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Human Anatomy, First EditionMcKinley & O'Loughlin
Chapter 27 :
UrinarySystem
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General Structure and Functions
of the Urinary System General Concept:
Waste products accumulate in blood
Are toxic Must be removed to maintain
homeostasis
Urinary System organs remove waste products from the blood
then from the body
Major homeostatic system
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General Structure and Functions
of the Urinary System Organs of the Urinary System:
Kidneys
Ureters
Urinary Bladder
Urethra
Primary organs: kidneys filter waste products from the bloodstream
convert the filtrate into urine.
The Urinary Tract: Includes:
ureters
urinary bladder
urethra
Because they transport the urine out of the body.
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Functions of the Urinary System 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 bloodunder 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
Erythropoietin (EPO): hormone produced by kidney Released if blood oxygen levels fall
Stimulates RBC production in red bone marrow
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Kidneys: Gross and Sectional
Anatomy Retroperitoneal
Anterior surface covered with peritoneum
Posterior surface against posteriorabdominal wall
Superior pole: T-12
Inferior pole: L-3 Right kidney ~ 2cm lower than left
Adrenal gland on superior pole
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Kidneys: Gross and Sectional
Anatomy Hilum: concave medial border
Renal sinus: internal space
Houses blood vessels, lymphatic vessels,nerves
Houses renal pelvis, renal calyces
Also fat
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Kidneys: Gross and Sectional
Anatomy Surrounding tissues, from deep to superficial:
Fibrous capsule (renal capsule) Dense irregular CT
Covers outer surface
Perinephric fat (adipose capsule) Also called perirenal fat
Completely surrounds kidney
Cushioning and insulation
Renal fascia Dense irregular CT
Anchors kidney to posterior wall and peritoneum
Paranephric fat Between renal fascia and peritoneum
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Kidneys: Gross and Sectional
Anatomy Sectioned on a coronal plane:
Renal Cortex Renal arches Renal columns
Renal Medulla Divided into renal pyramids
8 to 15 per kidney Base against cortex
Apex called renal papilla
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Kidneys: Gross and Sectional
Anatomy Minor calyx:
Funnel shaped Receives renal papilla 8 to 15 per kidney, one per pyramid
Major calyx Fusion of minor calyces 2 to 3 per kidney
Major calyces merge to form renal pelvis Renal Lobe
Pyramid plus some cortical tissue 8 to 15 per kidney
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Blood Supply to the KidneyAbout 20 to 25% of cardiac output to
kidneys
Path: Renal artery to segmental arteries to
interlobar arteries to arcuate arteries tointerlobular arteries to:
Afferent arteriole to glomerulus to efferentarteriole to peritubular capilaries and vasarecta
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Blood Supply to the Kidney Blood plasma is filtered across the glomerulus
into the glomerular space.
Once the blood plasma is filtered blood leaves the glomerulus
enters an efferent arteriole.
efferent arteriole is still carrying oxygenatedblood
a gas and nutrient exchange with the kidneytissues has not yet occurred.
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Blood Supply to the Kidney Path for veins:
Interlobar veins to arcuate veins to
interlobar veins to the renal vein
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Nephrons The functional filtration unit in the kidney. Consists of the following:
Renal corpuscle
Glomerulus Glomerular capsule (Bowmans capsule)
Proximal convoluted tubule (PCT) Nephron loop (loop of Henle)
Ascending loop of Henle Descending loop of Henle
Distal convoluted tubule (DCT) collectively called the renal tubule
In both kidneys: approximately 2.5 million nephrons. Are microscopic: measure about 5 centimeters in
length.
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Nephrons Cortical Nephrons
Near peripheral edge of cortex
Short nephron loops Have peritubular capillaries
Juxtamedullary nephrons
Near corticomedullary border Long nephron loops
Have vasa recta
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Urine Formation Three processes
Filtration
Renal corpuscle: forms filtrate From blood to tubule
Reabsorption Mostly PCT
Water and salt: rest of nephron From tubule to blood
Secretion From blood to tubule
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Renal Corpuscle Vascular pole
Afferent and efferent arterioles
Tubular pole Connects to PCT
Two structures: Glomerulus and glomerular capsule
Glomerulus Capillary bed
High pressure
fenestrations
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Renal Corpuscle Glomerular Capsule
Parietal layer Simple squamous epithelium
Visceral layer Podocytes
Pedicels
Filtration slits
Capsular space (Bowmans capsule): location offiltrate
Filtration membrane Fenestrations
Filtration slits
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Proximal Convoluted Tubule Begins at tubular pole of the renal corpuscle. Cells: simple cuboidal epithelium
actively reabsorb from the filtrate: almost all nutrients (glucose and amino acids) electrolytes plasma proteins
Osmosis: reabsorption of 60% to 65% of thewater in filtrate.
