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
