Kidneys

Overview

Gross anatomy

Location

  • Position:
    • Retroperitoneum
    • Projection: lateral to Th12–L2/3 (the right kidney is about half a vertebra lower than the left kidney because of the position of the liver)
  • Neighboring structures
    • Cranially: adrenal gland
    • Caudally: flexure of the transverse colon
    • Anteriorly
      • Left kidney: stomach, spleen, pancreas tail
      • Right kidney: right hepatic lobe, descending part of the duodenum
    • Posteriorly: major psoas muscle, lumbar quadrate muscle
      • Close proximity to the iliohypogastric nerve and ilioinguinal nerve → renal pathologies (e.g., kidney stones) can cause referred pain in the groin
  • Size: 10–12 cm long, 5–6 cm wide, and 3–4 cm deep
  • Weight: 150–180 g per kidney

Anatomy

Blood supply

  • Arterial blood supply: renal arteries (from the aorta) → segmental arteries → interlobar arteries → arcuate arteries → interlobular arteries → afferent arterioles → glomeruli → efferent arterioles
  • Venous drainage: blood from the efferent arteriolesvasa recta and peritubular capillaries → renal veins (merge into the inferior vena cava)

Every minute, 20% of the cardiac output (∼ 1200 mL in the average adult) is pumped through the kidneys!

The left renal vein passes between the abdominal aorta and the superior mesenteric artery; compression by these arteries can result in varicocele in men and lower abdominal pain in women.

Microscopic anatomy

Nephron

Renal corpuscle

Glomerular filtration barrier

  • Definition: a membrane consisting of 3 layers that allows selective filtration of blood plasma components based on their size and charge
  • Function
Layer Description Clinical relevance Function
Size barrier Charge barrier
Fenestrated glomerular capillary endothelium
  • Capillary endothelium with pores (fenestra)
  • Molecules (> 100–50 nm)
  • Negatively charged glycoproteins (e.g., with heparan sulfate on the BM) prevent entry of negatively charged molecules (e.g., albumin)
Glomerular basement membrane
  • Molecules (> 70 nm)

Podocytes

  • Foot processes that surround the capillaries and leave filtration slits (slit diaphragms)
  • In contact with the endothelium of glomerular capillaries
  • Anionic glycoproteins
  • Molecules (> 50–60 nm)

The glomerular filtration barrier ensures that large and/or negatively charged molecules from the blood are unlikely to filter through and into the Bowman capsule!

Renal tubules

Segments

Location

Microscopy Function
Proximal convoluted tubule Renal cortex
  • Each segment has a distinct epithelial lining and function
    • Receives the ultrafiltrate from the renal corpuscle
    • Transports and concentrates the ultrafiltrate
    • Forms urine via reabsorption, secretion, excretion of substances
    • For details see the learning card “Physiology of the kidney
Loop of Henle Thin descending loop of Henle Renal medulla
  • Smaller diameter than PCT
  • Flat epithelium (resembles capillary endothelium)
Thick ascending loop of Henle
  • Wider diameter than the descending segment
  • Epithelium with variable short brush border and tight junctions (impermeable to water)
Distal convoluted tubule Renal cortex
Connecting tubule and collecting duct Renal cortex and renal medulla

Juxtaglomerular complex

  • Definition: specialized structure that is located between the distal convoluted tubule and the afferent arteriole of the nephron
  • Parts
    • Extramesangial cells
    • Juxtaglomerular cells: modified smooth muscle cells located in the afferent arterioles
      • Function: synthesis of renin
    • Macula densa: composed of tall cuboid cells in the distal convoluted tubule
      • Monitors the NaCl concentration within the lumen of the DCT
        • Hypoosmolar urine triggers the release of reninvasoconstriction of the efferent arteriole → increase in GFR
        • Hyperosmolar urine triggers the release of adenosinevasoconstriction of the afferent arteriole → decrease in GFR

Function

Elevated EPO levels induce an increased hematocrit and improved oxygen-carrying capacity! Athletes may train in high elevation (hypoxia-induced EPO synthesis) or conduct EPO doping to increase their RBC count.

Patients with chronic kidney disease may develop renal anemia due to deficient EPO synthesis.

Embryology

The ureteropelvic junction canalizes last and is the most common site of obstruction.

Impaired renal function during fetal development results in oligohydramnios. The subsequent compression of the fetus causes characteristic abnormalities referred to as Potter sequence.

If the inferior poles of both kidneys fuse during fetal development, they form a horseshoe kidney and become trapped underneath the inferior mesenteric artery.

Clinical significance