2. Regulation of Body Fluid Osmolality – Regulation of Water Balance Flashcards
Is plasma ADH high or low in patients with central diabetes insipidus?
ADH is low – the posterior pituitary cannot produce it.
How does aldosterone increase sodium retention and potassium excretion?
It increases the presentation of ENaC channels in the collecting duct. This increases the reabsorption of sodium, and causes a concomitant ejection of potassium.
What is a medullary washout?
The event in which increased blood flow through the vasa recta dissipates the medullary gradient by literally washing out the solute.
What would diabetes insipidus do to plasma sodium levels?
Diabetes insipidus increases plasma sodium levels
(Diabetes insipidus = hypernatremia)
What two general mechanisms can be the cause of symptoms in nephrogenic diabetes insipidus?
Either a direct failure of the distal and collecting tubules to response to ADH,
or a failure of the countercurrent multiplier to establish a hyperosmotic medullary interstitium.
What is the osmolality of the tubular fluid entering the descending limb of the loop of Henle?
Isotonic
What four things (discussed along with polyuria) can increase the output of solutes in the collecting duct?
Diuretics, diabetes mellitus, hyperthyroidism, and hyperparathyroidism.
What is normal urine output?
1 to 2 L/day
What are the general functions of the intercalated cells?
Reabsorption of potassium.
Secretion of hydrogen.
What is obligatory urine volume?
How do we calculate it?
Obligatory urine volume is the minimum amount of urine that would have to be produced to secrete the minimal amount of solutes that must be excreted.
Take the minimum amount of solute (about 600 mOsm per day) and divide by 1200 mOsm per liter, the maximum concentration of urine.
(.5 L/day for a normal 70 kg human)
Interstitial fluid in the medulla is always at osmotic equilibrium with what portion of the nephron?
The descending limb of the loop of Henle.
(The medullary interstitial fluid and the descending loop of Henle will always have the same osmolality as they descend further into the medulla.)
What will the osmolality of the intratubular fluid of the distal tubule be in the presence of ADH?
Maximum of 300 mOsm / kg H20
(recall that the interstitial fluid of the cortex is isotonic)
In a patient with hypernatremia due to diabetes insipidus, what would we expect the osmolality of their urine to be?
Diluted (<300/<100)
Even though hypernatremia should increase the ability of the body to reabsorb water from the collecting ducts, the inability of the collecting ducts to respond to ADH makes this irrelevant.
What urea transporters are responsible for urea exit from the inner medullary collecting duct?
UT-A1 / UT-A3
What ADH related disease is a major cause of low sodium levels?
SIADH
Where does almost all of the urea leave the nephron?
The inner medullary collecting duct.