Have microvilli
Solutes and water: moved into blood plasma
via the peritubular capillaries.
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Nephron Loop (loop of Henle) originates at end of proximal convoluted tubule
projects toward and/or into the medulla.
Each loop has two limbs. descending limb:
from cortex toward and/or into the medulla
ascending limb:
returns back to the renal cortex
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Distal Convoluted Tubule begins at the end of the thick ascending limb of the
nephron loop adjacent to the afferent arteriole (important physiologically)
Juxtaglomerular apparatus.
primary function: Secretion From blood plasma to filtrate. secretes ions
potassium (K+) acid (H+)
Reabsorption of water also occurs: influenced by two hormones
Aldosterone antidiuretic hormone (ADH).
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Collecting Collecting Ducts Function in a well hydrated person:
transport the tubular fluid into the papillary duct and theninto the minor calyx.
Function in a dehydrated person: water conservation
more-concentrated urine is produced.
ADH can act on the collecting duct epithelium Cells become permeable to water
Water moves from filtrate into blood plasma
Involves vasa recta.
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Innervation of the Kidney innervated by a mass of autonomic nervous system
fibers
called the renal plexus.
The renal plexus
accompanies each renal artery
enters the kidney through the hilum.
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Urinary Tract : Ureters long, fibromuscular tubes
conduct urine from the kidneys to the urinarybladder.
average 25 centimeters in length retroperitoneal.
ureters originate at the renal pelvis
extend inferiorly to enter the posterolateral wall of
the base of the urinary bladder. wall is composed of three concentric tunics.
mucosa
muscularis
adventitia.
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Urinary Tract Urinary Bladder The urinary bladder:
expandable, muscular container serves as a reservoir for urine
positioned immediately superior and posterior to the pubic
symphysis. in females
the urinary bladder is in contact with the uterus posterosuperiorlyand with the vagina posteroinferiorly.
in males it is in contact with the rectum posterosuperiorly and is
immediately superior to the prostate gland.
is a retroperitoneal organ. when empty 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 Trigone
posteroinferior triangular area of the urinary bladder wall formed by imaginary lines
connect 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
directs urine into the urethra as the bladder wall contracts
four tunics mucosa submucosa Muscularis: called the detrusor muscle adventitia.
Internal urethral sphincter (smooth muscle)
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Micturition (Urination) The expulsion of urine from the bladder.
Initiated by a complex sequence of events called themicturition reflex.
The bladder is supplied by both parasympathetic andsympathetic nerve fibers of the autonomic nervous
system.
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Urethra Fibromuscular tube
exits the urinary bladder through the urethral opening at anteroinferior surface
conducts urine to the exterior of the body. Tunica mucosa: is a protective mucous membrane
houses clusters of mucin-producing cells called urethralglands.
Tunica muscularis: primarily smooth muscle fibers help propel urine to the outside of the body.
Two urethral sphincters: Internal urethral sphincter
restrict the release of urine until the pressure within the urinarybladder is high enough
External urethral sphincter
and voluntary activities needed to release the urine areactivated.
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Urethra The internal urethral sphincter
involuntary (smooth muscle) superior sphincter surrounding the neck of the bladder,
where the urethra originates. a circular thickening of the detrusor muscle controlled by the autonomic nervous system
The external urethral sphincter inferior to the internal urethral sphincter formed by skeletal muscle fibers of the urogenital
diaphragm. 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 Has a single function:
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 ofthe body at the external urethral orifice located in thefemale perineum.
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Male Urethra Urinary and reproductive functions:
passageway for both urine and semen
Approximately 18 to 20 centimeters long.
Partitioned into three segments: 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 malebladder, 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 thecorpus spongiosum
extends to the external urethral orifice
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Aging and the Urinary System 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 sphinctersand micturitionmay be lost.
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