperitoneal organ.
When empty, the urinary bladder exhibits an upside-down
pyramidal shape.
Filling with urine distends it superiorly until it assumes an oval
shape.
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Urinary Tract – Urinary Bladder
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A posteroinferior triangular area of the urinary
bladder wall, called the trigone is formed by
imaginary lines connecting the two posterior ureteral
openings and the anterior urethral opening.
The trigone remains immovable as the urinary
bladder fills and evacuates.
It functions as a funnel to direct urine into the
urethra as the bladder wall contracts to evacuate the
stored urine.
The four tunics that form the wall of the bladder are
the mucosa, submucosa, muscularis, and adventitia.
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Micturition (Urination)
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The expulsion of urine from the bladder.
Initiated by a complex sequence of events called the
micturition reflex.
The bladder is supplied by both parasympathetic and
sympathetic nerve fibers of the autonomic nervous
system.
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Urethra
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Fibromuscular tube that exits the urinary bladder through the
urethral opening from its anteroinferior surface and conducts
urine to the exterior of the body.
The luminal lining of the urethra is a protective mucous
membrane that houses clusters of mucin-producing cells called
urethral glands.
Bundles of primarily smooth muscle fibers surround the mucosa
and help propel urine to the outside of the body.
Two urethral sphincters restrict the release of urine until the
pressure within the urinary bladder is high enough and
voluntary activities needed to release the urine are activated.
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Urethra
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The internal urethral sphincter is the involuntary,
superior sphincter surrounding the neck of the
bladder, where the urethra originates.
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a circular thickening of the detrusor muscle and is controlled
by the autonomic nervous system
The external urethral sphincter is inferior to the
internal urethral sphincter and is formed by skeletal
muscle fibers of the urogenital diaphragm.
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a voluntary sphincter controlled by the somatic nervous
system
this is the muscle children learn to control when they
become “toilet-trained”
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Female Urethra
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Has a single function:
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to transport urine from the urinary bladder to the vestibule,
an external space immediately internal to the labia minora
3 to 5 centimeters long, and opens to the outside of
the body at the external urethral orifice located in the
female perineum.
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Male Urethra
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Urinary and reproductive functions:
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passageway for both urine and semen
Approximately 18 to 20 centimeters long.
Partitioned into three segments:
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prostatic urethra is approximately 3 to 4 centimeters long and is the
most dilatable portion of the urethra
 extends through the prostate gland, immediately inferior to the male
bladder, where multiple small prostatic ducts enter it
membranous urethra is the shortest and least dilatable portion
 extends from the inferior surface of the prostate gland through the
urogenital diaphragm
spongy urethra is the longest part (15 centimeters)
 encased within a cylinder of erectile tissue in the penis called the
corpus spongiosum
 extends to the external urethral orifice
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Aging and the Urinary System
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Changes in the size and functioning of the kidneys begin at 30.
Gradual reduction in kidney size.
Reduced blood flow to the kidneys.
Decrease in the number of functional nephrons.
Reabsorption and secretion are reduced.
Diminished ability to filter and cleanse the blood.
Less aldosterone or antidiuretic hormone.
Ability to control blood volume and blood pressure is reduced.
Bladder decreases in size.
More frequent urination.
Control of the urethral sphincters—and micturition—may be lost.
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