Urinary System

Sanjaya Adikari Department of Anatomy

• Urinary system

comprises of

– Two kidneys

– Two ureters

– Bladder

– Urethra

Position and size of the kidneys

Retro-peritoneal

Size 12 x 6 x 3 cm

Weighs about 130 g

Hilum is 5 cm from midline

12

6

Position and size …

Hilum of right Kidney

is just below

transpyloric plane

Hilum of left Kidney

is just above

transpyloric plane

Slight movement

with respiration

Back

Renal fascia

Perinephric fat

Renal capsule

Ureter

Renal vessels

Renal fascia separates the kidney from the suprarenal gland and surrounding tissues.

Limits the spread of a perinephric abscess

Structures at the hilum of the kidney

• Three structures enter each kidney via the hilum, from front to back

– the vein

– the artery

– the ureter

Renal vein

Renal artery

Ureter

Functions of the kidney

• Filtration

• Selective reabsorption

• Secretion

• Control of renin-angiotensin-aldosterone mechanism

• Secretion of erythropoietin

• Production of 1,25-dihydroxycholecalciferol

Structure of the kidney

• Capsulated

• Outer cortex and inner medulla

• Medulla is arranged into medullary

pyramids

• Apices of pyramids are called renal

papillae

• Papillae project into calyces

• Calyces converge to form the renal

pelvis

Minor Calyces

Renal Papilla

Structure of the kidney…

• Functional unit is the nephron

• Approximately 1.3 million nephrons

in each kidney

• Part of a nephron is in cortex and

part in medulla

Nephron

Nephron

Renal corpuscle

Renal tubule

Glomerulus

Bowman’s capsule

Proximal convoluted tubule

Loop of Henle

Distal convoluted tubule

Collecting tubule

Nephron…cont.

Location of nephrons

Cortical nephrons

Juxtamedullary nephrons

Renal corpuscle(Bowman’s space)

of the visceral layer

Vascular pole

Urinary pole

Bowman’s capsuleSingle layer of flattened cells resting on a basement membrane

GlomerulusCoiled network of anastomosing capillaries

MesangiumSupporting connective tissue

Renal corpuscle – microscopic view

Glomerular filtration barrier

• Capillary endothelium– Contain pores (fenestrations) large enough to permit

passage of all non-cellular elements of blood

• Common basement membrane (glomerular basement membrane)– Non-fenestrated, continuous layer. Allows passage of

molecules <65000 molecular weight. Eg. Free Hb. Albumin with mw of 68000 and other lager molecules are retained

• Podocyte layer– Has primary and secondary processes (pedicles).

There are slit pores between pedicles. Function of slit pores ?

Components of

glomerular filter

Proximal convoluted tubule (PCT)

• Longest section of the tubule

• Most convoluted section

• Makes up the bulk of cortex

• Simple cuboidal epithelium

• Brush border fills the lumen

• Cytoplasm is pink stained due to mitochondria

• >75% of GF is reabsorbed

Proximal convoluted tubule…

PCT

DCT

Loop of Henle

• Has thin and thick limbs

• Descending limb is lined by simple squamous epithelium (thin walled)

• Ascending limb is lined by simple cuboidal epithelium (thick walled)

• Closely associated with parallel capillary loops called vasa recta (forms a counter-current multiplier system)

• Generates a high osmotic pressure in the ECF of medulla

Loop of Henle…

Distal convoluted tubule (DCT)

• Shorter and less convoluted than PCT

• Found in the cortex

• No brush border

• Larger, clear lumen

• Cells smaller, so more cells than in PCT in section

• Na+ ions absorbed under the influence of aldosterone

Collecting tubule

• This is the terminal part of the DCT

• Opens into collecting ducts

• Together with collecting ducts form ‘medullary rays’

• Collecting tubules and ducts are not normally permeable to water

• Becomes permeable to water in the presence of ADH

• Site of reabsorption of water

Collecting ducts• Formed by fusion of collecting tubules

• They fuse with adjacent ducts and form larger ducts called ducts of Bellini

• Ducts of Bellini convey urine into the calyceal space through renal papillae

• Lined by tall, pale stained columnar cells

• No brush border

• Water reabsorption under ADH control

Collecting ducts…

Minor Calyces

Renal Papilla

Calyces and renal pelvis

Ureter

• About 25cm long

• In X-ray seem to lie

over the transverse

processes of L2 – L5

veterbrae

• Comprises the pelvis,

abdominal, pelvic and

intravesical portions

Course of the ureter• Abdominal part lies on medial edge of psoas

muscle

• Psoas muscle separates it from the tips of transverse processes of L2-L5

• Crosses into the pelvis at the bifurcation of common iliac artery in front of sacroiliac joint

