Hydrogen, Potassium, Calcium and Magnesium

Question 1

What are symptoms of hyperkalaemia

Answer 1

• High serum potassium leads to smaller potassium gradient
• Decreases membrane excitability as some Na channels already closed
• Risk of cardiac arrhythmias

Question 2

What are symptoms of hypokalaemia

Answer 2

• Low serum potassium leads to bigger potassium gradient between intracellular and extracellular compartment
  
  • Depolarization leads to increased excitability - risk of arrhythmias
  • U wave present on ECG
• Weakness
• Polyuria - low potassium causes ADH resistance
• Constipation - smooth muscle dysfunction

Question 3

Explain how vomiting leads to metabolic alkalosis

Answer 3

• Vomiting results in loss of HCl with the stomach contents
  
  • Loss of H+
• Severe vomiting also leads to loss of K and Na
• Kidney compensates by retaining sodium in the collecting ducts at the expense of H ions, leading to metabolic alkalosis

Question 4

State some causes of hypokalaemia

Answer 4

• Reduced dietary intake
• Increased entry into cells
  
  • Metabolic alkalosis
  • Increased ß-adrenergic activity - noradrenaline with stress
• Increased GI loss
  
  • Vomiting/diarrhoea
• Increased urine loss
  
  • Hyperaldosteronism
  • Increased urine flow
  • Diuretics

Question 5

Describe potassium movement in the proximal convoluted tubule

Answer 5

• 65-70% absorbed
• Early in the proximal tubule, potassium reabsorption mainly through solvent drag (transported with water)
• Later on in the proximal tubule, positive charge within lumen due to loss of Na causes paracellular diffusion

Question 6

Describe movement of potassium in the thick ascending limb

Answer 6

• 20% absorbed
• ROMK helps potassium secretion which is then reabsorbed through NaKCC
• Net movement of potassium reabsorption in interstitium

Question 7

Describe movement of potassium in the DCT

Answer 7

• Secretion of potassium dependent on amount of reabsorption of sodium
• Potassium secreted through ROMK

Question 8

Describe potassium movement in collecting duct

Answer 8

• Secretion
  
  • ENaC cause sodium to be reabsorbed, causing potassium to be secreted
    
    • BK channel are flow channels - good flow = more potassium lost
• Reabsorption
  - Hydrogen exits the cell, causing potassium to be reabsorbed in the opposite direction

Question 9

Describe the factors that affect potassium entry into cells

Answer 9

• Na/K ATPase activation moves potassium into cells

- Activity influenced by K concentration in plasma, insulin and noradrenaline effect on ß2 adrenoceptors

Question 10

Describe the factors that affect potassium exit from cells

Answer 10

• Potassium channels move potassium out of cells
• Decrease in ICF potassium through high osmolality, acidosis, cell damage
• Increase in ICF potassium during alkalosis

Question 11

What factors increase potassium secretion

Answer 11

1. High intracellular potassium
2. High electronegative lumen (increased sodium reabsorption)
3. Increase permeability of luminal membrane
4. Decrease in luminal potassium
   - Aldosterone upregulates the first 3 points
   - High flow increases potassium secretion due to 2,4

Question 12

Explain the aldosterone paradox

Answer 12

• The ability of the kidney to stimulate NaCl retention with minimal K secretion under conditions of volume depletion and maximize K secretion without Na retention in hyperkalaemia
• In volume depletion, RAAS activated
  
  • Angiotensin increases NaCl reabsorption in DCT and also inhibit ROMK channel (facilitates Na reabsorption and decreases K secretion)
• After eating a lot of potassium, blood volume normal so only aldosterone effect occurs without RAAS
  - Allows K secretion without having to hanging on to Na

Question 13

What is the risk of high calcium concentrations

Answer 13

• Problem with calcium is that it is difficult to keep in solution
• Crystallization would occur without inhibitors (eg. Magnesium)
• High calcium in urine increases risk of kidney stones

Question 14

In what form is calcium present as in the blood and which are filtered

Answer 14

• Only unbound calcium can be filtered by kidney - 55%
• Rest is bound to proteins such as albumin
• Only ionized form filtered
• 2% is excreted in urine

Question 15

Describe how calcium is reabsorbed in the kidney

Answer 15

• Calcium is dependent on sodium through paracellular reabsorption in PCT and thick ascending limb
• Transcellular reabsorption in DCT - immediately buffered when it enters cell (calmodulin)
• Collecting duct - calcium not normally absorbed in collecting duct
  
  • Will upregulate aquaporin and H-ATPase
    
    • Increase in acid formation reduces chance of stones formation

Question 16

What are symptoms of hypocalcaemia

Answer 16

• Pins and needles at finger tips/lips
• Tetany
• Reduced myocardial contractility

Question 17

What is the role of PTH in the kidney

Answer 17

• PTH increased in hypocalcaemia to cause more mobilisation from bone and decreased renal excretion of Ca
  
  • PTH also stimulates formation of calcitriol, which is then released into the circulation
• Calcitriol synthesis increases to help increase mobilisation of calcium, increased gut absorption and increased reabsorption from kidney

Question 18

What is the effect of PTH on phosphate

Answer 18

• PTH inhibits the reabsorption of phosphate, therefore decreasing plasma phosphate concentrations
• However PTH also stimulates production of calcitriol, which increases phosphate release from bones and increases reabsorption of phosphate in the gut
• Overall is slight decrease in serum phosphate levels