The anatomy and embryology of human urinary system.

1. URINARY SYSTEM
Dr. Moono Silitongo

2. Introduction
• urinary system is well known for eliminating wastes from the body (excretion) and homeostasis
• has a very close physiological relationship with the endocrine, circulatory and respiratory systems
• anatomically, the urinary system is closely associated with the reproductive system
• in many animals the eggs and sperm are emitted through the urinary tract and the two systems have
a shared embryonic development and adult anatomical relationship
• this is reflected in humans, where the systems develop together in the embryo and in the male, the
urethra continues to serve as a passage for both urine and sperm
• thus the urinary and reproductive systems are often collectively called the urogenital (U-G) system

3. Principal functions:
• filter blood plasma, separate wastes from useful chemicals and eliminate the
wastes e.g. urea,uric acid and creatinine; reabsorption of nutrients
• regulate blood volume and pressure by eliminating or conserving water
• regulate osmolarity of body fluids by controlling relative amounts of water
and solutes eliminated (control of fluid, electrolyte and acid-base balance)
• secrete the enzyme renin, which activates hormonal mechanisms that
control blood pressure and electrolyte balance

4. • secrete the hormone erythropoietin which controls the red blood cell count and
oxygen-carrying capacity of the blood
• function with the lungs to regulate the Pco2 and acid-base balance of the body fluids
• contribute to calcium homeostasis through their role in synthesizing calcitriol
(vitamin D)
• detoxify free radicals and drugs with the use of peroxisomes
• in times of starvation, they carry out gluconeogenesis; they deaminate amino acids
(remove the NH2 group), excrete the amino group as ammonia (NH3), and
synthesize glucose from the rest of the molecule

5. Principal organs:
• kidneys, ureters, urinary bladder and urethra

6. Embryology of the urinary system
• development of the urinary and genital systems is intimately
associated
• urinary system develops before the genital system
• three successive kidney systems develop:
• pronephroi (nonfunctional)
• mesonephroi (temporary excretory organs)
• metanephroi (permanent kidneys)

7. • metanephroi develop from two sources:
• metanephric diverticulum (ureteric bud), which gives rise to the ureter, renal pelvis,
calices and collecting tubules
• metanephrogenic blastema (metanephric mass of mesenchyme), which gives rise to
the nephrons
• the urinary bladder develops from the and the surrounding splanchnic
mesenchyme
• the female urethra and most of the male urethra have a similar origin

8. Kidney
• develops from the intermediate mesoderm
• longitudinal elevation of mesoderm-the urogenital ridge - forms on each side of
the dorsal aorta
• part of the urogenital ridge giving rise to the urinary system is the nephrogenic
cord the part giving rise to the genital system is the gonadal ridge
• develops from the last of three sets of kidneys:
• pronephros
• mesonephros
• metanephros

10. • Pronephros
• appears early (4th week)
• degenerates rapidly and never forms functional nephrons
• its development begins in the cranial (cervical region)
• pronephric ducts run caudally and open into the cloaca
• Mesonephros
• appear late in 4th week
• develops caudally or inferior to the pronephros in thoracolumbar region
• largely degenerates but forms the mesonephric (Wolffian) duct, which forms the ureteric bud
and contributes to the male reproductive tract

12. • Metanephros
• primordia of permanent kidneys
• begin to develop early in the fifth week
• start to function approximately 12th week
• develops from the ureteric bud and forms the permanent kidney, which
ascends from the sacral region to the upper lumbar region
• the permanent kidneys develop from two sources:
• metanephric diverticulum (ureteric bud)
• metanephrogenic blastema or metanephric mass of mesenchyme

13. • ureteric bud forms the ureter, which dilates at its upper end to form the
renal pelvis
• renal pelvis repeatedly divides to form the major and minor calyces
and collecting tubules
• metanephric mesoderm forms the nephrons of adult kidney
(glomerulus, renal capsule, proximal convoluted tubules and loop of
Henle), distal convoluted tubules and collecting tubules

15. Urinary bladder
• develops from the upper end of the urogenital sinus, which is
continuous with the allantois
• allantois degenerates and forms a fibrous cord in the adult called the
urachus
• trigone of the bladder is formed by the incorporation of the lower end
of the mesonephric ducts into the posterior wall of the urogenital sinus

