Microcirculation, Venous Blood Flow and Venous Return Flashcards
Is arterial pressure generally fluctuating or constant ? Why ?
Arterial P is generally constant because need to maintain it to ensure we can pump out CO around system in a way that works. if let P get too high, can damage vessels they’re in, too low won’t be able to pump blood effectively so overall within relatively tight range.
Does arterial P depend on local blood flow control ? on CO control ?
No, arterial P is independent of either local blood flow control or CO control, since it needs to be maintained in a tight range.
What are the main components of interstitium ?
Collagen and proteoglycan filaments which trap interstitial fluid
Describe the viscosity of interstitial fluid.
Fluid trapped amongst filaments, forming a tissue gel so only 1% of the water is free.
How quickly does diffusion occur in interstitial fluid ?
Diffusion occurs in gel ~95-99% as rapidly in free fluid
What processes of movement across the capillary membrane occur (between the capillary and interstitial fluid) ?
- Diffusion (depends on concentration gradients)
2. Bulk Flow/Filtration (depends on pressure gradient)
Distinguish between diffusion and bulk flow/filtration.
DIFFUSION
Net movement of nutrients, oxygen and metabolic end products
BULK FLOW/FILTRATION
Distribution of extracellular fluid
Distinguish between crystalloids and colloids.
CRYSTALLOIDS
Low mol. wt. solutes (e.g. Na+, Cl-, K+)
Can leak out of capillaries with fluid movement
COLLOIDS
Larger molecules like plasma proteins (e.g. albumin)
Cannot leak out of capillaries
Define and describe the main features of oncotic pressure.
= “the osmotic (the pressure required to stop osmosis through a semipermeable membrane between a solution and solvent) pressure of a colloid in solution “
Plasma Oncotic pressure of about ~28mmHg generated by plasma proteins (Predominately by albumin, lesser extent by globulins)
Interstitial oncotic pressure is much lower (~5-8mmHg)
Overall, pressure difference means fluid flow towards capillary.
Is capillary wall a perfect barrier to plasma proteins ? Give an example.
No, permeability for albumin is 1/1000th that of water (so some will leak out)
Define and describe the main features of hydrostatic pressure.
= “the pressure exerted by a liquid”
Capillary hydrostatic pressure (~30-40mmHg at arterial end, ~10-15mmHg at venous end)
forces fluid out of the capillaries and in to the interstitium
Interstitial hydrostatic pressure (can generally be assumed to be 0) will usually not contribute any movement of fluid. In come cases may be slightly positive or negative. If positive (e.g. in encapsulated organ), will force fluid to the capillary and draws fluid in if negative (e.g. in head).
IMPORTANT TO NOTE: flow to capillaries fluctuates (not all capillaries being perfused at all times), so changes in hydrostatic pressure will follow changes in capillary perfusion, will will affect fluid flow.
Identify the Starling forces. What does each force push towards ?
- hydrostatic pressure in the capillary (Pc) (tries to push fluid out)
- hydrostatic pressure in the interstitial fluid (Pif) (if positive, tries to push fluid to the capillaries and vice versa)
- Plasma colloid osmotic pressure (πc ) (pushes fluid towards capillary)
- Interstitial fluid colloid osmotic pressure (πif )(pushes fluid towards interstitial fluid)
Graph the pressure of different Starling Forces over time, as well as the net filtration pressure over time.
Refer to slide 10 of lectures on “Microcirculation, Venous Flow and Venous Return”
What is the overall result of all Starling forces ?
The predominant hydrostatic force is the capillary hydrostatic pressure (because Pif is usually 0), and it draws fluid towards the interstitium. However, it decreases at the end as it gets to the venous end.
The predominant oncotic force is the capillary oncotic pressure (πc) which pushes fluid inside the capillary.
Overall, at arterial end, net flow is towards interstitium (dominant force is hydrostatic pressure of capillary). By time get to venous end, net flow is towards capillary (hydrostatic pressure of capillary drops down). Overall, net loss of fluid.
How does the body deal with the net loss of fluid from capillaries ? What happens if this does not occur ?
Fluid accumulated is drained via lymphatic vessels, through lymph nodes, back into veinous circulation.
If this does not occur, accumulation of fluid in interstitial space –> swelling of soft tissue compartment –> oedema