Kidneys

Summary

The kidneys are paired retroperitoneal organs located on either side of the vertebral column extending between the 12th thoracic and the 3rd lumbar vertebral levels. They originate from the mesoderm. The kidneys play an important role in the excretion of waste products, the regulation of extracellular fluid volume and osmolality, the maintenance of acid-base balance, hormone synthesis (erythropoietin), and gluconeogenesis. They receive blood supply from the paired renal arteries and drain into the paired renal veins. The lymphatics drain into the para-aortic lymph nodes. The left renal vein passes between the abdominal aorta and the superior mesenteric artery. Compression of the left renal vein by the aorta and the superior mesenteric artery (nutcracker phenomenon) may cause varicocele in men and pelvic congestion or varicose veins in the lower abdomen with subsequent abdominal pain or heaviness in women. The kidneys are composed of the renal capsule, renal cortex, renal medulla, renal sinus, and renal hilum. The nephron is the functional unit of the kidney, and it is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of the glomerulus and the Bowman capsule, which are separated by the glomerular filtration barrier (GFB). The GFB, which is composed of the fenestrated glomerular capillary endothelium, the glomerular basement membrane, and the podocyte layer, is responsible for filtering blood plasma. The filtrate passes through the renal tubule, which is divided into the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct. In the renal tubule, urine is formed from the filtrate via reabsorption, secretion, and excretion of substances.

Gross anatomy

Location

  • Position:
  • Neighboring structures
  • Size: 10–12 cm long, 5–6 cm wide, and 3–4 cm deep
  • Weight: 150–180 g per kidney

Anatomy

Blood supply

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
  • 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

  • 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 article “Physiology of the kidney
Loop of Henle Thin descending loop of Henle Renal medulla
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

  • Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences; 2016.
  • Feher JJ. Quantitative Human Physiology. Academic Press; 2017.
  • Johns EJ, Kopp UC, DiBona GF. Neural Control of Renal Function. url: https://www.ncbi.nlm.nih.gov/books/NBK57113/ Accessed February 28, 2019.
  • Kurklinsky AK, Rooke TW. Nutcracker phenomenon and nutcracker syndrome. Mayo Clinic proceedings. 2010; 85(6): pp. 552–9. doi: 10.4065/mcp.2009.0586.
  • Vaziri ND. Mechanism of erythropoietin-induced hypertension. Am J Kidney Dis. 1999; 33(5): pp. 821–8. doi: 10.1016/s0272-6386(99)70413-0.
last updated 11/23/2020
{{uncollapseSections(['W21PgT0', 'p8cLnV0', 'I8cYLV0', 'q8cCnV0', 'r8cfLV0', 'J8csnV0'])}}