1 - Control of Sodium and Water Excretion Flashcards
Objectives: Explain the role of glomerular-capillary pressure, the plasma protein concentration, and the glomerular filtration coefficient in control of GFR
- Equation: GFR = Kf x (PC - PB - πC)
- Physiological Regulation of Glomerular-Capillary Pressure
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Physiological Changes in Plasma Protein Concentration
- Sweating / Diarrhea - Reduce Plasma Volume
- Increased Sodium Intake - Increase Plasma Volume
- Hemorrhage
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Physiological Control of Glomerular FIltration Coefficienct (Kf)
- Decrease volume causes increase Ang II, decrease in GFR
Objectives: Explain the role of the glomerulotubular balance, aldosterone, and other factors in the control of sodium tubular reabsorption
Glomerulotubular Balance
Aldosterone
RIHP
- Glomerulotubular Balance
- Change in GFR = change in reabsorption of sodium
- Intrarenal mechanism
- Aldosterone (most important)
- Produced in adrenal cortex
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Stimulates sodium reabsorption in principal cells in the cortical collecting duct
- Fine tuning control, most sodium already absorbed
- Stimulates sodium transport by sweat and salivary glands and by the intestine
- Increases mRNA synthesis for translation of sodium channes / NaK-ATPase Pumps
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Peritubular-capillary Starline Factors and role of Renal Interstitial Hydrostatic Pressure
- Increase RIHP = Decrease Sodium/Water Reabsorption
- Deacrease RIHP = Increase Sodium.Water Reabsorption
- Diarrhea
Objectives: Explain the role of ADH and its regulation in the control of water secretion
- Decreased Plasma Volume stimulates ADH secretion
- Increase permeability of cortical and medullary collecting ducts to water
- Increase sodium reabsorption (water will follow), increase plasma volume
How can one calculate excreted sodium?
What does this equation tell us?
- Equation:
- Na Excreted = Na Filtered - Na Reabsorbed
- = (PNa x GFR) - Na+ Reabsorbed
- These variable are open for control to regulate sodium excretion
- PNa - Not important for control; stays relatively constant
- GFR / Sodium Reabsorption:
- Reflexes Initiated by Baroreceptors
- Same as constant CV pressure
How are body-sodium levels and blood pressure related?
- Direct: Low sodium levels cause low pressures
- Via baroreceptors, this initiates reflexes
-
Reflexes:
- Decreased GFR
- Increased Sodium Reabsorption
- Net: Sodium Retention
- (Opposite starting point will have opposite results, e.g. High Sodium will drive Sodium Excretion)
How does sweating / diarrhea change plasma protein concentration?
- Fluid loss = reduced plasma volume
- Little protein is lost in fluid, thus:
- Plasma Protein Concentration Increases -> Oncotic Pressure Increases
- Net: Reduced Filtration Pressure, Reduced GFR
How does high sodium intake change plasma protein concentration?
- Increase plasma volume
- Decrease plasma protein concentration
- Decrease Oncotic Pressure, Increase GFR
How is oncotic pressure related to GFR?
Inverse, increase oncotic pressure decreases GFR
How does hemorrhage change plasma protein concentration?
How is this different that normal Oncotic Pressure v GFR relationships?
- No immediate change in plasma protein concentration; movement of interstitial fluid into the vascular compartment
- Decrease in plasma protein concentration, decrease in oncotic pressure (this normall causes decrease in GFR)
-
Reduced Arterial Pressure and increased sympathetic outflow = constriction of arterioles
- Decrease Glomerular capillary pressure > Oncotic Pressure fall
- Net: Decrease GFR
Explain physiological control of the Glomerular Filtration Coefficient (Kf)
- Decrease in Plasma Volume stimulates increase of Angiotensin II
- Angiotensin II stimulates constriction of mesangial cells
- Decrease Kf
- Decrease GFR
How do the renal sympathetic nerves control tubular sodium reabsorption?
Impact on Renin?
Vasoconstriction or Vasodilation?
GFR/RBF?
- Sympathetics stimulate renin secretion
- Act on B1-Receptors of Granular Cells
- Stimulates sodium reabsorption
- Act on tubular cells (proximal tubule)
- Stimulates afferent/efferent arteriolar constriction (a-adrenergic receptors)
- Decrease GFR, RBF
- Increase Sodium Reabsorption
- Decrease fluid to macula densa, increase reein secretion
How does Angiotensin II increase sodium reabsorption?
- Indirect:
- Increase Aldosterone Secretion
- Decrease RIHP
- Direct:
- Acts on Tubular Cells
What is pressure natriuresis?
- When renal arterial pressure increases, kidneys show rapid increase in sodium and water excretion
- Small change in GFR
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- Small change in GFR
What is the role of Atrial Natruiretic Peptide (ANP)?
What drives its release?
How does it act on sodium excretion?
- Increased Plasma Volume results in distension of cardiac atria
- Increase ANP Secretion
- Direct:
- Decreases Sodium Reabsorption (inhibits sodium channels)
- Indirect:
- Decrease Renin secretion
- Decrease Angiotensin-induced aldosterone secretion
- Afferent Dilation/Efferent Constriction
- Mesangial Cell dilation
- Net: Sodium Excretion (water will follow to lower pressure)
How do baroreceptors control blood volume?
How do osmoreceptors factor in?
- Sense increase or decrease in volume
- Decrease stimulates ADH secretion
- Collecting duct permeability increases, water reabsorbed (decreased excretion)
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Osmoreceptors:
- High Osmolarity ( high [Solute] ) - Increase ADH
-
Low Osmolarity ( low [Solute] ) - Decrease ADH
- Ex: High water intake; decrease ADH and increase excretion
