Microcirculation Flashcards
What are the 2 functions of microcirculation?
- Exchange of molecules from the blood to cells and vice versa, via DIFFUSION
- Regulation of blood volume by altering the distribution of fluid between the interstitial and vascular compartments, via FILTRATION from or ABSORPTION into capillaries
Do capillaries have smooth muscle cells?
NO! just endothelial cells
What controls the blood flow into capillary beds?
As the metarterioles give off capillaries, the junction is surrounded by precapillary sphincters that regulate the blood flow into the capillary bed and venules.
Where are precapillary sphincters most abundant?
mesenteric circulation
What molecules can cross capillaries via diffusion?
Oxygen and Carbon dioxide
What molecules can diffuse through clefts?
water and small solutes
How are proteins transported across capillaries?
1) Fenestrations (in liver)
2) taken up by endocytosis and transported across endothelial cells via exocytosis! (very low flux thorugh this system)
Can proteins usually cross capillaries?
NO
What is the equation for osmotic pressure?
Osmotic pressure π=RTσΔc
What is the reflection coefficient related to?
σ =1 - permeability coefficient
What is the reflection coefficient of proteins?
Albumin and Hemoglobin have permeability coefficient of around 0, so they are almost completely reflected (.999)! This means that they generate a significant osmotic pressure.
What is the reflection coefficient of water?
Water has permeability coefficient of 1, so it is NOT reflected (0)
What can change the refection coefficient of proteins?
The reflection coefficients of proteins can decrease significantly during inflammatory conditions (like burns)
What is the Starling Equation?
Jv = Kf [(Pc - Pi ) – (Πc – Πi) ]
What does Jv stand for?
transcapillary flow
What does a positive Jv mean?
(+) when flow is outward, filtration
What does a negative Jv mean?
(-) when flow is inward, reabsorption
What does Kf stand for?
filtration coefficient (hydraulic conductance)- equal to inverse of resistance
What regulates Kf?
- Regulated by precapillary sphincters that can block flow into capillaries and shunt blood around them
- Regulated by capillary type (fenestrated (high) v. continuous (low))
- Regulated by inflammation (which will increase capillary permeability and raise Kf)
What is [(Pc - Pi ) – (Πc – Πi) ] ?
Net transcapillary pressure
What is (Pc - Pi )?
capillary - interstitial hydrostatic pressure
Is hydrostatic pressure negative or positive?
+ PRESSURE- forces water OUT
What is (Πc – Πi)?
oncotic pressure: capillary - interstitial colloid osmotic pressure
Is oncotic pressure negative or positive?
- PRESSURE- high protein content within plasma pulls water IN from interstitium
Increasing Pc will do what to filtration?
INCREASE IT! (water is forced out of capillaries)
Decreased arteriolar hydrostatic RESISTANCE will have what effect?
increase blood flow and increase pressure in capillary
Increasing the venous hydrostatic pressure will have what effect on pressure in the capillary?
decreases blood flow and increases capillary pressure
What is the only factor in the Starling equation that changes from the arteriole to the venous side?
Pc is the only variable that really changes (DECREASES), because pressure as lost with forward movement of fluid
Is the net filtration or absorption on the arteriole side of the capillary?
net filtration
Is the net filtration or absorption on the venuole side of the capillary?
net absorption
What controls Pi?
Depends on interstitial fluid volume (fluid flowing in from the capillaries vs fluid flowing out via lymphatics ). Usually a vacuum!
A higher Pi says what about lymph flow?
HIGHER
Πc depends on what?
plasma protein concentration
What decreases plasma protein concentration (and decreases Πc)?
liver disease, starvation, kidney disease, transient dilution (IV infusion and fluid shifts)→ increased filtration
What increases plasma protein concentration (and increase Πc)?
lowering liquid component via dehydration, diarrhea, vomiting, prolonged sweating→increased absorption to replace fluid
Πi is regulated by what?
input (protein entering from capillaries)- output (protein leaving via lymphatic drainage)
Increased lymph flow does what to the interstitial protein concentration?
lowers it! flushes it out!
What opens flap valves?
increased interstitial pressure
What creates negative pressure (suction) that can pull fluid into the lymphatic vessel?
smooth muscle around lymph vessels
What is the most simplistic way to explain edema?
fluid entry is greater than fluid removal
List the edema safety factors.
- increase in lymph flow as interstitial fluid pressure increases (to a certain point)
How does increase in interstitial fluid pressure act as a safety factor to edema?
Negative interstitial pressure and Low compliance: flow will resist swelling until it is around 0mmHg (get about 5mm Hg of safety)
increase in interstitial volume → increase in Pi (goes from negative to more positive)→ decrease in (Pc – Pi) → reduce any further increase in interstitial volume
How does lymph flow act as a safety factor to edema?
Get around 7 mmHg of safety due to increasing lymph flow as interstitial fluid pressure increases
List the causes of edema.
Capillary pressure Interstitial pressure Plasma oncotic pressure Interstitial oncotic pressure Permeability – inflammation
What can increase capillary pressure?
heart failure; venous obstruction, inflammation (local vasodilation)
What can change interstitial pressure and lead to edema?
pulmonary– restrictive lung disease
What can decrease plasma oncotic pressure?
liver disease and malnutrition (reduced production), kidney disease (loss)
What can increase interstitial oncotic pressure?
lymphatic obstruction (only way for protein to get back into circulation is via lymphatics), inflammation (increased permeability)