16. • division of the cloaca (4th – 7th week) by the urorectal septum into a
dorsal rectum and a ventral urogenital sinus
• the urogenital sinus is divided into three parts:
• cranial vesical part that forms most of the bladder and is continuous with the
allantois
• middle pelvic part that becomes the urethra in the bladder neck, the prostatic
part of the urethra in males, and the entire urethra in females
• caudal phallic part that grows toward the genital tubercle (primordium of the
penis or clitoris)

18. Kidneys
• located in the upper abdominal cavity on either side of the vertebral
column
• kidneys are retroperitoneal and lie against the posterior abdominal
wall at the level of vertebrae T12 to L3
• extends from L1 to L4 vertebrae in the erect position
• right kidney lies a little lower than the left because of the large size of
the right lobe of the liver

19. • right kidney usually is related to rib 12 posteriorly, whereas the left one is
related to ribs 11 and 12 posteriorly
• human kidney is said to have lobes, each of which is a single renal pyramid
plus the cortical tissue that surrounds that pyramid
• there are 5 to 11 lobes and pyramids in each kidney
• has arterial segments including the superior, anterosuperior, anteroinferior,
inferior and posterior segments, which are of surgical importance

22. • each kidney is invested by a firm, fibrous renal capsule and is surrounded by the
renal fascia, which divides the fat into two regions:
• perirenal (perinephric) fat lies between the
renal capsule and renal fascia
• pararenal (paranephric) fat lies external
to the renal fascia
• the perirenal and pararenal fat layers
cushion the kidney against blows and
help hold the kidneys in place

23. • has an indentation - the hilus - on its medial border, through which the
ureter, renal vessels and nerves enter or leave the organ
• each kidney consists of an outer renal cortex and an inner renal medulla
• renal cortex is a continuous band of pale tissue that completely surrounds
the renal medulla
• extensions of the renal cortex (the renal columns) project into the inner
aspect of the kidney dividing the renal medulla into discontinuous
aggregations of triangular-shaped tissue the renal pyramids

24. • present in the medulla and cortex are 1 to 2 million nephrons which
are the anatomic and functional units of the kidney
• each nephron consists of a renal corpuscle, a proximal convoluted
tubule, Henle's loop and a distal convoluted tubule
• the bases of the renal pyramids are directed outward, toward the renal
cortex, while the apex of each renal pyramid projects inward, toward
the renal sinus

26. • the apical projection (renal papilla) is surrounded by a minor calyx
• the minor calices receive urine and represent the proximal parts of the
tube that will eventually form the ureter
• in the renal sinus, several minor calices unite to form a major calyx,
and two or three major calices unite to form the renal pelvis, which is
the funnel-shaped superior end of the ureters

27. Nephron
• Consists of :
i. Blood vessels
• afferent arteriole
• glomerulus
• efferent arteriole
ii. Renal tubules
• proximal convoluted tubule
• loop of Henle
• distal convoluted tubule

29. Glomerulus
• consists of a tuft of capillaries surrounded by a cup-shaped, hollow
glomerular capsule (Bowman’s capsule)
• this tuft of capillaries is supplied by an afferent arteriole and drained
by an efferent arteriole
• the endothelium of the glomerulus is fenestrated (has pores), and thus
these capillaries are highly porous allowing large quantities of fluid
and small molecules to pass from the capillary blood into the hollow
interior of the glomerular capsule, the capsular space

30. • this fluid is the filtrate that is ultimately processed into urine
• the external parietal layer of the glomerular capsule, which is a simple
squamous epithelium, simply contributes to the structure of the capsule and
plays no part in the formation of filtrate
• the capsule’s visceral layer clings to the glomerulus and consists of
unusual, branching epithelial cells called podocytes (“foot cells”)

32. • the branches of the octopus-like podocytes end in foot processes, or
pedicels (“little feet”), which interdigitate with one another as they
surround the glomerular capillaries
• the filtrate passes into the capsular space through thin clefts between
the foot processes called filtration slits

