Structure of kidney

[Ref: AV6:Chp1]

Nephrons

1 million nephrons

3 types of nephrones

1. Superficial cortical nephrons
   * Short loop of Henle
2. Midcortical nephrons
3. Juxtamedullary nephrons
   * All have long loops of Henle, extending into the inner medulla
   * Difference in loop lengths are all due to variabilities in thin descending and ascending limbs
   * All ascending limbs change from thin to thick at the junction of inner and outer medulla [AV6:p91]

NB:

• Before distal convoluted tubules, the cells in any given segments are homogenous.
• Beginning from DCT, 2 cell types are found in most of the remaining segments
  * Principle cells
  * Intercalated cells (A and B)
• Later portion of medullary collecting duct contains neither type and has a distinct cell type called inner medullary collecting duct cells.
• Nephrons remain separate until cortical collecting duct, where tubules from several nephrons merge

 

 

Renal parenchyma

1. Cortex
2. Medullary - outer and inner

NB:

• Beginning of the thick ascending limb mark the border of outer and inner medulla.
• i.e. No thick ascending limb in the inner medulla

Juxtaglomerular apparatus

• Granular cells (i.e. JG cells)
  * Secretes renin
  * Located in the media of the afferent arterioles as they enter glomeruli
• Macula densa
  * Monitors tubular content
  * Marks the end of the thick ascending limb and the beginning of distal convoluted tubule.
  * Essentially modified tubular epithelium
• Extraglomerular mesangial cells
  * [WG21:p461] aka lacis cells

Glomerular capsule

• Endothelial cells of renal capillary have large fenestrae
• Epithelial cells - podocytes, with foot processes (pedicels)
• Pedicels interdigitate with other pedicels from adjacent podocytes
• Slit diaphragm covers the space between pedicels
• Glomerular mesangial cells act as phagocytes

 

Blood vessels

• Glomeruli, and interlobar, arcuate, and interlobular arteries, are found only in the cortex.
• Vasa recta are found only in medulla

Vasa recta

• Descending vasa recta has smooth muscles at the beginning
• Ascending vasa recta has fenestrated endothelium like in glomerular capillaries.

Peritubular capillary

• Most of glomerular blood flow continue on in peritubular capillaries after leaving efferent arterioles

 

Innervation

• Rich sympathetic innervation
• No significant parasympathetic innervation

Sympathetic innervation

• Distributed to
  * Afferent arteriole
  * Efferent arteriole
  * JG apparatus
• From sympathetic celiac plexus