Regulation of Body Fluid Osmolarity (Rao) Flashcards
How is thirst centrally regulated?
hypothalamus lateral preoptic nucleus contains osmoreceptors that induce THIRST when high osmolarity is sensed (via cell shrinkage)
The kidney has a (high/low) capacity to excrete water and a (high/low) capacity to conserve water.
high
low
What is the formula for plasma osmolarity?
=2[Na] + [glu]/18 + [urea}/2.8
How much water can the body conserve in a day?
1L
How is ADH release regulated?
- in hypothalamus supraoptic and paraventricular nuclei, osmoreceptors sense increased osmolarity (cell shrinkage)
- signal travels to nerve endings located in posterior pituitary
- depolarization increases intracellular Ca concentration and ADH vesicles fuse/release hormone
What does AVP/ADH do?
vasoconstriction
increases water permeability in CT
How is the ADH signal turned “off”?
rapidly degraded in PT and liver
How does binding of ADH affect the nephron cells? (What part of the nephron?)
- ADH binds to basolateral V1/V2 receptor
- initiates a cascade, which generates cAMP
- cAMP causes aquaporin2-containing vesicles to fuse with lumenal membrane of the collecting duct epithelial cell
* this process allows the cell to be permeable or impermeable
When plasma osmolarity is high, levels of ADH are (high/low) and the nephrons (concentrate/dilute) urine.
high
concentrate
When plasma osmolarity is low, levels of ADH are (high/low) and the nephrons (concentrate/dilute) urine.
low
dilute
How do the release of ADH and the thirst response relate to increasing plasma osmolality?
ADH is released in small amounts under nml conditions
At 270mOsm, [ADH] increases as osmolality increases
Above 280 mOsm, up to 18pM of ADH is released
Thirst occurs at a higher plasma osmolality than ADH release
How does blood volume affect ADH?
an increase in ECF volume will increase venous filling and DECREASE [ADH]
A 10-15% decrease in ECF vol results in ADH secretion (i.e diarrhea)
How does GI fluid loss affect ADH?
even though blood becomes hypo- or iso-osmolar, which should decrease ADH, the DECREASE IN PV induces ADH secretion
*overrides the osmolality system
Which requires a less significant decrease in order to induce ADH secretion: ECF volume or blood osm?
blood osm (requires a 10-15% decrease in ECFV, but only a slight decrease in blood osm)
What is the appropriate treatment for hyponatremia due to GI fluid loss?
isotonic saline (avoid a quick change in Na levels)
Explain how a patient with severe vomiting becomes hyponatremic but has normal ECFV?
ADH released in response to the change in ECFV, resulting in concentrated urine.
Water conservation increases ECFV, but Na/etc will still be low
*this can be due to vomiting, heart failure, liver failure
What is nocturia? What causes it?
decreased ability to concentrate urine, resulting in frequent urination at night
causes: age, renal failure, infection, prostate hypertrophy
How do you quantify a patient’s ability to concentrate urine?
“osmolar clearance”
C-osm = (UF X U-osm) / P-osm
(nml = 1.5-2.5 mL/min)
How do you quantify free water clearance, and what is its significance?
difference between osmolar clearance and clearance of water:
C.H2O = [UF - C.osm] C.H2O = UF x (1 - U.osm/P.som)
- if urine osm is higher than plasma FWC>1
- if urine osm is lower than plasma FWC<1
How does the osmolality of the medula interstitium affect the movement of water?
medulla interstitium has high osmolarity, thus water moves out of the tubules and returns to blood
ADH acts on (what segments):
DT
CD -cortical
CD - inner medullary
What tubule segment is HIGHLY permeable to water, regardless of ADH?
thin desc limb
What tubule segment is highly permeable to urea?
CD - inner medullary
What tubule segment has ACTIVE NaCl transport?
thick asc limb (also K*)
The CD actively transports ___ into the ISF.
Na
What steps are involved in creating hyperosmotic medullary interstitium?
**look in notes–I couldn’t sum this up nicely…
What is the function of urea in the tubules?
contributes 40% of osm in medullary ISF-
tubule segments have different permeabilities which affects the hyperosmolarity of medullary ISF
Medullary blood flow is __% of renal blood flow.
1-2%
What 2 factors preserve hyperosmolarity of the medulla ISF?
- low medullary blood flow
2. counter current exchange via vasa recta
What are 3 causes of defective urine concentration or dilution?
- abnormal production or regulation of ADH secretion
- inability of CD to respond to ADH
- failure to form medullary osm gradient
What causes central diabetes insipidus?
inability of the pituitary gland to release ADH
What are possible causes of nephrogenic diabetes insipidus?
CD do not respond to ADH
V2 receptor mutation
Aquaporin-2 mutation
Lithium and tetracyclin
What causes loss of medullary hyperosmolarity?
- Diuretics (furosemide/ethacrylic acid, which inhibit Na transport)
- Excessive delivery of fluid to LOH
- Decreased urea production or filtered load
- Age and renal failure (fewer nephrons)
