Electrolyte balance Flashcards
How can receptors detect electrolyte levels?
Direct – e.g. extracellular fluid (ECF) potassium concentration [K+] has direct effect on release of aldosterone
Indirect – e.g. baroreceptors indicate ECF volume (marker of sodium levels)
How does the juxtaglomerular apparatus increase electrolyte reabsorption?
The three types of cells in the juxtaglomerular apparatus are:
1) Macula densa cells
2) Extraglomerular mesangial (Lacis) cells
3) Granular (juxtaglomerular) cells of afferent arteriole
JG Granular cells secrete an enzyme RENIN in response to falls in extracellular volume / low sodium
Falls in ECF volume detected by baroreceptors around the body
Aim of response is to increase sodium reabsorption and therefore water reabsorption
What 3 things cause the granular cells of the afferent arteriole to release renin?
Low afferent arteriole pressure
Activation of sympathetic nerves that supply JGA
Low [NaCl] in distal tubule
All of these are markers of a fall in blood pressure / fall in ECF volume and the body responds by aiming to increase sodium and water reabsorption to correct the situation
What does angiotensin II do?
Formed from enzymatic cleavage of angiotensin I by angiotensin converting enzyme (ACE)
Powerful vasoconstrictor
Has direct and indirect actions on kidney to promote sodium and water reabsorption
Direct effect on renal tubule cells to increase sodium reabsorption by increasing activity of sodium transporters
Indirect effects include promotion of thirst, release of aldosterone and antidiuretic hormone (ADH)
What does aldosterone do?
Aldosterone increases sodium reabsorption by its actions on principal cells in late distal / cortical collecting tubule
Secreted by adrenal cortex (zona glomerulosa) in response to either increased angiotensin II or increased extracellular potassium concentration
Binds to intracellular mineralocorticoid receptors (MR)
Binds to nucleus & increases production of proteins, e.g. ENaC, Na+/K+ATPase, that increase ability of these cells to reabsorb sodium
What does Atrial Natriuretic peptide (ANP) do?
In response to increased BP/volume.
This hormone inhibits sodium (and water) reabsorption by increasing excretion by the kidney.
Released by atrial muscle fibres in response to increased stretch of atria (as a result of excessive blood volume).
Causes small increases in GFR (dilates afferent arteriole) and decreases renal reabsorption.
What factors that shift K+ into cells (decrease extracellular K+)?
1) Insulin
2) Aldosterone
3) Beta-adrenergic stimulation
4) Alkalosis
What factors shift K+ out of cells (increase extracellular K+)?
1) Insulin deficiency (diabetes mellitus)
2) Aldosterone deficiency (Addison’s disease)
3) Beta-adrenergic blockade
4) Acidosis
5) Cell lysis
6) Strenuous exercise
7) Increased ECF osmolarity
How does potassium secretion occur?
Na+/K+ ATPase moves K+ into the principal cells creating a high intracellular concentration
K+ then passes through channels in luminal membrane into tubular lumen
What factors determine the RATE of K+ secretion?
1) Activity of Na+/K+ ATPase
2) [K+] gradient between blood, principal cell & lumen
3) Permeability of luminal membrane to K+
What factors REGULATE potassium secretion?
1) Plasma potassium concentration
2) Aldosterone
3) Tubular flow rate
Increasing all these three will increase the rate of potassium secretion
4) H+ concentration
Increasing this will decrease Na+/K+ activity and therefore the rate of potassium secretion
What is the normal range for extracellular [K+]?
3.5 - 5.3 mmol/L
What can cause hypokalaemia?
- Reduced intake
- Excessive losses eg. diuretics, severe diarrhoea, aldosterone excess
- Altered body distribution
What are the signs and symptoms of hypokalaemia?
- Often asymptomatic
- Muscle weakness
- Cardiac arrhythmias
How do you treat hypokalaemia?
- Address underlying cause
- Potassium supplementation may be needed
