Capillaries II - Fluid Exchange 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.
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
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 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
→ 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
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
→ Lymphatic problems
What are three situations where you would have Increased Capillary Pressure (Pc) ?
→ Dependent gravitational oedema (standing up for a long time)
→ Deep venous thrombosis
→ Cardiac failure
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
→ 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
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
