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Human Anatomy, First Edition

McKinley & O'Loughlin

Chapter 27 :

Urinary System

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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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Urinary System in Context

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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 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

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 posterior abdominal wall

Superior pole: T-12

Inferior pole: L-3

Right kidney ~ 2cm lower than left

Adrenal gland on superior pole

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Superior pole: T-12 Inferior pole: L-3

Surface anatomy of the Kidney

• Hilum is concave

located on the medial

surface.

• The renal sinus is a

cavity within the

kidney which is

occupied by the renal

pelvis, renal calyces,

blood vessels, nerves

and fat.

10 cm

5.5cm

3cm

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Kidneys: Gross and Sectional Anatomy

Renal sinus: internal space

Houses blood vessels, lymphatic vessels, nerves

Houses renal pelvis, renal calyces

Also fat

kidneys are covered by 4 layers: 1- the inner layer is called renal capsule 2-the middle layer is called perirenal fat (Perinephric) 3-the outer layer is called renal fascia. Paranephric 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

Macroscopic & Microscopic Anatomy of the Kidney

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Blood Supply to the Kidney

About 20 to 25% of cardiac output to kidneys

Path: Renal artery to segmental arteries to

interlobar arteries to arcuate arteries to interlobular arteries to:

Afferent arteriole to glomerulus to efferent arteriole to peritubular capilaries and vasa recta

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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 oxygenated blood

a gas and nutrient exchange with the kidney tissues has not yet occurred.

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Blood Supply to the Kidney

The efferent arterioles branch into one of two types of capillary networks:

peritubular capillaries

vasa recta

these capillary networks are responsible for the actual exchange of gases and nutrients

Peritubular capillaries: primarily in cortex

Vasa recta: surround the thin tubes that project into the medulla.

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Blood Supply to the Kidney

• Path for veins:

– Interlobular 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 (Bowman’s 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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0

m

Blood vessels from a human kidney. Arterioles and peritubular capillaries appear pink; glomeruli appear yellow.

Figure 44.14e

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Figure 44.14c

Nephron Types

Cortical nephron

Juxtamedullary nephron

Renal cortex

Renal medulla

Functional units of kidney are the nephrons.

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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

occurs at the distal convoluted tubule.

Summery

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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 (Bowman’s capsule): location of filtrate

– 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 the water 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).

juxtaglomerular (JG) apparatus, -

JG apparatus consists of

• a region of the DCT called the macula densa (simple columnar) located between the afferent & efferent arterioles.

Alteration in:

1-Blood pressure &/or

2-NaCl salt concentration in urine is detected by the JG apparatus.

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Collecting Ducts

Function in a well hydrated person:

transport the tubular fluid into the papillary duct and then into 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.

Osmolarity of interstitial

fluid (mOsm/L)

Key

Active transport

Passive transport

INNER MEDULLA

OUTER MEDULLA

CORTEX NaCl H2O

H2O

H2O

H2O

H2O

H2O

H2O

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

H2O

H2O

H2O

H2O

H2O

H2O

H2O

Urea

Urea

Urea

1,200

1,200 1,200

900

600

400

300 300

400

600

100

100

200

400

700 900

600

400

300

300

300

Figure 44.16-3

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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 urinary bladder.

• 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.

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 posterosuperiorly and 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. 27-49

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

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Urinary Tract – Urinary Bladder

• four tunics

– mucosa

– submucosa

– Muscularis: called the detrusor muscle

– adventitia.

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Internal urethral sphincter (smooth muscle)

Urinary Tract – Urinary Bladder

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Micturition (Urination)

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.

Urethra • Fibromuscular tube

– exits the urinary bladder

– 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 urethral glands.

• 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 urinary bladder is high enough

– External urethral sphincter

• and voluntary activities needed to release the urine are activated.

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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 of the body at the external urethral orifice

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Male Urethra

Urinary and reproductive functions:

passageway for both urine and semen

Approximately 18 to 20 centimeters long.

• Male Urethra is 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 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 • 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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