Capillaries Flashcards
Oncotic Pressure
- Higher osmolarity = greater pressure/force pushing water in osmosis
- BUT in capillary only protein conc differences create osmotic pressure (oncotic pressure)
- Oncotic pressure = 25-28 mmHg normally
- Oncotic pressure usually greater in capillary —> water INTO capillary (ABSORPTION)
Hydrostatic Pressure
- Pressure in blood from contraction of ventricle
* Hydrostatic pressure usually greater in capillary —> water OUT of capillary (FILTRATION)
Starling Equation
- Jv = Kf (Pc-Pi) - sigma (PIc - PIi)
- Jv - net filtration
- Kf is filtration coefficient
- Sigma is reflection coefficient (correction for protein leakage)
- Pc/Pi - hydrostatic pressure in cap & interstitial space
- PI c/PI i - osmotic pressure in cap & interstitial space
- Kf and sigma usually constant for given vascular bed SO…
Jv = (Pc-Pi) - (PIc - PIi)
+ Jv means filtration
- Jv means absorption
Kf
Filtration Coefficient
how permeable is the capillary?
largely based on #aquaporin channels
inc permeability will inc overall magnitude of net absorption/filtration
How does Starling Equation normally play out in human?
Pc dec as you move across capillary so go from filtration to absorption
Usually net filtration into interstitial fluid (lose 2-4 L fluid a day to this space)
What happens to Starling Equation if you inc venous pressure?
Only thing that changes is Pc which inc so inc filtration
More fluid lost –> edema
Ex) standing for prolonged time
What happens to Starling Equation if you have hypoproteinemia?
Both PIc and PIi decrease so inc filtration overall
More fluid lost –> edema
Ex) liver disease or starvation
What happens to Starling Equation if you inc capillary permeability?
Inc Kf will inc magnitude of net change
Net change is normally filtration so greater filtration –> edema
Ex) histamine release
What happens to Starling Equation if you dec arterial pressure?
Lower Pc so less filtration –> net absorption to inc blood volume in the cap (DILUTE - low hematocrit)
Ex) hemorrhage (combo of dec blood volume and compensatory baroreceptor inc in resistance really slows flow through capillary to favor absorption)
Does changing hematocrit change oncotic pressure in the capillary?
NO - RBC proteins are in separate compartment from plasma
Starling Equation in the Lungs
- Overall lower pressure system so Pc lower
- Same oncotic pressure in pulmonary circulation (about 25 mmHg) BUT much higher interstitial fluid oncotic pressure —> more filtration
- Luckily, lymph system is very efficient in lungs
How does fluid get into lymph system and then flow within it?
- Valves so only flow into lymph vessel not back out
- Force driving fluid into lymph is high interstitial pressure
- Fluid pumped by skeletal muscle compression and valves (similar to veins)
2 Hemodynamic Roles of Lymph System
- Maintains slightly negative interstitial pressure (like a vacuum); to prevent edema; P i = -3 usually
- Removes interstitial protein so that the conc of protein in interstitial space does not exceed that in the capillary
4 Ways to Inc Flow to Lymph
- 1- Capillary pressure elevated
- 2- Plasma oncotic pressure reduced
- 3- Interstitial protein concentration is increased
- 4- capillary permeability is increased (Kf)
What happens if lymph not working? 2 ex
Back up of fluid in interstitial space –> EDEMA
1- in injury, damage —> inc permeability —> edema but if lymph also damaged then little drainage
2- Elephantiasis - parasitic infection of lymph blocks drainage