33. Renal tubules
Proximal convoluted tubule (PCT)
• confined to the renal cortex, is most active in resorption and secretion
• its walls are formed by cuboidal epithelial cells whose luminal surfaces
have long microvilli that seem to fill the tubule lumen with a “fuzz
• microvilli increase the surface area of these cells and maximizing their
capacity for resorbing water, ions and solutes from the filtrate
• cells of the proximal tubule contain many mitochondria which provide
the energy for resorption and a highly infolded plasma membrane on their
basal and lateral cell surfaces that contains many ion- pumping enzymes
responsible for resorbing molecules from the filtrate

35. Loop of Henle
• consists of a descending limb and an ascending limb
• first part of the descending limb is continuous with the PCT and has a similar structure
• the rest of the descending limb is the thin segment, the narrowest part of the nephron, with
walls consisting of a permeable simple squamous epithelium
• the thin segment continues into the ascending limb joining the thick segment of the
ascending limb
• thick ascending limb joins the DCT in the cortex
• the cell structure of this thick segment resembles that of the distal convoluted tubule

36. Distal convoluted tubule (DCT)
• like the proximal convoluted tubule is confined to the renal cortex
• has walls of simple cuboidal epithelium and is specialized for the
selective secretion and resorption of ions
• it is less active in resorption than the proximal tubule and its cells do
not have an abundance of absorptive microvilli

37. • cells of the distal tubule have many mitochondria and infoldings of the
basolateral membrane features that are typical of all ion-pumping cells
in the body
• urine passes from the distal tubules of the nephrons into the collecting
ducts, each of which receives urine from several nephrons and runs
straight through the cortex into the deep medulla
• at the papilla of the pyramid adjacent collecting ducts join to form
larger papillary ducts which empty into the minor calices

40. Ureters
• are muscular tubes about 25 cm (10 inches) long that carry urine from the
kidneys (renal pelvis) to the urinary bladder
• each ureter begins superiorly at the level of L2
• are continuous superiorly with the renal pelvis which is a funnel-shaped
structure in the renal sinus
• renal pelvis is formed from a condensation of two or three major calices
which in turn are formed by the condensation of several minor calices

41. • minor calices surround a renal papilla
• is retroperitoneal and descends on the transverse processes of the
lumbar vertebrae and the psoas muscle
• it is crossed anteriorly by the gonadal vessels and crosses the
bifurcation of the common iliac artery
• enter the pelvic cavity from the abdomen by passing through the
pelvic inlet

43. • on each side, the ureter crosses the pelvic inlet and enters the pelvic cavity
in the area anterior to the bifurcation of the common iliac artery
• from this point, it continues along the pelvic wall and floor to join the base
of the bladder
• in the pelvis, the ureter is crossed by:
- the ductus deferens in men
- the uterine artery in women
• is innervated by the lumbar(sympathetic) and pelvic (parasympathetic)
splanchnic nerves

44. Histology of the ureter
• similar histological structure as that of renal calices and renal pelvis
• the walls having three basic layers: mucosa, muscular and adventitia
• the lining mucosa is composed of a transitional epithelium that stretches when the ureters
fill with urine and a lamina propria composed of a stretchy fibroelastic connective tissue
• the middle muscularis consists of two layers: inner longitudinal layer and outer circular
layer of smooth muscle
• a third layer of muscularis an external longitudinal layer appears in the inferior third of
the ureter
• the external adventitia of the ureter wall is a typical connective tissue

46. Urinary bladder
• a collapsible, muscular sac that stores and expels urine
• it lies inferior to the peritoneal cavity on the pelvic floor just posterior
to the pubic symphysis
• in males, the bladder lies anterior to the rectum; in females,the bladder
lies just anterior to the vagina and uterus
• a full bladder is roughly spherical and expands superiorly into the
abdominal cavity; an empty bladder lies entirely within the pelvis

47. • when full, the bladder becomes firm and can be palpated through the
anterior abdominal wall just superior to the pubic symphysis
• empty bladder is shaped like a inverted three-sided pyramid
• it has an apex, a base, a superior surface and two inferolateral surfaces
• apex of the bladder is directed towards the top of the pubic symphysis;
a structure known as the median umbilical ligament (urachus; remnant
allantois of foetus) continues from it superiorly up the anterior
abdominal wall to the umbilicus

48. • at the bladder’s apex is a fibrous band called the urachus (“urinary
canal of the foetus”), the closed remnant of an embryonic tube called
the allantois
• the inferior angle (neck) drains into the urethra

