- osmolarity: total solute concentration of a solution
- hypoosmotic: total solute concentraiton less than that of normal extracellular fluid (~300mOsm/L)
- isoosmotic: totaly solute concentration equal to that of normal
- hyperosmotic: solute concentration si greater than normal.
2renal regulation of water¶
- water is readily absorbed (~99%)
- majority at proximal tubule
- hormal control (1/3 of absorption) at collecting duct ONLY.
- (remember for sodium, it's at both collecting duct and distal convoluted tubule.
- absorption mechanism : follow sodium.
- movement of sodium increases local osmolarity in the interstitium, so water flows into the interstitium.
- flow of wateR: vi atubular cells' membrane and tight junction.
- when water intake is small, kidney reabsorb more , vice versa.
- regulation at collecting duct is made possible by
- high osmolarity of the medullary interstitium done by counter current multiplier system
- permeability (regulated by vasopressin) of CD to water
3countercurrent multiplier system¶
- mechanism of how kidney concentrate the urine at medullary collecting ducts.
- urine concentration depends on the hyperosmolarity of the interstitual fluid.
- presence of vasopressin promotes water diffusion out of the ducts inot the interstitial fluid.
- medullary interstitual fuid becomes hyperosmotic through the function fo Henle's loop,
- ...which includes proximal tubule, descending limb, ascending limb, distal convoluted tubule.
- countercurrent multiplier system's mechanism:
- isoosmotic fluid enter proximal tubule,
- isomotic fluid enters descending limb, which allows water diffusion but does not reabsorb NaCl
- water flows out of the tubule.
- as fluid goes down the limb, it becomes more and more hyperosmotic because interstitutual fluid becomes more and more hyperosmotic
- it's because the nearby ascending limb actively reabsorb sodium into the interestitual fluid.
- hyperosmotic fluid enters ascending limb, which is impermeable to water but actively reabsorb NaCl.
- Na concentration goes down as the fluid ascends in the limb.
- fluid is isoosmotic when it's at distal convoluted tubule.
- as fluid descends down the collectnig duct, water flows out into the interstitual space
- because of the ascending limb, deeper in the medulla the more hyperosmotic is the interstitual fluid.
- eventually, the urine output is very hyperosmotic because water flows out of the collecting doct.
- btw, in addition to NaCl, urea also contributes to medullay hyperosmolarity.
- vasa recta : blood vessel in the medulla
- specially designed to be hairpin so it doesn't carry solute away from the interstitua fluid and destroy the hyperomolarity.
- same shape as henle loop :hairpin --> structure prevents it from carrying the solutes away.
- while descending limb of the vessel absorb solute
- ascending limb spits the solute back to the medullary
- proximal tubule is highly permeable to water.
- water permeability in CCD and MCD (cortical collecting duct and medullary collecting duct) depends on hormonal control.
- vasoppressin, which inserts water channels (aquaporins) into the epithelium.
- a peptide hormone, aka antidiuretic hormone
- reduce urine output and increase blood pressure
- made by some hypothalamic neurons and released from post. pituitary.
- coupeld to GPCR V1 (smooth muscle) and V2 ( kidney) : basically two different kinds of g-protein coupled receptors.
- stimulates insertion of aquaporins in the luminal membrane of collecting duct.
- promote water reabsorption in collecting duct.
- diabetes insipidus: a condiction in which kidney cannot conserve water.
- malfunction of vasopressin
- loss of water in body leads to high glucose.
- regulation of vasopressin:
- osmoreceptor control (more important)
- sense decrease in osmolarity, decrease vasopressin release, more urine (vice versa)
- baroreceptor control (less sensitive)
- more about Na regulation than water regulation
- sense decrease in blood pressure, increase vasopressin release, less urine output (vice versa)
- osmoreceptor control (more important)
- promoted by :
- decrease in plasma volume (sensed by baroreceptor) --> angiotensin --> decrease in Na excretion, thus less water excreted.
- increase in plasma osmolarity
- dry mouth or throat
- sweating : loss of hypoosmotic salt solution
- inhibited by water intkae by GI tract