Lecture 29 - Potassium and Hydrogen Homeostasis Flashcards
What is homeostasis?
Homeostasis is defined as maintenance of the internal environment in a constant state despite external changes.
Give 5 examples of homeostasis?
Water Balance
Blood Pressure
Blood pH
Body Temperature
Blood glucose
Which ion is the major intracellular ion?
Potassium (K+)
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 metabolic acidosis
How does alkalosis affect the movement of K+?
K+ is pumped into cells cause hypokalaemia metabolic alkalosis
What are the 2 major areas for K+ control?
- The total body K+
- The distribution between intracellular and extracellular spaces
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
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
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 potential making it harder for cells to fire
What % of K+ is reabsorbed before the filtrate enters the distal convoluted tubule?
95%
What is a major determinant of the resting cell- membrane potential?
Ratio of intracellular to extracellular K+
What 3 things does the resting potential play a vital role in?
- Function of skeletal muscle
- Function of cardiac muscle
- Function of nerve
What does decreased K+ mean for resting potential?
increased resting potential
decreased excitability
What does increased K+ mean for resting potential?
decreased resting potential
increased excitability
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