49. • base of the bladder is shaped like an inverted triangle and faces
postero - inferiorly
• two ureters enter the bladder at each of the upper corners of the base
and the urethra drains inferiorly from the lower corner of the base
• inside, the mucosal lining on the base of the bladder is smooth and
firmly attached to the underlying smooth muscle coat of the wall-
unlike elsewhere in the bladder where the mucosa is folded and
loosely attached to the wall

50. • smooth triangular area between the openings of the ureters and urethra on the
inside of the bladder is known as the trigone
• in males, the prostate, a reproductive gland, lies directly inferior to the bladder
where it surrounds the urethra
• inferolateral surfaces of the bladder are cradled between the levator ani muscles
of the pelvic diaphragm and the adjacent obturator internus muscles above the
attachment of the pelvic diaphragm
• superior surface is slightly domed when the bladder is empty; it balloons upward
as the bladder fills

53. In both sexes:
- internal urethral sphincter - under involuntary control
- external urethral sphincter - under voluntary control
• although the bladder is considered to be pelvic in the adult, it has a higher
position in children
• at birth, the bladder is almost entirely abdominal; the urethra begins
approximately at the upper margin of the pubic symphysis
• with age, the bladder descends until after puberty when it assumes the adult
position

55. Histology of the urinary bladder
• the wall of the urinary bladder has three layers:
1. a mucosa with a distensible transitional epithelium and a lamina propria
2. a thick muscular layer
3. a fibrous adventitia (except on the superior surface of the bladder which is
covered by parietal peritoneum)
• the muscular layer called the detrusor muscle, consists of highly
intermingled smooth muscle fibres arranged in inner and outer longitudinal
layers and a middle circular layer

56. • the bladder’s great distensibility makes it uniquely suited for its function of
storing urine
• when there is little urine in it the bladder collapses into its basic pyramidal
shape, its walls thick and its mucosa thrown into folds known as rugae but
as urine accumulates, the rugae flatten and the wall of the bladder thins as it
stretches, allowing the bladder to store larger amounts of urine without a
significant rise in internal pressure
• a full adult bladder holds about 500 ml of urine, 15 times its empty volume

57. Empty urinary bladder Full urinary bladder

58. Urethra
• begins at the base of the bladder and ends with an external opening in
the perineum
• paths taken by the urethra differ significantly in women and men
In women
• urethra is short, being about 4 cm long
• travels a slightly curved course as it passes inferiorly through pelvic
floor into the perineum, where it passes through the deep perineal
pouch and perineal membrane before opening in the vestibule that lies
between the labia minora

59. • urethral opening is anterior to the vaginal opening in the vestibule
• inferior aspect of the urethra is bound to the anterior surface of the
vagina
• two small para-urethral mucous glands (Skene's glands) are associated
with the lower end of the urethra
• each drains via a duct that opens onto the lateral margin of the external
urethral orifice

60. In men
• urethra is long, about 20 cm and bends twice along its course
• beginning at the base of the bladder and passing inferiorly through the
prostate, it passes through the deep perineal pouch and perineal
membrane and immediately enters the root of the penis
• as the urethra exits the deep perineal pouch, it bends forward to course
anteriorly in the root of the penis

61. • when the penis is flaccid, the urethra makes another bend, this time
inferiorly, when passing from the root to the body of the penis
• during erection, the bend between the root and body of the penis
disappears
• urethra in men is divided into preprostatic, prostatic, membranous and
spongy parts

63. Pre-prostatic part
• this part of the urethra is about 1 cm long
• extends from the base of the bladder to the prostate and is associated
with a circular cuff of smooth muscle fibres (the internal urethral
sphincter)
• contraction of this sphincter prevents retrograde movement of semen
into the bladder during ejaculation

64. Prostatic part
• this part of the urethra is 3-4 cm long and is surrounded by the prostate
• in this region the lumen of the urethra is marked by a longitudinal midline
fold of mucosa (the urethral crest)
• the depression on each side of the crest is the prostatic sinus; the ducts of
the prostate empty into these two sinuses
• midway along its length, the urethral crest is enlarged to form a somewhat
circular elevation (the seminal colliculus aka verumontanum)

