Urinary tract

Kidney

• Paired bean-shaped organs that regulate fluid balance and filter wastes from the body

Embryology

• • Three separate excretory organs appear in development, which succeed each other in time and space, such that only the last is retained
    • Pronephros
      • Present only as clusters of cells at the cranial part of the urogenital ridge
      • Only the duct persists, which becomes known as the mesonephric (Wolffian) duct, which opens caudally into the cloaca
    • Mesonephros
      • A ridge becomes visible, the mesonephric ridge or mesonephros
      • Mesonephric tubules, the first tubules, develop individually here and open into the mesonephric duct
      • Some of these ducts develop into genital structures such as epididymis
    • Metanephros (the true precursor of the kidney)
      • Forms caudal to the mesonephros as a mass of new tubules
      • This mass induces a bud, the ureter, to grow from the caudal end of the mesonephric duct
      • Ureteric bud then separates from the mesonephric duct, leaving it to form part of the bladder and ductus deferens
      • Metanephros continues to develop in the pelvis and is supplied by internal iliac artery. It gradually migrates cranially, acquiring new blood supply from aorta.
      • At first, the hilum is anterior, but the kidney rotates 90 degrees medially

Structure

• • Lies in the paravertebral gutter between T12 and L3
  • Right kidney is lower due to the liver
  • Hilum
    • Contains renal vessels, renal pelvis and a variable amount of fat (usual order of vein, artery, ureter from front to back)
    • In the right kidney it lies just below the trans-pyloric plane, and in the left it lies just above
  • Renal pelvis
    • Funnel-shaped commencement of the ureter
    • Usually the most posterior of the three main structures
    • Receive two or three major calyces
    • Lined by transitional epithelium (like the ureter)
  • Coverings
    • Covered by a strong capsule - gives glistening appearance
    • Perinephric fat lies outside the renal capsule - plays a part in retaining the kidney in position
    • Renal fascia surrounds the perinephric fat and separates the kidney from the adrenal gland - firmly attached at the hilum to the renal vessels and ureter (also stabilises the kidney)
  • Internal structure
    • Cortex - beneath the capsule, extends towards the pelvis as the renal columns
    • Medulla - lies between the columns as the renal pyramids (darker and triangular striated areas)
    • The apices of several pyramids open together into a renal papilla, each of which projects into a minor calyx
    • The minor calcyes unite to form two or three major calyces, which opens into the renal pelvis

Relations

• • Posteriorly
    • Diaphragm
    • Quadratus lumborum muscles, with overlap medially onto psoas and laterally onto transversus abdominis
    • Costodiaphragmatic recess of the pleura
    • Subcostal vein, artery and nerve
    • Iliohypogastric and ilioinguinal nerves - as they emerge from lateral border of psoas
  • Anteriorly
    • Peritoneum of the posterior abdominal wall
    • Right - D2, hepatic flexure, ascending branch of right colic artery, hepatorenal pouch of the greater sac
    • Left - tail of pancreas, splenic flexure, upper left colic artery, peritoneum of the lesser sac

Arterial supply

• • Renal arteries (from aorta, L2)
  • Five segmental branches to each kidney
    • Anterior division - apical, upper, middle and lower segments
    • Posterior division - posterior segment
  • Aberrant renal arteries - segmental vessels from aorta to lower pole

Venous drainage

• • Renal veins lie in front of the renal arteries, and behind the pancreas, and join the IVC at right angles, at level of L2
  • Each emerges from the hilum as five or six tributaries which soon unite
  • Left:
    • 7.5cm long
    • Crosses in front of aorta
    • Receives left suprarenal vein and gonadal vein
    • Connected with left ascending lumbar, and hence connected to azygos system
  • Right
    • 2.5cm long
    • Drains only its own kidney, except for sometimes the right testicular vein

Lymphatics

• • Para-aortic nodes
  • Surface of upper pole may drain through the diaphragm into nodes in the posterior mediastinum

Innervation

• • Sympathetic preganglionic (T12-L1) - through the coeliac, renal and superior hypogastric plexuses
  • Afferent fibres (pain) accompany the sympathetic nerves and radiate to the back
  • Some PNS supply from the vagus of uncertain function

