Capillaries II Flashcards
What is the importance of fluid exchange?
→ Important for normal physiological function
→ Fluid reabsorption from tissues to blood can maintain circulation during haemorrhage
Where does fluid move in the capillary wall?
→ moves across the membrane into interstitial space due to blood flow which exerts hydraulic pressure.
What kind of pressure do large molecules exert and why?
→ Exert an osmotic pressure termed oncotic pressure
→ because they are too large to pass through the membrane
What does oncotic pressure create?
→ A suction force to move fluid into the capillary
What does fluid movement across capillary walls depend on?
→ Balance between hydraulic and oncotic pressures across the capillary wall.
These forces aren’t in balance so the net movement is toward fluid being moved out.
The interstitial fluid is drained into lymphatic system that drains fluid back into the capillary
What are the four pressures that determine filtration rate?
HYDROSTATIC PRESSURES:
→Pc: capillary blood pressure
→ Pi: interstitial fluid pressure
ONCOTIC PRESSURE:
→ πp: plasma proteins
→ πi: interstitial proteins
What is the fluid flux equation?
→Jv = Lp x A {(Pc - Pi) - σ(πp - πi)}
→Lp is the hydraulic conductance of the endothelium, ie. how leaky the endothelium is to the fluid.
→A is the wall area.
→σ is the reflection coefficient = fraction of osmotic pressure that is exerted
→ JV is the movement of fluid or ‘flux’
What is the equation for effective osmotic pressure?
→ σ x potential osmotic pressure
σ for plasma protein is 0.9
ie. 10% plasma proteins are conducted across capillary wall into interstitial space
What do Starling’s forces normally favour in capillaries?
Starling’s forces favour filtration in capillaries.
What is favored filtration or reabsorption?
→Taking into account factors that affect both filtration and reabsorption
→the balance is tipped into filtration
→However, there are other factors involved, so the balance can change.
What do well perfused capillaries do?
→Well-perfused capillaries will filter along their entire length.
What does the lymphatic circulation do?
→ Returns excess tissue fluid/solutes back to the CVS
How many liters per day are filtered by the lymphatic system?
→ About 8 liters per day are filtered
What do lymph vessels have?
→ Valves to ensure unidirectional flow and smooth muscle
What contributes to lymph flow?
→ Spontaneous contractions of smooth muscle contributes to lymph flow
→ Surrounding skeletal muscle contractions and relaxations also contribute to lymph flow
What does lymph contain?
→ Immune cells
→ at the lymph nodes
What does control of extracellular fluid balance depend on?
→ Capillary filtration
→ Capillary reabsorption
→Lymphatic system
What happens in the capillaries during hypovolemia (decreased blood volume)?
→ A bit of filtration to begin with but then reabsorption
→ Osmotic pressure is now able to overcome the hydraulic pressure
→Caused by sympathetic nerve-induced vasoconstriction of pre-capillary arterioles leading to drop in downstream capillary pressure and hence shift the balance of force for reabsorption.
→ More fluid is absorbed back into circulation
→ Increasing blood volume
What does a drop in CO cause?
→ Drop in BP (BP = CO x TPR)
→ so Pc is reduced
Overall balance no longer favours filtration which is good in this situation to increase blood volume
What is oedema?
→ Excess of fluid within interstitial space
→ Imbalance between filtration, reabsorption + lymph function
What are the causes of oedema?
→ Increased capillary pressure
→ Decreased plasma protein oncotic pressure
→ Inflammatory response- capillaries become leaky and plasma proteins will leave so reduction in oncotic pressure- favouring filtration- oedema
→ Lymphatic problems
What are three situations where you would have Increased Capillary Pressure (Pc) ?
→ Dependent gravitational oedema (standing up for a long time)- increased venous pressure
→ Deep venous thrombosis-prevention of venous return due to occlusion that blocks a deep vein- so you are going to get a net filtration into the interstitial space and swelling of the lower limb.
→ Cardiac failure- a build up of pressure in pulmonary artery which backs up to lung causing pulmonary oedema.
What is the equation for net filtration?
P = (Pc - Pi) - σ (πp - πi)
How can reduced plasma protein concentration cause oedema?
→ reduced plasma oncotic pressure
→ greater influence of Pc and πi. Less of reabsorptive force
→ Fluid efflux from capillaries into the interstitial fluid
How can malnutrition cause oedema?
→ Not enough protein intake to make plasma proteins
How can nephrotic syndrome cause oedema?
→ Urinary protein loss - replaced by liver production
How can liver disease cause oedema?
→ Not enough endogenous albumin produced
How does inflammatory mediated oedema arise?
→ Swelling is triggered by local chemical mediators of Inflammation
→ Large increase in capillary permeability
→reduction in reflection coefficient
What are the effects of inflammation?
→ Increased Lp
→ Increased protein permeability
How is lymphatic obstruction caused?
→Filariasis/elephantitis
→ nematode infestation
→ Larvae migrate to lymph system
→ grow and block lymph drainage
What is lymphatic removal?
→ caused by surgery to treat testicular cancer
→ removal of lymphatics
What can abnormalities in fluid exchange lead to?
→ Abnormalities in fluid exchange can lead to oedema / tissue swelling
What are hydraulic conductance(Lp) and wall are(A) in normal conditions?
In normal conditions, Lp and A are constant
Factors promoting filtration and reabsorption…
Factors promoting filtration:
Pc- capillary blood pressure pi i- interstitial proteins
Factors promoting reabsorption
pi p- plasma proteins
What happens to Pc along the length of a capillary in well perfused capillaries?
Pc but not pi p alters along length of capillary
This will be opposed by the net osmotic pressure gradient.
Due to blood flow, pressure is lost but on the other hand osmotic pressure is constant because plasma proteins are not lost favouring reabsorption back into capillary
What happens to filtration force along the capillary?
Outward filtration force declines along the capillary but its net value remains above force favouring re-absorption so well perfused capillaries filter along their entire length
