Chapter 29: Renal Regulation of K+, Ca++, Na+/Fluid Volume (Discussion 3)

Question 1

Location of K+ in the body

Answer 1

Primarily in cells (4.2 mEq/L ECF vs. 140 mEq/L ICF) High ECF [K+] can cause cardiac arrhythmias/death

Question 2

Hypokalemia

Answer 2

Low plasma [K+]

Question 3

Hyperkalemia

Answer 3

High plasma [K+]

Question 4

Factors that shift K+ into cell

Answer 4

Aldosterone (excess = Conn’s syndrome) Insulin B-adrenergic stimulation Alkalosis

Question 5

Factors that shift K+ out of cell

Answer 5

Deficient Aldosterone (Addison’s disease) Diabetes mellitus B-adrenergic blockade Acidosis (H+ inhibits Na-K pump) Cell lysis Strenuous exercise ↑ECF osmolarity

Question 6

Excretion/Renal Regulation of K+

Answer 6

65% reabsorbed in proximal tubule

25-30% reabsorbed in loop of Henle (relatively constant; Na/2Cl/K pump action in thick ascending limb)

Reabsorb (minimally) by intercalated cells (H/K ATPase)

Secrete by principal cells (diffusion)

Question 7

Factors that control excretion/renal regulation of K+

Answer 7

1. ECF [K+]: Increase in ECF concentration will:
   
   1. Stimulate Na-K pump
   2. Reduce back-leakage
   3. Stimulate aldosterone
2. Aldosterone: Increase in aldosterone will:
   
   1. Stimulate Na-K pump
   2. Increase in the # of luminal K+ channels
3. Tubular flow rate: increase in flow rate
   
   1. “Washdown” of K+ –> Increase in diffusion

Question 8

Location of Ca++ in the body

Answer 8

• Higher concentrations in ECF but much lower quantities than Na+
• Most ECF Calcium bound to plasma proteins/non-ionized –> 2.4 mEq/L active Ca++
  
  • Binding affected by pH (acidosis –> decreased binding)
  • Only free Ca++ filtered into nephron (~50%)
    
    • 99% reabsorbed (~65% at proximal tubule)
• ~99% stored in bone (acts as Ca reservoir)

Question 9

Hypocalcemia

Answer 9

Decrease [Ca++] ECF

Cause nerve/muscle hyperexcitability –> tetany

Question 10

Hypercalcemia

Answer 10

Increase [Ca++] ECF

Depress neuromuscular excitability –> arrhythmias

Question 11

Parathyroid Hormone (PTH)

Answer 11

Stimulated by low ECF [Ca⁺⁺] or high [PO₄^3-]

Regulate [Ca++]

• Increase bone resorption (release Ca++)
• Increase reabsorption of Ca++ in kidneys (acts at thick ascending loop/distal tubule)
• Activate vitamin D –> increase intestinal reabsorption of Ca++

Regulate PO₄^3-

• Increase bone resorption (release PO₄^3-)
• Decrease PO₄^3- reabsorption in renal tubules

Question 12

Na+/Volume Regulation

Answer 12

Major regulator is pressure natriuresis & diuresis (more powerful in chronic hypertension)

1. slight increase in GFR
2. High peritubular capillary hydrostatic pressure –> decrease reabsorption of Na/H2O
3. High arterial pressure –> decrease angiotensin II/aldosterone formation –> decrease reabsorption of Na/H2O

Question 13

Sympathetic Nervous Control of Excretion

Answer 13

Baroreceptors sense low BP –> sympathetic stimulation

–> constrict renal arterioles –> decrease GFR –> increase reabsorption of water/salts –> increase renin/angiotensin II/aldosterone –> increase tubular reabsorption

Question 14

Angiotensin II

Answer 14

Increase NaCl/H2O reabsorption

Amplify pressure natri/diuresis (thus important for controlling NaCl/H2O reabsorption)

ACE inhibitors/angiotensin II receptor antagonists block its effects and shift excretion curve to lower pressures

Question 15

Aldosterone

Answer 15

Increase Na+/H2O reabsorption and K+ secretion

During over-secretion of aldosterone, rise in BP will be minimized by pressure natriuresis

Question 16

ADH

Answer 16

Increase water reabsorption

Excessive ADH only slightly increases ECF volume and BP because of pressure diuresis –> significant decrease in [Na+] ECF

Question 17

ANP

Answer 17

Increase GFR and decrease Na+/H2O reabsorption

Question 18

Response to adjust volume/Na+ output

Answer 18

Output lags behind input –> rise in ECF –> response

• Inhibition of pressure receptors –> inhibit sympathetic stimulation
• Pressure natriuresis
• Suppression of Ang II –> decrease water/Na+ reabsorption and aldosterone release
• Stimulate of ANP

Question 19

Conditions that increase ECF volume - Cardiovascular Failure

Answer 19

Increase Na/H2O retention

• Valvular disorders (less effective pumping/pressure)
• Congenital heart defects
• Increased circulatory capacity
  
  • Vascular dilation
  • Pregnancy
  • Varicose veins

Question 20

Conditions that increase ECF volume - Nephrotic Syndrome

Answer 20

Loss of proteins into urine

• Caused by tubule protein failure (could have lost negative charge, membranes became porous, etc)
• Normal blood volume but large ↑ECF volume
• Glomerulus fills large # of proteins into tubules –> ↓ in capillary osmotic pressure
  
  • capillaries filter large amounts of fluid into interstitial spaces –> extracellular edema
• –> ↓blood volume triggers water/Na retention

Question 21

Conditions that increase ECF volume - Liver Cirrhosis

Answer 21

Fibrous tissue in liver w/o protein production capabilities

• –> decrease in plasma proteins
• Fibrotic tissue obstructs portal vein –> increase in P_c (abdominal) –> edema (ascites)