Fluid and Electrolyte Homeostasis

Question 1

How does ADH work?

Answer 1

Causes insertion of aquaporins into distal tubule increasing water permeability from tubule into cells -> blood.
Causing more concentrated urine

Question 2

What does isotonic mean?

What does hypertonic mean? What happens if this occurs?

What does hypotonic mean?

Explain what hypertonic saline is and what it’s indications are?

Answer 2

Isotonic = same osmolarity in intracecellular and extracellular fluid (therefore fluid doesn’t move either way)

Hypertonic = higher osmolarity in extracellular fluid than intracellular fluid (so fluid moves out from cells into extracellular space)

Hypotonic = lower osmolarity in the ECF compared to ICF (so fluid in extracellular space moves into cells)

Hypertonic saline is saline with a higher conc. of NaCl than in blood plasma e.g. 3% NaCl compared to 0.9% in isotonic saline. So causes fluid in ICF to move into ECF
Indicated in raised ICP, severe hyponatreamia

Question 3

What are the important electrolytes of the body?

Answer 3

Na+
K+
Ca2+
Cl-
Phosphate
Bicarbonate
Mg2+

Question 4

Concentrations of Na+ and K+ in relation to intracellular and extracellular space?

Answer 4

Na+ = higher extracellularly
K+ = higher intracellularly
Because of Na+/K+ pump which pumps 3 Na+ out and 2 K+ in

Question 5

What is the significance of sodium in physiology?

Answer 5

Na+ accounts for half of the osmolarity of ECF

Na+ involved in conduction of action potentials in neurons

Question 6

What are the 2 main electrolytes in ECF (and therefore urine)? Why?

Answer 6

Na+, Cl-
Na+ because of sodium potassium pump
Cl- because it follows Na+ due to + and -ve attraction. Cl- goes from inside cell to ECF because there is more sodium outside.

Question 7

What are the main electrolytes in the ICF?

Answer 7

Phosphate
Potassium
Magnesium

Question 8

Phosphate is the most common anion in the ICF - why?

Answer 8

Phosphate needed in cell to make ATP
Phosphate needed in cell to make G6P

Question 9

What is the most common electrolyte imbalance in refeeding syndrome? Why?

Answer 9

Hypophosphatemia

Food intake -> insulin release -> mitochondria activity increased -> lots of ATP used up -> low phosphate in cells -> phosphate moves from ECF to ICF -> low phosphate in ECF

Question 10

What electrolyte abnormality do beta agonists cause?

Answer 10

Hypokalaemia

Because Beta agonists increase Na+/K+ pump activity causing more K+ to go in

Question 11

Function of calcium?

Answer 11

Bone and teeth hardness
Blood clotting
Excitability of muscle and nervous system through adjusting cell permeability to Na/K