Urinary System

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The Department of Human anatomy
Urinary System

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Development
 The development of the kidney
proceeds through a series of
successive phases: the pronephros,
mesonephros, and metanephros.

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Pronephros
 It develops in the cervical region of the embryo.
During approximately day 22 of human gestation, the
paired pronephron appear towards the cranial end of
the intermediate mesoderm. In this
region, epithelial cells arrange themselves in a series
of tubules called nephrotomes and join laterally with
the pronephric duct. This duct is fully contained within
the embryo and thus cannot excrete filtered material
outside the embryo; therefore the pronephros is
considered nonfunctional in mammals.

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Mesonephros
The development of the pronephric duct
proceeds in a cranial-to-caudal direction.
As it elongates caudally, the pronephric
duct induces nearby intermediate
mesoderm in the thoracolumbar area to
become epithelial tubules
called mesonephric tubules. The
mesonephric tubule forms a capsule
around the capillary tuft, allowing for
filtration of blood. This filtrate flows
through the mesonephric tubule and is
drained into the continuation of the
pronephric duct, now called
the mesonephric duct or Wolffian duct.

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Metanephros
 During the fifth week of gestation, the
mesonephric duct develops an outpouching,
the ureteric bud, near its attachment to the
cloaca. This bud, also called the
metanephrogenic diverticulum, grows
posteriorly and towards the head of the
embryo. As the cranial end of the bud
extends into the intermediate mesoderm, it
undergoes a series of branchings to form
the collecting duct system of the kidney.

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Migration
 After inducing the metanephric mesenchyme the
lower portions of the nephric duct will migrate
caudally (downward) and connect with the bladder,
thereby forming the ureters. The ureters will carry
urine from the kidneys to the bladder for excretion
from the fetus into the amniotic sac. As
the fetus develops, the torso elongates and
the kidneys rotate and migrate upwards within
the abdomen which causes the length of
the ureters to increase.

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Development of metanephros

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General Structure and Functions
of the Urinary System
Organs of the Urinary System:
- Primary organs: Kidneys
 filter waste products from the bloodstream
 convert the filtrate into urine.
- The Urinary Tract: Ureters, Urinary Bladder,
Urethra
 they transport the urine out of the body.

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Functions of the Urinary System

Removing waste product from the blood, then from the
body
 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: Th-12
 Inferior pole: L-3
 Right kidney ~ 2cm lower than left

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Anatomy
 Surrounding tissues, from deep to
superficial:
 Fibrous capsule (renal capsule)
 Perinephric fat (adipose capsule)
 Renal fascia
 Paranephric fat

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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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Anatomy
 Minor calyx:
 Funnel shaped
 Receives renal papilla
 8 to 15 per kidney
 Major calyx
 Fusion of minor calyces
 Major calyces merge to form renal pelvis
 Renal Lobe
 Pyramid plus some cortical tissue

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Blood Supply to the Kidney
 About 20 to 25% of cardiac blood flow
to kidneys (1500-2000 L/day)
 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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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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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
Glomerulus
• Capillary bed - rete mirabile
arteriosum (wonderful network )
• High pressure
• fenestrations

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Proximal Convoluted Tubule
 Begins at tubular pole of the renal
corpuscle.
 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

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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 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.

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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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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 urethral
glands.
 Tunica muscularis: primarily smooth muscle fibers
 help propel urine to the outside of the body.
 Two urethral sphincters:
 Internal urethral sphincter
 External urethral sphincter

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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 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.

Urinary System

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Urinary System