Electrolytes 1

Question 1

What composition of the body is water?

Answer 1

60%

Question 2

What is the distribution of body water between ICF and ECF?

Answer 2

ECF = 1/3rd 
ICF = 2/3rds

Question 3

True or false?

‘Cell membranes are permeable to water’

Answer 3

True - cell membranes are permeable to H2O - remember electrolytes dissolved in water cannot freely cross cell membranes

Question 4

What is osmolality?

Answer 4

Number of (mmol) of a substance dissolved in a kg of solution

Question 5

True or false

“Osmolarity’ is identical in all fluid compartments’

Answer 5

True - equilibrium is rapidly re-established with movement of water

Question 6

What is the normal osmolality of sodium in the ECF?

Answer 6

Normal = 150 mmol/L

Question 7

True or false

‘Absolute number of Na+ molecules within an individual determines their ECF volume’

Answer 7

True

Question 8

True of false

‘sodium is usually gained or lost irrespective of water’

Answer 8

FALSE - sodium is rarely gained or lost without water

Question 9

What are the two stimulators of change (in WATER)?

Answer 9

1. Osmolality - directly proportional to [Na+] - quite fragile
2. Low cardiac output - baroreceptors - move independently of osmolality - responds to larger changes

Both result in ADH release + thirst –> increase H20

Question 10

What system is responsible for modifying the absolute amount of sodium?

Answer 10

RAAS system

Question 11

Explain the mechanism of the RAAS system:

Answer 11

1. Low kidney perfusion = Renin released from juxtaglomerular apparatus
2. Hydrolyzes angiotensinogen –> angiotensin I –> angiotensin 2

Function of angiotensin II:

1. Stimulates release of aldosterone from adrenal + increases Na+ re-absorption in proximal tubule = sodium + water
2. Stimulates thirst centre in the hypothalamus + release of ADH from the posterior pituitary gland

Question 12

What is the effect of aldosterone and how does it effect

i. Absolute number of ECF sodium molecules
ii. ECF volume
iii. sodium concentration

Answer 12

Aldosterone - distal nephron to retain sodium (with H2O)

i. Increases absolute number of ECF sodium molecules
ii. Increases ECF
iii. Doesn’t change sodium concentration

Question 13

Explain the mechanism of ANP and it’s effect on the concentration of sodium:

Answer 13

ANP is released from atria when stretched (excessive ECF), blocking sodium (+ water) resorption in the distal nephron

Does not change sodium concentration - just decreases absolute number of ECF sodium molecules + ECF volume

Question 14

What are the mechanisms of urgent handling intracellular movement of potassium?

Answer 14

Insulin + Catecholamines

Question 15

What is the effect of aldosterone on potassium?

Answer 15

Acts on the distal nephron (and gut to excrete potassium) - acting to decrease potassium concentration

Question 16

What is the impact of an aldosterone deficiency?

Answer 16

1. Hyperkalaemia
2. Metabolic acidosis
3. Hyponatremia (free water excess)

Question 17

Provide three examples of a cause of hyponatremia = Excess WATER

Answer 17

1. Pseudohyponatremia - measurement error
2. Decreased “effective circulating volume” - actual isotonic fluid is replaced with pure water - diarrhoea, polyuria, profuse sweating
3. Perceived isotonic loss replaced with pure water - e.g. CHF

Question 18

Provide an example of a cause of hypernatremia (=deficient WATER):

Answer 18

1. Diarrhoea (calves)
2. Profuse panting
3. Diabetes insipidus - central or nephrogenic in nature

Question 19

What are three cause of hypokalaemia?

Answer 19

polyuria, vomiting or diarrhoea

Question 20

What are three examples of intake related causes of hyperkalaemia?

Answer 20

1. Anuria/oliguria - all things that decrease urine output
2. Hypoaldosteronism
3. Accidental overdose

Question 21

What are two examples of translocation related causes of hyperkalaemia?

Answer 21

1. Massive cellular necrosis

2. Inorganic metabolic acidosis