Regulation of Water and Osmolality Flashcards
Are salt and water regulation by the kidney dependent or independent of each other
Independent
Sodium regulation determines ___
Water regulation determines ___
Na: ECF volume
WaterL body osmolality
What is the tonicity in the medullary interstitium? (hyper, iso, hypo)
Hypertonic
Allows us to concentrate our urine
Tonicity
Effective plasma osmolality
Refers to the ability of the solution to actually influence the movement of water across the cell membrane
(ECF solute + ICF solute)/Total body water
Normal osmolality of Na
285-290 mosm/kg
Are disorders of plasma osmolality normally due to abnormalities of water or Na?
Water! 99% of the time
Abnormalities in Na handling may co-exist
Body compartment volume disorders are usually due to water or Na?
Na
Insensible losses
Any way of losing fluids that is not the urine
Ex: skin, membranes, lungs, feces
Vasa recta
Major blood vessels that carry blood into and out of the renal medulla
Flow in the opposite direction as urine
Countercurrent exchange
Urine and blood flow in opposite directions
Promotes exchange of solutes and water to preserve medullary interstitial hypertonicity
2 requirements for forming concentrated urine
Hypertonic medullary interstitium
High levels of ADH
4 factors that contribute to the excess buildup of solute in the interstitium
Thick ascending LoH (active transport of Na out)
Collecting ducts (active transport of Na out)
Passive urea diffusion/recycling
Diffusion of only a small amount of water fro medullary tubules into interstitium
Main function of
- Descending LoH
- Ascending LoH
- Concentration of filtrate
2. Dilution of filtrate
Urea
Also helps to generate the hypertonic medullary interstitium
It is passively reabsorbed from the tubule
Recirculation between the CD and the LoH
When ADH is present, more urea is reabsorbed from the inner medullary CDs
2 features of the renal medullary blood flow that prevent the countercurrent multiplier system from dissipating
Medullary blood flow is low
Vasa recta serve as countercurrent exchangers
Plasma osmolarity is regulated by 2 main systems
Osmoreceptor-ADH system
Thirst mechanism
ADH is released in response to which 2 stimuli
Change in plasma osmolarity (detected by osmoreceptors in anterior hypothalamus)
Change in BP or in blood volume (detected by arterial baroreceptors and atrial stretch receptors)
How does an increase in plasma osmolarity cause ADH release?
Increase in the osmolarity causes osmoreceptors to shrink
Initiates an AP that eventually gets to the posterior pituitary
Stimulates an influx of Ca ions causing a release of ADH from secretory granules
Signals from the osmoreceptors also induce the thirst mechanism
2 places where ADH increases water permeability
Late distal tubules
Cortical and inner medullary collecting ducts
How does
1. Decreased BP/volume
2. Increased blood volume
influence ADH release?
- Detected by arterial baroreceptors, stimulate ADH release
2. Detected by cardiopulmonary reflexes (atrial stretch receptors), decrease ADH release, stimulate ANP release
How is water reabsorbed?
In the presence of ADH
Passive diffusion through aquaporin channels down an osmotic gradient
Without ADH the collecting duct is impermeable to water
What is the obligatory urine volume?
0.5L/day
Minimal volume of water needed to excrete ingested and waste produced osmoles
