Electrolyte balance Flashcards Preview

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Flashcards in Electrolyte balance Deck (18)
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1

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)

2

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

3

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

4

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)

5

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

6

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.

7

What factors that shift K+ into cells (decrease extracellular K+)?

1) Insulin
2) Aldosterone
3) Beta-adrenergic stimulation
4) Alkalosis

8

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

9

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

10

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+

11

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

12

What is the normal range for extracellular [K+]?

3.5 - 5.3 mmol/L

13

What can cause hypokalaemia?

- Reduced intake
- Excessive losses eg. diuretics, severe diarrhoea, aldosterone excess
- Altered body distribution

14

What are the signs and symptoms of hypokalaemia?

- Often asymptomatic
- Muscle weakness
- Cardiac arrhythmias

15

How do you treat hypokalaemia?

- Address underlying cause
- Potassium supplementation may be needed

16

What can cause hyperkalaemia?

- Excessive intake
- Inadequate losses eg. kidney disease, aldosterone deficiency
- Altered body distribution eg. acidosis

17

What are the signs and symptoms of hyperkalaemia?

- Often asymptomatic
- Cardiac arrhythmias - ECG may show characteristic tall T waves

18

How do you treat hyperkalaemia?

- Address underlying cause
- Restrict intake
- Calcium gluconate (to stabilise myocardium)
- Insulin (along with glucose) to drive potassium into cells
- Aid excretion - fluids, ion-exchange resins, dialysis