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Human Anatomy, First Edition McKinley & O'Loughlin. Chapter 27 : Urinary System. 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

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Human anatomy first edition mckinley o loughlin l.jpg

Human Anatomy, First EditionMcKinley & O'Loughlin

Chapter 27 :

Urinary System

General structure and functions of the urinary system l.jpg
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 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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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 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:

    • Interlobar veins to arcuate veins to interlobar veins to the renal vein

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  • 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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  • 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 (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).

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Collecting 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.

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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.

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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 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.

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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 the micturition reflex.

  • The bladder is supplied by both parasympathetic and sympathetic nerve fibers of the autonomic nervous system.

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  • 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 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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  • 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 located in the female 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 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.