oedema

Question 1

capillaries

Answer 1

have no smooth muscle in their walls- only endothelial cells are sites of exchange. the number of open capillaries is determined by pre-capillary vessels.

Question 2

blood flow velocity in capillaries is not uniform

Answer 2

• depends on contractile state of arterioles/pre-capillary vessels
• can vary from 0 to ~ 8mm/sec (average 1mm/sec)

Question 3

solute/solvent movement across capillaries is not uniform

Answer 3

• depends on permeability which can vary between tissues, within tissues at different times and along the capillary bed
  determined by diffusion; filtration ; pinocytosis
  (vesicles – movement of large lipid-insoluble molecules)

Question 4

diffusion

Answer 4

• exchange of substances (O2 & CO2 etc.) occurs primarily by diffusion down concentration gradients
  capillary permeability of substance and surface area are prime determinants.
  Fick’s Law J = -PS (Co-Ci)
  J – quantity moved per unit time
  P – capillary permeability to the substance
  S – capillary surface area
  C – concentration outside (o) and inside (i)
  lipid soluble substances such as O2 & CO2 readily pass through endothelial cells but large lipid-insoluble proteins are excluded
  They need another mechanism - pinocytosis

Question 5

filtration/ reabsorption (Bulk flow)

Answer 5

• movement of a volume of protein-free fluid out of the capillary (filtration) and back (reabsorption)
• important in determining distribution of ECF

0.06ml/100g/min = entire plasma volume/day

Question 6

capillary hydrostatic pressure (Pc)

Answer 6

major determinant of fluid movement.
Depends on: pre/post capillary resistances, venous and arterial pressure.
if an arteriole constricts- increase pressure upstream, decrease pressure downstream therefore precapillary constriction reduces Pc.
if a venule constricts: increase pressure upstream, decrease pressure downstream- increase pressure upstream, decrease pressure downstream, therefore postcapillary constriction increases Pc

Question 7

filtration is favoured by:

Answer 7

capillary hydrostatic pressure (Pc)

interstitial fluid colloid osmotic pressure (i)

Question 8

reabsorption is favoured by:

Answer 8

capillary (plasma) colloid osmotic pressure (c)

interstitial fluid hydrostatic pressure (Pi)

Question 9

interstitial fluid colloid osmotic pressure ( i)

Answer 9

normally minor determinant of fluid movement.

depends on the presence of protein in interstsitium hence capillary permeability to protein- normally very low

Question 10

Capillary colloid osmotic pressure ( c)

Answer 10

major determinant of fluid movement. depends upon:

• synthesis/breakdown of protein (liver)
• capillary permeability to protein
• abnormal protein loss (kidney damage)

Question 11

interstitial fluid hydrostatic pressure (Pi)

Answer 11

normally minor determinant of fluid movement.
depends upon: interstitial fluid volume, compliance of organ, effective drainage.
Lymphatic system provides drainage
Lymphatic vessels are valved & highly permeable to protein
Lymph flow rate ~ 2-4 litres/day - returns excess filtered fluid and 95% of protein lost from vascular system back to the circulation (subclavian vein)

Question 12

Since  i and Pi are normally negligible, capillary fluid transfer depends upon
:

Answer 12

1. capillary hydrostatic pressure
   increase in Pc leads to increased filtration
2. plasma colloid osmotic pressure
   increase in  c leads to increased reabsorption