Describe the renal handling of water including the modulation of water excretion.

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College Answer

This question required a brief introduction of the role the kidney plays in water balance; a more detailed description of how water is handled as it passes through the various segments of the nephron (glomerulus, PCT, Loop of Henle, DCT and Collecting Duct); the modulation of water excretion by the kidney due to ADH (vasopressin) and how this operates; and the stimuli (osmotic and non-osmotic) for ADH secretion. Although worth mentioning in the context of the effect they have on water movement through the kidney, detailed explanations of Starling's forces in the glomerulus, and of the operation and maintenance of the counter-current mechanism, were not required. More important was describing the control of water reabsorption in the collecting ducts (and thus modulation of water excretion by the kidney) under the influence of ADH.

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).