S2: Capillaries - Fluid Movement Flashcards
Importance of fluid exchange
- Fluid exchange is important for normal physiological function
- H2O is needed for chemical reactions
- Abnormalities in fluid exchange can lead to oedema/tissue swelling
- Controlling blood volume, and interstitial and cell volumes are important goals in medicine following drop in blood pressure/poor end organ perfusion e.g. haemorrhage, sepsis, during surgery, dehydration
Explain fluid movement at the capillary wall
What pressure is exerted?
Capillary wall is a semi permeable membrane and it allows H2O to pass through
- Fluid moves across membrane into interstitial space due to blood flow which exerts hydraulic pressure (pressure trying to move fluid out of capillaries)
- Large molecules (e.g. plasma proteins) cannot pass through membrane
These in the capillary exert an osmotic pressure/oncotic pressure which creates a suction force (trying to move fluid from interstitial space into capillary) - Fluid movement depends on balance between Hydraulic and Oncotic pressures across the capillary wall
What are the two types of hydrostatic pressures and osmotic pressures?
Hydrostatic:
- Pc = Capillary blood pressure
- Pi = Intersititial fluid pressure
Easy movement across membrane
Osmotic:
- π p = Plasma Proteins
- πi- Interstitial proteins
Movement through intracellular gaps
What is Starlings Principle of Fluid exchange
Jv = Lp A [(Pc - Pi ) - σ (πp - πi)]
Jv is the net filtration (flux)
Pc - Pi is hydraulic pressure difference
πp - πi is the osmotic pressure difference
Lp = Hydraulic conductance of the endothelium (how leaky it is to fluid)
A = wall area
σ = reflection coefficient - related to intracellular gaps so how much osmotic pressure molecules can exert ( smaller σ , larger gap, higher the JV)
What does starling’s principle tell us about fluid exchange at capillaries?
- There us a pressure drop across capillaries from arterial to venous end
- If Pc is larger than πp there will be filtration
- As capillary pressure drops πp becomes more dominant
- Filtration on the arterial side as hydraulic pressure is higher
- Re absorption of the venous side as hydraulic pressure is less
What is incorrect with starling’s principle?
- Fluid filtration generally occurs throughout the length of capillaries and reabsorption does not occur even though it is important for fluid replacement.
- Interstitial osmotic pressure is not small so sometimes πp= πi
- The glycocalyx is not taken into account
- · Starling’s principle states that increasing πp and reabsorption with colloid fluids should increase blood volume
But, colloid fluids do not significantly expand plasma volume so, we need to revise Starling’s principle
What is the revised starling’s principle of fluid exchange?
Jv = Lp A [(Pc - Pi ) - σ (πp - πg)]
- Includes glycocalyx (lumen side)
- Plasma proteins move from lumen into interstitial space via vesicle system not via intracellular space as glycocalyx acts as a barrier
- Stream of fluid filtration into interstitial carries plasma proteins away from endothelium into πi creating low πg region - now πp equals πi
- πg is plasma proteins in endothelium
- · Osmotic gradient is πp - πg (not πp - πi)
What does revised starling’s principle tell us about fluid exchange at capillaries?
- Filtration occurs across the length of capillaries
- Less Pc at venous end means πi moves into πg – less (πp - πg) gradient promoting filtration at venous end
- This explains why a significant decrease in Pc e.g. haemorrhage will initially produce reabsorption in an attempt to maintain blood volume.
- In sick patient, shedding of glycocalyx explains why colloid fluid do not expand blood volume
What is hypovolemia?
It is a state of decreased blood volume or more specifically, decrease in volume of blood plasma
Describe the relationship between low capillary pressure pc and hypovolemia
Big drop in BP as blood volume is lost e.g. in injury. Also sympathetic medicated vasoconstriction causes larger pressure drop so Pc is reduced
- Plasma protein levels are much higher
- Larger oncotic drive so little filtration and more reabsorption (balance shifts)
- πp will reduce in concentration if blood volume increases and then filtration will occur again
- This small window of reabsorption period is a life saving mechanism (maintains CO, starlings law, perfusion of major organs)
Where does the fluid from constant filtration go to?
The lymphatic circulation
Describe the lymphatic circulation
- Lymphatic circulation returns excess tissue fluid/solutes back to the CVS (same amount of fluid is just shifted to a different part of the body).
- Lymph vessels have valves and smooth muscle
- Spontaneous contractions of the smooth muscle contributes to lymph flow
- Surrounding skeletal muscle contractions/ relaxation also contributes to lymph flow
What controls ECF balance?
· Capillary filtration
· Capillary reabsorption
. Lymphatic system
Changes in:
· Starling’s factors
· Efficiency of lymphatic system
. Influence fluid balance between the intravascular and interstitial spaces
What happens if balance of ECFV is not maintained?
Imbalance between filtration, reabsorption, lymphatic function, glycocalyx function
Excess interstitial fluid – Oedema
Factors from starlings principle promoting filtration and reabsorption
Factors promoting filtration:
- Increase Pc
- Increase πg
- Increase Lp
- Increase A
Factors promoting reabsorption:
- Increase πp
