Potassium and Hydrogen Homeostasis

What is the major intracellular cation?

Potassium

What is the average intracellular concentration of potassium?

160 mmol/L

What is the normal plasma concentration range of potassium?

3.5-5.2 mmol/L

Why does serum have 0.4mmol/L more K+ than plasma?

serum is left to clot, to remove clotting factors, increasing K+

How does acidosis affect the movement of K+?

K+ leaches out into extracellular fluid causing hyperkalaemia

How does alkalosis affect the movement of K+?

K+ is pumped into cells cause hypokalaemia

How does insulin affect the movement of K+?

insulin drives K+ into cells by stimulating the Na+/K+ ATPase pump

Give three examples of catecholamines that affect the movement of K+:

1) adrenaline

2) noradrenaline

3) dopamine

How do catecholamines affect the movement of K+? (3)

1) they trigger beta adrenergic receptors which promote cellular K+ uptake

2) they trigger alpha adrenergic receptors which promote cellular K+ loss

3) this leads to an overall net uptake

Describe how increase tonicity affects the movement of K+:

K+ passively leaches out of cells causing hyperkalaemia

Give 3 examples of when extracellular fluid osmolality would be increased:

1) infusion of hypertonic saline

2) hyperglycaemia in hyperosmolar hyperglycaemic state

3) mannitol

How does hyperkalaemia affect membrane potential?

hyperkalaemia raises the resting potential towards the threshold, making is easier for cells to fire

How does hypokalaemia affect membrane potential?

hypokalaemia lowers the resting potnetial making it harder for cells to fire

What % of K+ is reabsorbed before the filtrate enters the distal convoluted tubule?

95%

What is the effect of systemic acidosis on kidney K+ secretion?

reduced K+ secretion

What is the effect of hyperkalaemia on aldosterone release?

hyperkalaemia triggers aldosterone release

What is the effect of aldosterone on K+ levels?

aldosterone causes Na+ retention for K+ excretion as well as H+ secretion from cells in the collecting duct and distal convoluted tubule so that H+ can be excreted with K+

Give three key outputs of K+:

1) urinary output

2) faecal excretion

3) skin loss

What is the threshold for hyperkalaemia?

above 5.5 mmol/L

Give two common causes of fictitious hyperkalaemia:

1) leaving a blood sample out at room temperature allowing clotting to occur, increasing K+

2) using an EDTA blood sample bottle which already has K+ in it

Give 8 clinical signs of hyperkalaemia:

1) cardiac arrhythmia

2) fatigue

3) vomiting

4) weakness

5) sudden death

6) paraesthesia

7) ECG changes

8) palpitations

Give 4 ECG changes seen in hyperkalaemia:

1) tall T waves

2) prolonged PR intervals

3) widened QRS intervals

4) flattened P waves

Give 8 causes of hyperkalaemia:

1) hypoaldosteronism

2) tissue damage

3) fictitious

4) renal failure

5) K+ sparing diuretics

6) hyperinsulinism

7) acidosis

8) drugs

Give an example of a condition that causes hypoaldosteronism:

Addison's disease

Give an example of a K+ sparing diuretic:

Spironolactone

Give the three steps used in hyperkalaemia treatment:

1) give 10ml of calcium gluconate

2) give insulin or salbutamol to shift K+ into cells

3) stop potassium intake (halting K+ rich drugs and food)

Why is calcium gluconate given to treat hyperkalaemia?

to protect the cardiac membrane

What is hypokalaemia?

Potassium level of <3.5mmol/L

Give 8 clinical signs of hypokalaemia:

1) lethargy

2) respiratory failure

3) ventricular arrythmias

4) decreased tendon reflexes

5) cardiac arrest

6) tubular resistance to ADH

7) weakness and paralysis

8) ECG changes

Give 4 ECG changes seen in hypokalaemia:

1) flattened T waves

2) ST segment depression

3) prolonged QT interval

4) tall U wave

What is does U wave represent on an ECG?

repolarization of the purkinje fibres

Give 4 causes of hypokalaemia:

1) insulin therapy

2) alkalosis

3) diuretic therapy

4) vomiting and diarrhoea

When replenishing low K+, what other mineral is important to consider?

magnesium

Give the two steps used to treat hypokalaemia?

1) place patient with suspected hypokalaemia on a heart monitor

2) direct K+ replacement therapy (oral or IV if severe)