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 … Back Hilum of right Kidney is just below transpyloric plane Hilum of left Kidney is just above transpyloric plane Slight movement with respiration

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 artery Renal vein 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 Glomerulus Renal corpuscle Bowman’s capsule Nephron Proximal convoluted tubule Renal tubule Loop of Henle Distal convoluted tubule Collecting tubule

Nephron…cont.

Location of nephrons Cortical nephrons Juxtamedullary nephrons

Renal corpuscle Urinary pole Vascular pole (Bowman’s space) of the visceral layer Vascular pole

Mesangium Glomerulus Bowman’s capsule Supporting connective tissue Coiled network of anastomosing capillaries Bowman’s capsule Single layer of flattened cells resting on a basement membrane

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

Glomerulus – high power view E – capillary endothelial cell M – mesangial cell MS – mesangeal substance BM – glomerular basement membrane P - podocyte

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…

Calyces and renal pelvis 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

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

Efferent arteriole Peritubular Capillaries. Afferent arteriole

Renal Veins The left renal vein receives blood from The left suprarenal vein The left gonadal vein The inferior phrenic vein The right renal vein collects blood only from its own kidney.

Renal artery Segmental arteries Renal vein Interlobar arteries Interlobar veins Arcuate arteries Arcuate veins Interlobular arteries Interlobular veins Afferent arterioles Peritubular capillaries Glomerulus Vasa recta Efferent arterioles