Anatomy of Kidneys/Urinary Tract

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The Nephron[edit]

  • A renal tubule plus a glomerulus
  • Human kidney has 1 million nephrons

The Glomerulus:

  • 200 μm diameter, formed by invagination of a tuft of capillaries into Bowman's capsule (the dilated, blind end of the nephron)
  • Capillaries are supplied by an afferent arteriole and drained by the efferent arteriole
  • The diameter of the afferent arteriole is greater than the efferent arteriole
  • Filtrate is formed from the glomerulus, crossing the capillary endothelium of the capillary and the specialized epithelium of the capsule
  • The glomerular endothelium is fenestrated, with pores 70-90nm in diameter, completely surrounded by glomerular basement membrane with specialised cells called podocytes
  • Podocytes have pseudopodia which interdigitate to form filtration slits along the capillary wall, each 25nm wide and closed by a thin membrane
  • The glomerular basement membrane (the basal lamina) does not contain visible gaps or pores
  • Stellate cells called mesangial cells are located between the basal lamina and the endothelium (similar to pericytes found elsewhere in the body).
  • These are common between two adjacent capillaries, where the basement membrane forms a sheath shared by both capillaries
  • The mesangial cells are contractile and play a role in glomerular filtration regulation
  • Mesangial cells secrete ECM, take up immune complexes and are involved in glomerular disease progression
  • The glomerular membrane permits free passage of neutral substances up to 4 nm in diameter, and excludes anything with a diameter greater than 8 nm
  • Charge on molecules and their diameter affects passage into bowman's capsule
  • Total area of glomerular capillary endothelium is about 0.8m2

The Tubule:

  • Proximal Convoluted Tubule:
  • 15mm long, 55μm diameter, made up of a single layer of cells that interdigitate with one another and are united by apical tight junctions
  • Between the cells are extensions of extracellular space the lateral intercellular spaces
  • The luminal edges of the cells have a striated brush border of microvilli
  • Loop of Henle:
  • Descending and proximal ascending limbs made up of thin, permeable cells
  • Thick portion of the ascending limb made up of thick cells with many mitochondria
  • Nephrons with peripheral glomeruli have short loops of Henle (cortical nephrons), while those in the cortex (juxtamedullary nephrons) have long loops extending into medullary pyramids
  • Distal Convoluted Tubule:
  • 5mm long, starting at the macula densa
  • Contains a few microvilli but no distinct brush border
  • Distal tubules coalesce to form collecting ducts - 20mm long, pass through the renal cortex and medulla to empty into the renal pelvis at the apexes of the medullary pyramids
  • Collecting duct epithelium is made up of principal cells (P cells - small, few organelles, involved in Na+ and vasopressin stimulated water reabsorption) and intercalated cells (I cells - more microvilli, vesicles and mitochondria, involved in acid secretion and HCO3- transport)
  • Total nephron length 45-65mm
  • Renal medullary interstitial cells - fibroblast-like cells which contain lipid droplets and synthesise COX-2 and Prostaglandin synthase PGES which produces PGE2 - important in regulation of salt and water homeostasis
  • Prostacyclin PGI2 and other prostaglandins are secreted by the arterioles and glomeruli

Blood Vessels[edit]

  • Afferent Arterioles - short, straight branches of the interlobular arteries
  • Divide into multiple capillary brancehs to form the vessel tuft of the glomerulus
  • These capillaries then coalesce to form the efferent arteriole, which then breaks up into capillaries which supply the tubules (peritubular capillaries)
  • Arterial segments between glomeruli and tubules are therefore a portal system, and the glomerular capillaries are the only capillaries which drain into arterioles
  • Capillaries draining tubules of the cortical nephrons form a peritubular network
  • Efferent arterioles from juxtamedullary glomeruli drain into the peritubular network, but also into vasa recta - vessels that form hairpin loops into the medullary pyramids alongside the loops of Henle
  • Descending vasa recta have non-fenestrated endothelium containing a fenestrated endothelium, allowing solutes to be conserved
  • Each efferent arteriole from each glomerulus breaks up into capillaries supplying a number of different nephrons, therefore supplying different tubules
  • Total surface area of renal capillaries = total surface area of tubules = 12m2
  • Total volume of blood in renal capillaries = 30-40ml
  • Innervation
  • Renal nerves travel with renal blood vessels as they enter the kidney
  • Contain many post-ganglionic sympathetic efferent fibres and a few afferent fibres arising from lower thoracic and upper lumbar segments of the spinal cord with ganglions in the sympathetic ganglion chain, superior mesenteric ganglion and along the renal artery
  • Sympathetic fibres are distributed to afferent and efferent arterioles, proximal and distal tubules, and the JGA
  • Nociceptive afferents mediating pain in kidney disease parallel the sympathetic efferents, entering the spinal cord in the thoracic and upper lumbar dorsal roots
  • Other renal afferents mediate the reno-renal reflex - increase in ureteral pressure in one kidney decreases efferent nerve activity to the other kidney, increasing excretion of Na+ and water
  • Also contains vagus nerve innervation of unknown function

The Capsule

  • Renal capsule is thin but tough
  • Limits swelling in oedematous kidneys causing an increase in renal interstitial pressure. This decreases the GFR and enhances and prolongs anuria in ARF.

The Lower Urinary Tract[edit]

  • The Ureters contain smooth muscle arranged in spiral, longitudinal and circular bundles
  • Regular peristaltic contractions occuring one to five times per minute moving the urine from the renal pelvis
  • The ureters pass obliquely through the bladder wall, and this angulation keeps the ureters closed except during peristaltic waves, preventing urine from refluxing from the bladder
  • The bladder has a similar arrangement of smooth muscle arranged in spiral, longitudinal and circular bundles
  • Contraction of the circular detrusor muscle causes bladder emptying during micturition
  • Muscle bundles pass on either side of the urethra, and are sometimes called the internal urethral sphincter (although these do not encircle the urethra)
  • Further along the urethra is a sphincter of skeletal muscle - the external urethral sphincter
  • The bladder epithelium is made up of a superficial layer of flat cells, and a deep layer of cuboidal cells


  • A spinal reflex, facilitated and inhibited by higher brain centres
  • Urine entering the bladder does not cause much increase in intra-vesical pressure until the viscus is well filled
  • During micturition, perineal and external urethral sphincter are relaxed, the detrusor muscle contracts, and urine passes through the urethra
  • Voluntary initiation of micturition begins with relaxation of pelvic floor muscles, which tug downwards on the detrusor muscle which in turn initiates its contraction
  • During micturition, perineal muscles and external sphincter can be contracted voluntarily, preventing urine from passing down the urethra or interrupting flow once urination begins
  • The bands of smooth muscle of the internal urethral sphincter are not involved - these are for prevention of semen reflux into the bladder during ejaculation