Regulation of Extracellular Fluid Volume Flashcards
Maintainance of body fluid volume is closely tied to regulation body ______ content.
Sodium
The Na-K-ATPase pump maintains a high sodium concentration _____ the cell and a high potassium concentration _______ the cell.
High sodium outside the cell
High potassium inside the cell
What happens to ECF volume when extracellular Na+ increases?
Increased ECF volume
Na excretion = ?
Na excreted = (GFR * [Na]plasma) - Na reabsorbed
(GFR * [Na]plasma) = Na filtered
Describe a positive Na imbalance.
Na excreted < Na ingested
Describe a negative Na imbalance.
Na excreted > Na ingested
What two sensors control GFR and Na balance?
Extrarenal baroreceptors
Juxtaglomerular apparatus
What are the two extrinsic systems that control GFR?
Renal sympathetic nerves
Renin-angiotensin system
What is the effect of increasing renal sympathetic nerve activation? (5)
Increased renal arteriolar constriction
Decreased GFR
Increased filtration fraction
Increased reabsorption
Increased plasma volume
What is the effect of increased renin release?
Increased plasma AngII
Decreased GFR
Decreased RBF
Tubular reabsorption is regulated by? (5)
Aldosterone
Renal sympathetic nerves
Angiotensin II
ANP
ADH
How does aldosterone act on the kidney? What cellular changes occur?
Binds to intracellular receptors
Increases number of luminal-membrane sodium channels as well as basolateral Na-K-ATPase pumps in the cortical collecting duct (principal cells)
Increases ATP synthesis
What are the three major inputs to the adrenal gland that stimulate aldosterone secretion?
ACTH
Increased plasma potassium
Angiotensin II
Does hemorrhage inhibit or stimulate aldosterone secretion? Does this lead to an increase or decrease in Na reabsorption?
Stimulates aldosterone secretion
Increases Na reabsorption
How do renal sympathetic nerves stimulate renin secretion? What direct effect does this have?
Direct action on β1-receptors of granular cells
Stimulates Na reabsorption in proximal tubules
How do renal sympathetic nerves affect filtration fraction and peritubular capillary Starling forces?
Constriction of arterioles decreases RBF
Decreased PPTC
Increase πPTC
Increases Na reabsorption
What is the direct effect of angiotensin II on proximal tubule cells?
Stimulate Na reabsorption by stimulating Na-H exchanger
What effects does angiotensin II have on filtration fraction and peritubular capillary Starling forces?
Increases filtration fraction
Increases Na and water reabsorption in proximal tubule
What effect does angiotensin II have on aldosterone secretion?
Increases aldosterone secretion.
Describe atrial natriuretic peptide (ANP). Where is it made? Function?
A peptide hormone secreted by cardiac atrial cells; released during distention of the atria
Inhibits sodium reabsorption by increasing cGMP (inhibits Na channels)
*evolutionary remnant; cannot overcome effects of Angiotensin II*
Water excreted = ?
Water excreted = GFR - Volume of water reabsorbed
What is the main determinant of water excretion? What is it secreted by?
ADH levels
Baroreceptors and/or osmoreceptors
What is the major function of antidiuretic hormone (ADH)? How does this affect water excretion?
Increases permeability of collecting ducts to water
Decreases water excretion by increasing the number of aquaporin channels
How do baroreceptors control ADH secretion?
When blood plasma volume decreases, less firing of baroreceptors occurs, which stimulates ADH secretion by the posterior pituitary gland
How do osmoreceptors control ADH secretion?
They sense osmolarity and stimulate secretion of ADH accordingly
Why is a thirst response necessary when the kidneys are able to reabsorb a majority of the 180 L/day they secrete?
Reabsorption of water alone cannot restore total body water to normal levels, which can only be achieved by increasing water intake.
Where are the centers that mediate thirst located?
Hypothalamus
What effects does angiotensin II have on the thirst response?
Stimulates thirst
Stimulates ADH release to increase water reabsorption
What is euvolemia?
Normal ECF volume
Describe Glomerulotubular balance (GTB).
The ability of each successive segment of the proximal tubule to reabsorb a constant fraction of glomerular filtrate and solutes delivered to it
How much Na initially filtered actually makes it to the distal tubule?
10%
90% reabsorbed
Na reabsorption in the distal tubule and collecting duct is regulated so that amount excreted in urine _____.
closely matches the amount ingested in the diet
What five mechanisms maintain constant Na delivery to the distal tubule (during euvolemia)?
Autoregulation
GTB
Sympathetic stimulation
Angiotensin II
Local metabolites (prostaglandins, NO, dopamine)
What happens to renal function in patients with congestive heart failure? (4)
Decreased renal perfusion pressures
Decreased GFR
Increased renin-angiotensin-aldosterone system
Increased sympathetic stimulation
What is hyponatremia?
Low blood sodium
< 135 mEq/L
< 275 mOsm/L
Describe syndrome of inappropriate ADH secretion (SIADH). What is it caused by?
Hyponatremia caused by increased ADH secretion
What is hypernatremia?
High blood sodium
> 145 mEq/L
> 295 mOsm/L
Describe central diabetes insipidus. Cause? Effect?
Decreased ADH secretion from the posterior pituitary
Results in massive outputs of hypoosmotic urine and hypernatremia
Describe nephrogenic diabetes insipidus. Cause? Effect?
Normal ADH, but tubules do not respond to it
Results in hypoosmotic urinary output and hypernatremia
