4.4. Regulation of Extra-Cellular Volume - Complexities of Real Physiology

Question 1

What will happen to a persons ADH-secreting cells, if they have lost 3 litres of H2O and NaCl due to diarrhoea, but have drank 2 litres of pure H2O?

Answer 1

There will be opposing inputs:

1. Decrease in ECF osmolarity - Causing an inhibiton of ADH secretion (via Osmoreceptors)
2. Decrease in ECF Volume - Causing secretion of ADH (Via Baroreceptors)

Question 2

Which considerations have primacy, Volume or Osmolarity?

Answer 2

Volume - if the extra-cellular volume is compromised, ADH will increase due to Baroreceptors even though this is associated with Hypo-osmolarity

Question 3

What does the Volume having primacy represent?

Answer 3

An emergency mechanism for the Brain

Question 4

What is normally the main determinant for ADH concentration?

Answer 4

Osmolarity

Question 5

What can change the main determinant of ADH concentration?

Answer 5

Sufficient Volume change to compromise brain perfusion - You will tolerate disturbed osmolarity to maintain volume

Question 6

What happens once the Volume is restored?

Answer 6

Osmolarity will be normalized again