Topic 5: Interstitial fluid (ISF), Transport through the capillary wall, Measurement and composition of ISF

Question 1

What should be mentioned in this topic?

Answer 1

• important factors determining the formation of ISF
• Diffusion
• Electroneutrality
• Thermodynamic Rule
• GibbsDonnan balance
• Measurement of ISF
• Composition of ISF

Question 2

Two important factors determining the formation of ISF are:

Answer 2

• Transport through the capillary wall

- Forces determining transport

Question 3

Transport through the capillary wall:

Answer 3

• Water, electrolites and anelectrolites with small molecular weight can permeate the
  capillary wall without restriction.
• Protein movement is restricted, only a csmall amount of it can get through to the ISF
  mainly by pinocytosis and exocytosis.
• Protein conc of the ISF is high due to proteins being large enough that they cannot
  return to the capillary.
• Capillary wall is a considerable barrier for colloids. They are transported with the
  help of specific carrier systems
• Special capillaries such as sinusoids in the liver are permeable for proteins.

Question 4

Forces determining transport:

Answer 4

Diffusion, osmotic conditions, electric forces and hydrostatic forces

Question 5

Diffusion:

Answer 5

Diffusion is the most important force to ensure transport of substances.

Question 6

If movement of some component is restricted between 2 compartments:

Answer 6

If movement of some component is restricted between two compartments, the concentration of the diffusible ions will be different in the two compartments after balance develops.
Principles

Question 7

Electroneutrality

Answer 7

Total amount of anions and cations have to be equal in certain compartments
Electroneutrality prevails.

Question 8

Thermodynamic Rule

Answer 8

Product of the concentrations of diffusible ions must be equal to both sides of the
membrane.

Question 9

Gibbs Donnan balance:

Answer 9

According to the rules of GibbsDonnan balance the diffusable ion concentrations in the intravasal compartment and the interstitium may hardly differ.This ratio is much smaller than the one between the intracellular compartment and the interstitium. This is mainly because there is a much smaller difference between the protein concentration in the plasma and the ISF than between the protein concentration of the IC compartment and that of the ISF. The second reason for it is the fact that the permeability of the cell membrane for electrolites is regulated.

Question 10

Measurement of ISF:

Answer 10

ISF = EC – Intravasal volume

Question 11

Composition of ISF:

Answer 11

Na+ 135 mmol/L
K+ 4 mmol/L
Mg+ 1.5 mmol/L
Ca2+ 1.25 mmol/L

Cl-              114 mmol/L
HCO3-        28 mmol/L
Protein:
 - filtrated:  0.6 g/l
 - finally:     20-30 g/l