crrm2.3: renal structure12/02/08

Learning Outcomes

Describe the major structural features of the kidney

  • The kidneys are retroperineal structures that lie between T12 and L3
  • There are two functional layers within the kidney:
  • The cortex is the granular outer layer containing convoluted tubules andglomeruli
  • The medulla contains parallel tubules within 8 to 18 subdivisions: the renal pyramids
  • A fibrous capsule covers the outermost surface of the kidney and reflects into an opening called the renal sinus
  • The hilum penetrates the capsule into the sinus allowing passage of:
  • Renal artery and nerves
  • Renal vein, lymphatics and ureter
  • The ureter broadens in the sinus to form the renal pelvis and divides to form major and minor calyces
  • The minorcalyces join to thepapillaeof therenal pyramids which are perforated to allow passage of urine
  • Cortical extensions between the medullary pyramids are called columns of Bertin
  • Medullary rays extend into the cortex creating the cortical labyrinth

Explain the importance of the nephron as the functional unit of renal function

  • There are approximately 1 million nephrons in each kidney
  • A nephron comprises a vascularglomerulus and an epithelial tubule
filtration pathway
  • Afferent arterioles supply the glomeruli which form the initial filtrate
  • The glomeruli are encapsulated in Bowman’s capsuleswhich capture the filtrate in the Bowman’s space,contiguous with the tubule lumen
  • The filtrate passes into the proximal convoluted tubule (PCT)which leads to the proximalstraight tubule(PST)
  • The proximal straight tubule narrows and becomes the thin descending loop of Henle(tDLH)
  • The thin ascending loop of Henle (tALH) follows, broadening to the thick ascending loop of Henle(TAL)
  • The thick ascending loop of Henle becomes the distal convoluted tubule (DCT) level with the glomerulus
  • The distal convoluted tubule leads to a short connectingtubule(CNT) which then becomes an initial collecting tubule (ICT)
  • The initial collecting tubules feed into the common cortical collecting tubule (CCT) which eventually leads to the terminal papillary duct
juxtaglomerular apparatus (JGA)
  • The juxtaglomerular apparatus is composed of:
  • Extraglomerular matrix
  • Macula densa
  • Granular cells
  • The macula densa are located along the epithelium of the TAL and mark the beginning of the DCT
  • They have large nuclei and are closely packed
  • They monitor glomerular filtration rate (GFR) and alter the tone of afferent arterioles
  • The granular cells form part of the wall of the afferent arterioles that produce, store and release renin
nephron subtypes
  • Superficial nephronsare the majority of nephrons (80%) and are characterised by:
  • Longproximal straight tubule
  • Short thin loop of Henle that doesn’t extend into the inner medulla
  • Juxtamedullary nephrons are specialised for concentrating urine and are characterised by:
  • Short proximal straight tubule
  • Long thin loop of Henle that penetrates the inner medulla
renal corpuscle
  • The renal corpuscle is made up of the glomerulus and the Bowman’s capsule
  • The filtration barrier between the vascular and tubule lumens comprises:
  • Glomerular capillary endothelium
  • Glomerular basement membrane
  • Podocyte epithelium
  • The glomerular capillary endothelium isheavily fenestrated, filtering only large cellular elements
  • The basement membrane has three layers and restricts intermediate-sized molecules
  • The podocytes have foot processes that interdigitate and cover the basement membrane
  • Filtration slits separate adjacent foot processes and further limit the size of filtered molecules
  • Negative charge of the podocytes favours filtration of positively charged solutes