66. Membranous part
• of the urethra is narrow and passes through the deep perineal pouch
• during its transit through this pouch, the urethra, in both men and
women, is surrounded by skeletal muscle of the external urethral
sphincter

67. Spongy urethra
• is surrounded by erectile tissue (corpus spongiosum) of the penis
• it is enlarged to form a bulb at the base of the penis and again at the
end of the penis to form the navicular fossa
• two bulbourethral glands in the deep perineal pouch are part of the
male reproductive system and open into the bulb of the spongy urethra
• external urethral orifice is the sagittal slit at the end of the penis

68. Applied anatomy
Urinary tract stones (calculi)
• occur more frequently in men than in women, are most common in
people aged between 20 and 60 years
• are usually associated with sedentary lifestyles
• stones are polycrystalline aggregates of calcium, phosphate, oxalate,
urate and other soluble salts within an organic matrix
• urine becomes saturated with these salts and small variations in the pH
cause the salts to precipitate

69. Hydronephrosis
• is a fluid filled enlargement of the renal pelvis and calyces
• is as a result of obstruction of the ureter
• it is due to an obstruction:
- urine flow by kidney stones in the ureter
- by compression on the ureter by abnormal blood vessels
- by the developing foetus at the pelvic brim
• this condition may be corrected by the pyeloplasty
• which is a surgical reconstruction of the renal pelvis and ureter to
correct an obstruction at the ureteropelvic junction by removing the
obstructed portion of the ureter and then reattaching the healthy ureter
to the renal pelvis

70. Urinary tract cancer
• most tumours that arise in the kidney are renal cell carcinomas
• these tumours develop from the proximal tubular epithelium.
• approximately 5% of tumours within the kidney are transitional cell
tumours, which arise from the urothelium of the renal pelvis
• most patients typically have blood in the urine (haematuria), pain in
the infrascapular region (loin) and a mass

71. Nephrostomy
• is a procedure where a tube is placed through the lateral or posterior
abdominal wall into the renal cortex to lie within the renal pelvis
• function of this tube is to allow drainage of urine from the renal pelvis
through the tube externally
• done in patients with distal ureteric obstruction the back pressure of
urine within the ureters and the kidney significantly impairs the
function of the kidney which will fail to function

72. Bladder infection
• the relatively short length of the urethra in women makes them more
susceptible than men to bladder infection
• primary symptom of urinary tract infection in women is usually
inflammation of the bladder (cystitis)
• in children under 1 year of age, infection from the bladder may spread
via the ureters to the kidneys, where it can produce renal damage and
ultimately lead to renal failure

73. Suprapubic catheterization
• in certain instances it is necessary to catheterize the bladder through
the anterior abdominal wall
• e.g. when the prostate is markedly enlarged and it is impossible to pass
a urethral catheter a suprapubic catheter may be placed
• procedure of suprapubic catheterization is straightforward and
involves the passage of a small catheter on a needle in the midline
approximately 2 cm above the pubic symphysis
• catheter passes easily into the bladder without compromise of other
structures and permits free drainage

74. Diabetes insipidus
• is caused by inadequate ADH secretion
• due to the shortage of ADH, water reabsorption in CD is
compromised, leading to polyuria

75. • Horseshoe Kidney (Cake or fused kidney)
• where the two developing kidneys fuse into a single horseshoe-shaped
structure
• occurs if the kidneys become to close together during ascent from the pelvis to
the abdomen
• usually stuck underneath the inferior mesenteric artery
• usually asymptomatic and can be prone to obstruction

76. • Exstrophy of the bladder results from a rare ventral body wall defect
through which the posterior wall of the urinary bladder protrudes onto
the abdominal wall
• Epispadias is a common associated anomaly in males; the urethra
opens on the dorsum of the penis

77. • A - Unilateral renal agenesis
• B - Right side, pelvic kidney;
• left side, divided kidney with a bifid ureter
• C - Right side
• malrotation of the kidney; left side, bifid
ureter and supernumerary kidney
• D - Crossed renal ectopia.
• left kidney crossed to the right side and
fused with the right kidney
• E - Discoid kidney
• resulting from fusion of the kidneys while
they were in the pelvis
• F - Supernumerary left kidney
• resulting from the development of two
metanephric diverticula