Lecture 29 - Potassium and Hydrogen Homeostasis Flashcards by Niamh Millar

What is homeostasis?

Homeostasis is defined as maintenance of the internal environment in a constant state despite external changes.

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Give 5 examples of homeostasis?

Water Balance
Blood Pressure
Blood pH
Body Temperature
Blood glucose

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Which ion is the major intracellular ion?

Potassium (K+)

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What is the average intracellular concentration of potassium?

160 mmol/L

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What is the normal plasma concentration range of potassium?

3.5-5.2 mmol/L

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

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

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How does acidosis affect the movement of K+?

K+ leaches out into extracellular fluid causing hyperkalaemia metabolic acidosis

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How does alkalosis affect the movement of K+?

K+ is pumped into cells cause hypokalaemia metabolic alkalosis

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What are the 2 major areas for K+ control?

The total body K+
The distribution between intracellular and extracellular spaces

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What 3 factors influence the balance between the passive loss out of cells and the active Na, K-ATPase pumping K+ in against the concentration gradient?

Acid-base status
Hormones- Insulin,Catecholamines
ECF Osmolality

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How does insulin affect the movement of K+?

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

This is exploited in the treatment of hyperkalaemia

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

1) adrenaline

2) noradrenaline

3) dopamine

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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

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Describe how increase tonicity affects the movement of K+:

K+ passively leaches out of cells causing hyperkalaemia

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

1) infusion of hypertonic saline

2) hyperglycaemia in hyperosmolar hyperglycaemic state

3) mannitol

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How does hyperkalaemia affect membrane potential?

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

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How does hypokalaemia affect membrane potential?

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

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

95%

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What is a major determinant of the resting cell- membrane potential?

Ratio of intracellular to extracellular K+

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What 3 things does the resting potential play a vital role in?

Function of skeletal muscle
Function of cardiac muscle
Function of nerve

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What does decreased K+ mean for resting potential?

increased resting potential
decreased excitability

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What does increased K+ mean for resting potential?

decreased resting potential
increased excitability

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Aside from membrane potential, what 3 other major things does K+ play a role in?

Protein Synthesis
Acid- base balance
Maintenance of intracellular osmolarity

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+ in exchange for Na+

Give three key outputs of K+:

1) urinary output 2) faecal excretion 3) skin loss

What does urine potassium concentration vary between?

5- 150 mmol/L

How are clinical disorders of potassium classified?

based on the serum potassium

What are the main causes of clinically significant changes of K+?

Transcellular shifts rather than total body K loss

How much potassium is excreted through faeces per day?

5-10 mmol/day

How much potassium is excreted through skin loss per day?

5-10 mmol/day

What is the threshold for hyperkalaemia?

above 5.5 mmol/L

What are the stages of hyperkalaemia?

Mild (serum potassium 5.5 - 6.0 mmol/L) Moderate (6.1 - 6.9 mmol/L) Severe (≥7.0 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

Name 3 features of Hyperkalaemia in the neuromuscular system?

Weakness Paraesthesia Paralysis

Name 3 features of Hyperkalaemia in the Cardiovascular system?

Conduction defects Arrhythmias Cardiac arrest

Name 3 features of hyperkalaemia in the GI tract?

Nausea Vomiting Pain Ileus

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)

Name 2 features of Hypokalaemia in the neuromuscular system?

Weakness Paralysis

Name 3 features of Hypokalaemia in the Cardiac system?

Conduction defects Arrhythmias Digoxin toxicity

Name 3 features of Hypokalaemia in the renal system?

Impaired concentrating ability Tubular defects (resistance to ADH) Metabolic alkalosis

If the potassium level is less than 2.5 mmol/L, what should be given?

intravenous potassium

What is difficult to replenish if serum magnesium levels are low?

Serum Potassium