4. 27-4
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.
6. 27-6
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.
7. 27-7
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.
8. 27-8
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.
13. 27-13
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.
17. 27-17
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
18. 27-18
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
22. 27-22
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
23. 27-23
Kidneys: Gross and Sectional
Anatomy
Minor calyx:
Funnel shaped
Receives renal papilla
8 to 15 per kidney
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
25. 27-25
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
28. 27-28
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.
29. 27-29
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
31. 27-31
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
33. 27-33
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
34. 27-34
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
36. 27-36
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).
37. 27-37
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.
38. 27-38
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.
40. 27-40
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.
41. 27-41
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)
44. 27-44
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
45. 27-45
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”
46. 27-46
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.
48. 27-48
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
50. 27-50
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.
