8. Fluid and electrolyte balance

Question 1

Major divisions of fluid compartments?

Answer 1

Intracellular

Extracellular: Plasma, interstitial, synovial, intra-ocular, CSF

Question 2

Barriers to fluid movement between the compartments?

Answer 2

Barriers:
 Between plasma and interstitial fluid: capillary wall
 Between extracellular fluid and intracellular fluid: plasma membrane

Question 3

Composition of the compartments?

Answer 3

Na OUTSIDE of cell
Cl- OUTSIDE of cell

K+ INSIDE the cell

Remember, there are also differences between ISF and plasma: protein (oncotic pressure)

Question 4

Gains and losses of fluid?

Answer 4

Gains: Food and water intake, oxidation of food

Losses: 
Urine (av 1500ml)
Faeces (av 100ml)
Sweat (av 50ml)
Insensible losses (av 900ml)
Total = 2250ml

Question 5

What is insensible water loss?

Answer 5

Transepidermal diffusion: Water that passes through the skin and is lost by evaportation
Evaporative loss from resp tracts

Insensible losses are solute free

Question 6

What are the 4 routes of insensible loss?

Answer 6

Resp
GIT
Urinary
Skin

Question 7

Insensible losses from respiratory system, how?

Answer 7

Already moisture rich
Need to ensure that gas put on is humidified so ensure not on DRY gas
This dries out resp tract more

Question 8

Insensible losses from GI system, how?

Answer 8

In disease..
Ion rick loss (Na, Cl, HCO3)
Diarrhoea

In treatment intervention…
“Bowel prep” = industrial laxative prior to colonoscopy

Question 9

Insensible losses from urinary system, how?

Answer 9

Sugar is osmotic active particle
If diabetes poorly controlled, more urinary losses
Diuretics = urinary losses

Question 10

Insensible losses from skin, how?

Answer 10

Long operation if bowel exposure, leads to increase in losses
Pyrexia leads to increase in insensible losses

Question 11

Which compensatory mechanisms are linked to volume change?

Answer 11

If low = Low GFR, stimulation of JGA

If high= Increased GFR, release of ANP

Question 12

Key driver of total volume is…

Answer 12

total NA

Question 13

If Na intake is high or low, is controlled by…

Answer 13

Only controllable route of Na loss is via urine, which is under hormonal control

Non-hormone controlled routes:

• Exercise/heat = sweating
• Diarrhoea causing increased loss via faeces

Question 14

What are the options of IV fluids (crystalloids)?

Answer 14

5% dextrose (glucose)

• Initially distributes through ISF and plasma, the glucose is metabolised so effectively adding just water
• Further distributes into cells as well as ISF and plasma

0. 18% NaCl 4% dextrose
   - Maintainance fluid
1. 9% NaCl (isotonic saline)
   - Ion rich

Plasmalyte

• Electrolyte rich
• Distributes through ISF and plasma
• Does not enter cells
• Better resus fluids

Question 15

What are the options of IV fluids (colloids)?

Answer 15

4. 5% albumin
   - Supplied in 0.9% NaCl
   - Tends to stay in plasma, does not enter cells
   - Blood product

Hydrolysed gelatin

• Supplied in 0.9% NaCl
• Initially tends to stay in plasma
• Does not enter cells
• Protein metabolised over time so then equivalent to 0.9% NaCl

Blood
-Stays in the vasculature and increased blood volume

Question 16

questions to ask before prescribing fluid?

Answer 16

1. What is my patients starting volume?
2. Does my patient need IV fluids?
3. Am i prescribing…
   - Maintenance fluid?0.18% NaCl 4% dextrose
   - Replacement fluid? 0.9% NaCl (isotonic saline, so no fluid shift)
   - Resuscitation fluid? Na containing crystalloids
4. What volume and what type?

Question 17

What is a fluid challenge?

Answer 17

Fluid resus to replace pre-existing deficits is done in stat boluses. This is called giving a fluid challenge
To do this, give a bolus (e.g. 500ml 0.9% saline stat) and then reassess hydration status changes
A suitabl

Question 18

Net sodium excretion =

Answer 18

Net sodium excretion = [Na+ filtered] – [Na+ reabsorbed]

Question 19

Where is the area of control in kidney?

Answer 19

In the nephron DCT

No Na+ detection, controlled indirectly via volume sensors

Question 20

Gains and losses of K+

Answer 20

Gain: food/drink

Losses:
Urine
Sweat
Faeces

Question 21

Control of K+

Answer 21

At the kidney
Reabsorbed at PCT + controlled secretion at the DCT.
Na+-K+ pump for secretion

Question 22

98% of K+ is ____ cells

Answer 22

98% of K+ is inside cells

Question 23

How does increase in plasma K+ lead to Conn’s syndrome

Answer 23

Increase K+ leads to increase in K+ entering the tubular fluid of nephron.

• -> ALDOSTERONE SECRETION –>
• Increase Na-K pump activity at basolateral surface
• More potassion secreted and sodium reabsorption at apical surface
• -> CONN’s SYNDROME
  i. e. hyperaldosteronism leading to:
  1. Hypertension from increased fluid volume
  2. Hypokalaemia