B5.040 Renal Physiology III: Regulation of Extracellular Fluid Volume and Osmolarity Flashcards
(56 cards)
what is regulated to maintain ECF volume
Na+ intake and excretion
what is regulated to maintain ECF osmolarity
water intake and excretion
water balance in the body
2.4 L/day intake (2 L ingested, 0.4 L from metabolism)
loss of 2.4 L/day (1.5 L from kidney, 0.4 L from skin and lungs, and 0.1 L from intestine)
what controls water uptake
mechanism of thirst
what controls water elimination
reabsorption in the kidney
discuss the mechanisms involved when there is an increase in extracellular fluid osmolarity
rise in osmolarity > stimulated thirst center and osmoreceptors > water ingestion stimulated by thirst center > ADH production stimulated by osmoreceptors > ADH stimulates reabsorption of water from collecting duct > urine osmolarity increases and volume decreases
what portions of the nephron are the key players in water reabsorption
proximal tubule- automatic fashion
distal/collecting tubule- regulated by ADH
other name for ADH
AVP
arginine vasopressin
where is ADH synthesized
hypothalamus
discuss the relationship between plasma oncotic concentration and ADH release
linear relationship, a small variation in osmolality can cause immediate changes in ADH
basal ADH = 2ish
max effective concentration of ADH on kidneys = 4ish
pressure/volume changes effects on ADH release
decreases in pressure and volume both cause release of ADH
ADH is more sensitive to osmotic changes than to pressure/volume changes, however
general function of ADH
establishes a high water permeability at the distal and predominantly collecting tubules
causes water to be rapidly reabsorbed
urine volume diminishes and osmolarity increases
relationship between plasma ADH levels, urine osmolality and flow rate
low ADH = max flow, min osmolality
high ADH = min flow, max osmolality
mechanism of ADH in renal tubule
binds to V2 receptor on BM
activates cAMP and thus PKA
phosphorylation of aquaporin 2 molecules cause them to move to the membrane
once in membrane, aquaporins facilitate reabsorption of water from lumen
mechanism of ADH in vasculature
binds to V1 receptor on smooth muscle cell
stimulates Ca2+ entry into cell and release from sarcoplasmic reticulum in cell
increased Ca2+ conc increased actin-myosin coupling
vasoconstriction is triggered
discuss the countercurrent exchange mechanism of the loop of henle
descending loop: permeable to salt and water, water flows out and salt flows in to equilibrate with the medulla, generating an extremely hypertonic solution at the bottom of the loop
ascending loop: permeable to salt but NOT water, salt pumped out against gradient by NKCC yielding an extremely hypotonic solution at the top of the loop
discuss the effect of the countercurrent mechanism on the distal and collecting tubules
receive hypotonic solution from loop of henle
in presence of ADH, transport excess fluid out of tubule into ISF down the conc gradient into the salty medulla
descending loop of henle
concentrating segment
ascending loop of henle
diluting segment
discuss the vasa recta as a part of the countercurrent exchange mechanism
very slow rate of blood flow in a hairpin formation
allows blood to flow through cortex without disturbing osmotic gradient
both limbs of vasa recta permeable to salt and waterr
recycling of urea as a part of the countercurrent exchanger
urea concentrated in medullary ISF and in the tubular fluid
inner medullary collecting duct is permeable to urea and there is a gradient for passive diffusion of urea into medullary ISF
urea then diffuses from ISF into vasa recta
as vasa recta ascends, urea diffuses back into the ISF and then back into the descending capillary
primary engine of the countercurrent mechanism
reabsorption of salt without water by the thick ascending limb of the loop of henle
where are salt and urea trapped and why
medulla
countercurrent flow arrangement traps them while minimizing the water that enters
establishes a gradient to be capitalized on by the collecting tubule when ADH is present
factors affecting the kidney’s ability to concentrate the urine
- length of loops of henle (modifies vertical osmotic gradient)
- ADH levels that control water permeability in the collecting tubule
- blood flow rate through the medullary capillaries (when increased, more salt is reabsorbed from the interstitium and countercurrent effect decreases)
- factors that influence concentration of urea, higher urea in medulla favors countercurrent mechanism
- drugs and diuretics that affect salt reabsorption in the loop of henle inhibit countercurrent effect
