5 Oedema Flashcards
Describe the microcirculation that takes place in capillary beds
- as well as how circulation is controlled, and determined
Capillaries have no smooth muscle in their walls
- only endothelial cells are the site of the exchange
Regulation of blood flow into the capillary happens before and after the capillary bed
- at pre and post-capillary sphincters
The number of open capillaries is determined by pre-capillary vessels
- small arterioles - surrounded by smooth muscle cells
- which can contract - vasoconstriction
- also by veins (which can contract too)
Explain why blood flow velocity is not uniform in capillaries
It depends on the contractile state of arteriole/pre-capillary vessels
- it can vary from 0-8mm/s (avg is 1)
Explain why Solute/solvent movement across capillaries is not uniform
This depends on permeability which can vary between tissues, within tissues at different times and along the capillary bed
It is determined by
- diffusion; filtration; pinocytosis
- pinocytosis - the movement of large lipid-insoluble molecules in vesicles
Describe the method by which the exchange of substances takes place
Exchange of substances (O2 + CO2) occurs primarily by diffusion down concentration gradients
prime determinants:
- Capillary permeability
- surface area
Lipid soluble substances such as O2 and CO2 readily pass through endothelial cells, but large lipid-insoluble proteins are excluded
o They need another mechanism – pinocytosis
Describe the bulk flow mechanism (filtration/reabsorption)
Bulk flow
- The movement of a volume of protein-free fluid out of the capillary (filtration) and back (reabsorption)
It is important in determining the distribution of Extra-cellular fluid (ECF)
Describe what conditions Filtration is favoured by
Filtration is favoured by:
- Capillary hydrostatic pressure (Pc) - OUT
- Interstitial fluid colloid osmotic pressure (πi)
Describe what conditions Filtration is favoured by
Filtration is favoured by:
- Interstitial fluid hydrostatic pressure (Pc) IN
- Capillary (plasma) colloid osmotic pressure (πc)
Describe capillary hydrostatic pressure (Pc), and what it depends on
It is a major determinant of fluid movement. It depends on:
- Pre/post-capillary resistances
- Venous pressure
- Arterial pressure
If an arteriole constricts:
- Increase in pressure upstream
- Decrease in pressure downstream
Hence, precapillary constriction reduces Pc
If a venule constricts:
- Increase pressure upstream
- Decrease pressure downstream
Hence postcapillary constriction increases Pc
Describe interstitial fluid colloid osmotic pressure (πi), and what it depends on
Normally a minor determinant of fluid movement
- Depends on the presence of protein in the interstitium
- hence capillary permeability to protein is normally very low
Describe Capillary colloid osmotic pressure (πc), and what it depends on
A major determinant of fluid movement. Depends on:
- Synthesis of protein (from the liver)
- Capillary permeability to proteins
- Abnormal protein loss (kidney damage)
Describe Interstitial fluid hydrostatic pressure (Pi), and what it depends on
Normally a minor determinant of fluid movement. It depends on:
- Interstitial fluid volume
- Compliance of the organ
- Effective drainage
Describe how the lymphatic system provides drainage
The lymphatic system provides drainage
- lymphatic vessels are valves
- and highly permeable to proteins
- lymph flow rate around 2.4l/day
- and they return excess filtered fluid + 95% of proteins lost from the vascular system back to the circulation (through the subclavian vein)
Describe how all the 4 determinants of fluid movement across a capillary can come together to impact capillary fluid transfer overall
Fluid movement (Q) is proportional to - filtration forces - reabsorption forces
Q = Kf[Pc + πi) – (πc + Pi)]
- Where Kf is the filtration coefficient - constant that depends on permeability + surface area availability
Since πi and Pi are normally negligible, capillary fluid transfer really depends on:
- Capillary hydrostatic pressure
(increase in Pc leads to increased filtration)
- Plasma colloidal osmotic pressure
(increase in πc leads to increased reabsorption)
Describe the hydrostatic pressure and capillary colloidal osmotic pressure differences at either end of a (normal) capillary bed
and the resulting outcomes
Hydrostatic pressure varies across a capillary
- it is greatest at arterial side
- it decreases towards the mid capillary
- lowest at the venule end
Capillary colloidal osmotic pressure remains stavle
So, at the arterial end, there is higher hydrostatic pressure
- There is NET FILTRATION
AT the venous end, there is higher osmotic pressure
- There is NET REABSORPTION
Describe the hydrostatic pressure and capillary colloidal osmotic pressure differences at either end of a capillary bed, where there is DECREASE in Hydrostatic pressure
and the resulting outcomes
E.g. if there is pre-capillary vasoconstriction
- increase in reabsorption
Reason:
- there is an increase in pressure upstream, decrease in pressure downstream
- so there is less filtration, and more time spent in net reabsorption