• Pelvic part runs on the lateral wall of pelvis towards the ischial spine

• At ischial spine turns forwards and medially to enter the bladder

• Intravesical part runs obliquely through the bladder wall for 2 cm. This creates a valvular effect

Narrow parts in the ureter

• Ureter is relatively narrowed at three sites

– Junction between the renal pelvis and abdominal

part

– At the pelvic brim

– At the ureteric orifice (narrowest of all)

• A ureteric stone can get lodged at one of these

places

• Usually stones <5mm in diameter tend to pass

naturally

Ureter…

• Lined by transitional epithelium

• Muscular tube

– Inner longitudinal

– Outer circular

• Urine is propelled by peristalsis

Transitional epithelium

• Also called urinary epithelium/

urothelium

• Plasma membrane of superficial cells

are thicker

• Impermeable to urine (potentially toxic)

• Prevents water leaking out towards

concentrated urine

• Allows greater stretching of epithelium

Transitional epithelium…

Bladder• Lined by transitional epithelium• Wall has three smooth muscle layers (Detrusor

muscle)

Urethra• In males, has three parts

– Prostatic urethra

– Membranous urethra

– Spongy urethra (penile urethra)

• Prostatic urethra is lined by urinary epithelium

• Urethral meatus is lined by stratified squamous epithelium

• Other parts are lined by stratified or pseudostratified columnar epithelium

• External orifice is the narrowest part

UrethraCompare the distance between the anus and the urethral opening in male with that of female. In females the urethra opens into the vestibule of vulva where the vagina is also opened.

Females are more prone to get urinary tract infections (UTI)

Juxtaglomerular apparatus (JGA)

Juxtaglomerular apparatus…

• Has 3 components:

– Juxtaglomerular cells

• Derived from smooth muscle cells of afferent arteriole

– Macula densa

• Modified DCT cells

– Lasis cells

• Modified extraglomerular mesangial cells

Juxtaglomerular cells

• Derived from smooth muscle cells of afferent

arteriole. Cytoplasm contains granules of

enzyme renin. Directly sensitive to BP in the

afferent arteriole

Macula densa

• Modified DCT cells. Closely associated with

juxtaglomerular cells. Has taller cells with

prominent nuclei. Sensitive to [Na+] within

DCT. When [Na+] is low stimulates the release

of renin from juxtaglomerular cells

Lasis cells

• Modified extraglomerular mesangial

cells. Lie between macula densa

and Bowman’s capsule. Produce

the hormone erythropoietin

Blood supply of the kidney

Five segments of the kidney

Apical

Upper

Middle

Inferior

Anterior view

Posterior

Posterior view

They are supplied by five segmental branches of the renal artery.

In the region of the hilum, the artery divides into anterior and posterior branches

The posterior branch supplies the posterior segment of the kidney.

The anterior branch further divides into four branches and supply the remaining 4 segments

Both renal arteries give off a suprarenal branch and a

ureteric branch before they enter the kidneys.

The pattern of

branching of

vessels may vary,

but always five

segmental arteries

are present.

Branches of renal

arteries do not

have collateral

circulations

between them.

Each segmental artery gives several interlobar arteries.

They ascend between the pyramids of the renal medulla.

At the corticomedullary junction, they branch to form the arcuate arteries.

Arcuate arteries give rise to interlobular arteries.

Afferent arterioles of the glomeruli originate from these interlobular arteries.

Efferent arterioles give rise to peritubular

capillaries and vasa recta

Vasa recta are found in association with the

loops of Henle of juxtamedullary nephrones

that extend down into the medulla

The cortical and medullary capillaries drain

via interlobular veins to arcuate vein at the

corticomedullary junction and through

interlobar veins to the renal vein

Afferent

arteriole

Efferent

arteriole Peritubular

Capillaries.