Outline the role of the kidney in body water homeostasis.

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College Answer

The main concept required was that the renal excretion of water is basic to the maintenance
of constant body water conditions. This renal water excretion is controlled by multiple factors
that influence glomerular filtration and tubular reabsorption. Also the kidney has mechanisms
that allow it to eliminate excess water by excreting a dilute urine or to conserve water by
excreting a concentrated urine.
Better answers provided details of the large GFR and the renal tubular handling of water.
Also the creation of the hyperosmolar medullary interstitium by the counter current multiplier
system, the special characteristics of the Loop of Henle that cause solutes to be trapped in the
renal medulla and the resulting delivery of a hypoosmolar tubular fluid to the collecting
ducts. Finally the variation in water permeability of the collecting ducts under the influence
of ADH.
Extra marks were awarded for details on ADH including its origin, secretion, regulation and
mechanism of action and the concept of electrolyte free water clearance.
Some candidates confused aquaporins with vasopressin receptors. Other candidates produced
long and irrelevant descriptions of the renin angiotensin system which gained no extra marks.

Discussion

Water Handling in the Nephron
Segment What happens to water Regulatory mechanisms
Glomerulus
  • Filtered freely in the glomerulus (~180L/day)
  • Rate of filtration is related to glomerular blood flow.

Main mechanism is to influence glomerular blood flow:

  • Tubuloglomerular feedback
  • Renal blood flow autoregulation
  • Sympathetic nervous system
  • Vasoactive substances which affect the afferent and efferent arterioles
Proximal convoluted tubule
  • Reabsorbed through the highly permeable tubule wall
  • Absorption is driven by sodium gradient which is generated by Na/K ATPase
  • Glomerular filtration rate
  • Natriuretic peptides
Descending thin limb
  • Reabsorbed through the highly permeable tubule wall
  • Absorption is driven by the osmotic pull of the increasingly hypertonic medullary interstitium
  • Not under any direct regulatory control
  • Absorption here is iititated by by the countercurrent  multiplier mechanism, and maintained by the coutercurrent exchange mechanism
Ascending thin and thick limbs
  • Diluted by the removal of solutes
  • Natriuretic peptides
Distal convoluted tubule
  • Diluted by the removal of solutes
  • Aldosterone (increases solute removal and therefore tubular fluid dilution)
Connecting tubule and collecting duct
  • Reabsorbed through aquaporin channels
  • Driven mainly by the hypertonic medullary interstitium

Variable:

  • With maximal vasopressin stimulus, maximally concentrated urine can be produced (~1200 mOsm/kg)
  • In absence of vasopressin, maximally dilute urine (50 mOsm/kg)
  • Vasopressin (increases water reabsorption)
  • Secreted in response to
    • Osmotic stimuli (hypoosmolar state)
    • Non-osmotic stimuli (hypotension, sympathetic activation)
  • Aldosterone (increases osmotic gradient for water reabsorption)

References

Schafer, James A. "Renal water reabsorption: a physiologic retrospective in a molecular era." Kidney International 66 (2004): S20-S27.

McDonald, Keith M., et al. "Hormonal control of renal water excretion." Kidney international 10.1 (1976): 38-45.

Lassiter, William E., Carl W. Gottschalk, and Margaret Mylle. "Micropuncture study of net transtubular movement of water and urea in nondiuretic mammalian kidney." American Journal of Physiology-Legacy Content 200.6 (1961): 1139-1147.

Boone, Michelle, and Peter MT Deen. "Physiology and pathophysiology of the vasopressin-regulated renal water reabsorption." Pflügers Archiv-European Journal of Physiology 456.6 (2008): 1005-1024.

Sansom, Steven C., et al. "Water absorption in the proximal tubule: effect of bicarbonate, chloride gradient, and organic solutes.Proceedings of the Society for Experimental Biology and Medicine 172.1 (1983): 111-117.

Berry, C. A. "Water permeability and pathways in the proximal tubule." American Journal of Physiology-Renal Physiology 245.3 (1983): F279-F294.

Corman, B., N. Roinel, and Ch de Rouffignac. "Dependence of water movement on sodium transport in kidney proximal tubule: a microperfusion study substituting lithium for sodium." The Journal of membrane biology 62.1 (1981): 105-111.

Bishop, J. H. V. "Osmolality of Renal Cortical Tissue Fluid in Hydropaenic and Diuretic Rats." Kidney and Blood Pressure Research 1.5 (1978): 263-267.

Aukland, K. N. U. T., RONALD T. Bogusky, and EUGENE M. Renkin. "Renal cortical interstitium and fluid absorption by peritubular capillaries." American Journal of Physiology-Renal Physiology 266.2 (1994): F175-F184.

Scott, Jonathan H., Mohammed A. Menouar, and Roberta J. Dunn. "Physiology, aldosterone." (2017).