PHYS - Osmotic and Volume Regulation

Question 1

EFFECT OF VOLUME ON ADH RELEASE

Answer 1

• Decrease in plasma volume
  
  • Decrease P in circulation
    
    • Decrease baroreceptor activity
      
      • Stimulate ADH release
        
        • Increase tubular permeability to H₂O
          
          • Increase H₂O reabsorption

Question 2

VOLUME STIMULUS VS OSMOTIC STIMULUS

Answer 2

• Osmotic stimulus → drink water → decrease P_osm → decrease ADH → excrete H₂O
• Volume stimulus → IV saline → increase EFV → decrease ADH → excrete H₂O → increase P_osm → increase ADH
• Regulate volume by regulating sodium reabsorption

Question 3

CONTROL OF EFV BY SODIUM

Answer 3

• Volume (stretch) receptors:
  
  • Intrarenal – JG cells of afferent arteriole
  • Venous stretch receptors
  • Baroreceptors
• Regulate extracellular volume by regulating sodium reabsorption which then regulates water reabsorption
  
  • The following mechanisms that act on Na+ retention in the kidney
    
    • GFR
    • Aldosterone
    • Natriuretic hormone
    • Peritubular pressure
• Glomerulotubular Balance (GTB) – ensures a constant percent of the filtered load of sodium gets reabsorbed
  
  • Always about 67%, while the amount in the 67% will change based on sodium intake
• Increased Na+ intake → increased P_osm
  
  • First → shift of fluid from IFC compartment to EFC → increased water reabsorption → increased EFV
  • → ADH release → increased EFV
  • → thirst → consume water → increased EFV

Question 4

ALDOSTERONE

Answer 4

• Intercellular receptors in principle cells of the collecting duct
• Release stimulated by:
  
  • Blood pressure/volume → Angiotensin II
  • Hyperkalemia
• Affects protein synthesis
  
  • Increases Na+ channels on apical (tubular) membrane
    
    • Allows Na+ to rush in and depolarize membrane so that K is stimulated to diffuse into tubule
  • Increases Na+/K+ ATPases on basal membrane (blood side)
  • Enhances ATP synthesis
• Functions:
  
  • Na+ reabsorption
  • K+ excretion
  • H+ excretion

Question 5

CS: CHANGES IN ALDOSTERONE LEVEL

Answer 5

• Hypoaldosteronemia (adrenal insufficiency; Addison’s disease)
  
  • Hyperkalemia
  • Acidosis (K+ put into cells)
  • Hypotension (salt wasting)
• Hyperaldosteronemia
  
  • Hypokalemia
  • Alkalosis (K+ withdrawn from cells for pump function)
  • Hypertension (without overt volume expansion)
  • Primary – overstimulation of adrenal gland by a tumor
  • Secondary – caused by RAS

Question 6

HORMONES AFFECTING SODIUM/H2O REABSORPTION

Answer 6

• Natriuresis – excretion of excess sodium in water
• Aldosterone - Na+ reabsorption in CD
• ADH - H2O reabsorption in CD
• Angiotensin II
  
  • Secreted in response to low GFR
  • Stimulates adrenal gland to secrete aldosterone
  • Direct increase in proximal Na+ reabsorption
• ANP (Atrial natriuretic hormone)
  
  • Binds membrane receptors in CD that GC*
  • Prevents Na+ reabsorption
  • Increases GFR in glomerulus (decreased Na+ reabsorption)
  • Diminishes effects of renin, angiotensin II, and aldosterone

Question 7

CONGESTIVE HEART FAILURE

Answer 7

• CHF patients exhibit positive and increasing sodium balance
• Arterial baroreceptors function to maintain ECV (effective circulating volume)
• CHF → decreased CO → shift in blood volume to venous side → decreased P/V on arterial baroreceptors → renin/angiotensin II/aldosterone release → Na+ retention → increases P_osm → increase ADH → H₂O reabsorption
• With volume trapped in venous circulation → increased edema
• Treatment: diuretics to try to prevent edema

Question 8

CHRONIC KIDNEY DISEASE

Answer 8

• Progressive decline in GFR due to loss of nephrons
• However, sodium balance and plasma osmolarity are still maintained!
• Sodium – overall filtered load decreases because of the decreased number of nephrons working to filtered the blood
  
  • So, less plasma filtered per cycle through kidneys, and more sodium drawn out per cycle to maintain constant excretion rate
  • Decreased filtered load
  • Increased fractional excretion
  • No change in excretion rate
• Creatinine – since less plasma is being filtered, creatinine concentration builds up more quickly in plasma creating a steeper gradient for filtration
  
  • More concentrated filtered load
  • No change in excretion rate

Question 9

ERYTHROPOIETIN

Answer 9

• Kidneys function in regulation of RBC production
• Release erythropoietin in response to reduced tissue oxygenation
  
  • 90% from kidney
  • 10% from liver
• Erythropoietin stimulates HSCs in the bone marrow to produce proerythroblasts→ RBCs → increased tissue O₂ → decreased erythropoietin release