Lec 16 Microcirculation Flashcards
What controls flow to individual capillary beds?
precapillary sphincters control flow, constrict to turn off flow to capillary bed
What is metarteriole?
type of shunt pathway
What do shunts do in skin?
- they are pathways to bypass capillaries and connect arterioles/venules
What is structure of a single capillary? What is the function of this structure?
- capillary = 5-10 um diameter
- surrounded by single endothelial cell
- small size optimizes for gas and nutrient transfer [max surface area to volume ratio]
What is law of laplace?
Wall stress = (pressure * radius) / thickness
wall stress = P*r/w
Why do capillaries not burst?
reasonable wall stress since very small radius even with high P
= law of laplace
How does wall stress of capillary compare with that of aorta?
6x greater wall stress in aorta
What are two main mech of gas/nutrient movement across capillaries?
- diffusion
- filtration
Where is most O2 transfer in capillary?
at beginning of capillary
What are 3 things that mainly transfer out of capillary by diffusion
- O2
- CO2
- glucose
How does velocity in capillary allow effect?
slow velocity through capillaries allows diffusion
What are normal values of hydrostatic and oncotic pressures?
hydrostatic
- Pc: capillary hydrostatic P, high at beginning of capillary, low at end of capillary
- Pi: close to zero
oncotic
- πc: high through capillary since plasma proteins in capillary but not in interstitial space
- πi: close to zero
What is πc? Where is it high? Value?
- πc is oncotic pressure in capillary created by plasma proteins [albumin]
- πc = 25 mmHg
- high through capillary since plasma proteins don’t exit capillary
What is Pc? Where is it high/low? Values?
- Pc = hydrostatic pressure in capillary
- high at beginning of capillary [35 mmHg]
- low at end of capillary [15 mmHg]
What is normal value of πi?
close to zero since no plasma proteins in interstitial space
What is normal value of Pi?
close to zero
What is the starling equation?
Q = k[(Pc - Pi) - (πc - πi)]
Q = fluid movement [mL/min] k = filtration constant
What does positive Q mean in starling equation?
movement out of capillary
What does negative Q mean in starling equation?
movement into capillary
What part of capillary has filtration and what has absorption?
- filtration on arterial end
- absorption on venous end
What is NFP?
- net filtration pressure
- NFP = Pc - πi
What causes increase in albumin in interstitial space? What effect?
cause: damage to capillary may allow filtration of protein from capillary to interstitium
effect: increased πi, decreased absorption, edema
What causes increase in capillary hydrostatic pressure? What effect?
cause: increased pressure in veins, propagates back to capillaries and increases Pc
effect: increased Pc, increased filtration, edema
What effect of decrease in plasma protein?
decreased πc, reduced absorption, edema
Does increased arteriolar resistance alter Pc?
No!
What happens to Pc and pressure drop across arterioles when you constrict arterioles?
- same Pc
- bigger pressure drop across arterioles
- increased MAP
What is autoregulation of blood flow? where does it occur?
- in kideneys, heart, brain, muscle
- way to keep flow through vessel relatively constant
if high local BP –> increased local flow –> vasoconstriction –> decreased flow
if low local BP –> decreased local flow –> vasodilation –> increased flow
What are 3 ways of local regulation of blood flow?
- autoregulation
- active hyperemia
- endothelial-vascular communication
What is active hyperemia? Where does it occur?
- usually in skeletal muscle
- metabolic activity –> releases agents that cause vasodilation –> increased blood flow
What are a few important dilating factors in active hyperemia?
- lactic acid
- intravascular O2
- adenosine
- CO2
- H+
- extracellular K+
What is endothelial-vascular communication?
- turbulent flow –> increased shear stress causes endothelial cells to produce NO [Endothelium derived relaxing factor]
- NO diffuses from endothelial to smooth muscle cell –> cGMP to inhibit MLCK –> vasodilation
laminar flow vs turbulent flow
- laminar flow is straight through
- turbulent flow in all directions
- most blood flow is laminar
Is most blood flow laminar or turbulent?
laminar
What 3 things encourage turbulent flow?
- high velocities [v = Q/A]
- low fluid viscosity [decreased hematocrit]
- narrow blood vessels [bc increases velocity]
What is path of function of NO [EDRF]
turbulent flow –> increased endothelial shear stress –> production endothelial NO –> vasodilation –> decreased endothelial shear stress
What is ultimate fate of fluid filtered in capillaries?
- returns to blood via lymphatic system, leads to edema