Variant anatomy

• • Persistence of fetal arteries (30%) - especially a vessel to the lower pole from aorta
  • Horseshoe kidney - fusion of the lower poles (1 in 800). Ascent of kidney limited by IMA.
  • Renal agenesis (1 in 500)

Functions of the kidney:

• • Eliminate water-soluble waste products of metabolism
  • Eliminate water-soluble drugs
  • Fluid and electrolyte homeostasis
  • Acid-base balance
  • Blood pressure control (renin-angiotensin system)
  • Endocrine function - erythropoietin and vitamin D production

Ureter

• Paired fibromuscular tubes that convey urine from the kidneys to the bladder

Embryology

• • Mesodermal origin
  • Derived by a process of budding from the caudal end of the mesonephric duct

Structure

• • 25cm long
  • Narrow points (most common sites of calculus obstruction): pelviureteric junction, where it crosses the pelvic brim, and as it passes through the bladder wall
  • Whitish, non-pulsatile cord that vermiculates when gently pinches by forceps

Sections

• • Abdominal - from renal pelvis to pelvic brim
  • Pelvic - from pelvic brim to bladder
  • Intravesical - within bladder wall

Course

• • Retroperitoneal, along surface of psoas major, in front of tips of transverse processes
  • Crosses IN FRONT of genitofemoral nerve
  • Crosses BEHIND the gonadal vessels, to lie medial to them
  • Right: upper part is protected behind D3. Lower down crossed anteriorly by right colic and ileocolic vessels and root of mesentery. Protect it during RHC, even if it's not visualised, by ensuring gonadal vessels lie posteriorly.
  • Left: lateral to IMA and IMV, crossed anteriorly by left colic vessels, and by the apex of the sigmoid mesocolon at the pelvic brim.
  • On both sides, leaves the psoas muscle at the bifurcation of the common iliac artery, crossing anteriorly, and passes into the pelvis anterior to SIJ. Course anteriorly to internal iliac vessels down the lateral pelvic sidewall.
  • At the level of the ischial spine, turn forwards and medially to enter the posterolateral wall of the bladder, where it runs an oblique 1-2cm course, before opening into the bladder at the internal ureteric orifice.
    • In men, they are posterior to the vas.
    • In women, the ureters are immediately posterior to the ovaries, then cross in the base of the broad ligaments, where they pass behind and then under the uterine arteries.
    • Ureters tend to lie anterior to dissection around the rectum.
  • Injuries most commonly occur near the lower pole of the kidney on the left, or in the pelvis if anatomy is difficult and distorted

Relations

• • Abdominal
    • Posterior - psoas, genitofemoral nerve, tips of L2-L5 TPs, common iliac vessels
    • Anterior left - gonadal vessels, left colic vessels, sigmoid mesentery and colon
    • Anterior right - descending duodenum, gonadal vessels, root of mesentery, right colic, ileocolic and superior mesenteric vessels
  • Pelvic
    • Posterior - SIJ, IIA
    • Anterior - vas deferens/uterine artery
    • Inferior - seminal vesicle/lateral fornix of vagina

Surface markings

• • On anterior abdo wall: from tip of 9th costal cartilage to the bifurcation of the common iliac artery
  • Radiographic course: medial to tips of the transverse processes of the lumbar vertebrae; crosses pelvic brim at SIJ; then passes to ischial spine; then to the pubic tubercle.

Blood supply

• • Upper end: ureteric branch of renal artery
  • Middle: branches from abdominal aorta, gonadal, common iliac and internal iliac arteries
  • Lower: branches from inferior and superior vesical and uterine arteries
  • All blood vessels anastomose with each other in ureteric adventitia.

Lymphatic drainage

• • Abdominal portion - para-aortic nodes below the renal arteries
  • Pelvic portion - nodes on the sidewall of the pelvis, alongside the internal iliac arteries

Innervation

• • SNS - coeliac and hypogastric plexuses
  • PNS - pelvis splanchnic nerves

Variant anatomy

• • Duplex ureter
  • Bifid ureter
  • Ureterocoele

Microscopic anatomy

• • Lined by transitional epithelium