Lec 16 Microcirculation

Question 1

What controls flow to individual capillary beds?

Answer 1

precapillary sphincters control flow, constrict to turn off flow to capillary bed

Question 2

What is metarteriole?

Answer 2

type of shunt pathway

Question 3

What do shunts do in skin?

Answer 3

• they are pathways to bypass capillaries and connect arterioles/venules

Question 4

What is structure of a single capillary? What is the function of this structure?

Answer 4

• capillary = 5-10 um diameter
• surrounded by single endothelial cell
• small size optimizes for gas and nutrient transfer [max surface area to volume ratio]

Question 5

What is law of laplace?

Answer 5

Wall stress = (pressure * radius) / thickness

wall stress = P*r/w

Question 6

Why do capillaries not burst?

Answer 6

reasonable wall stress since very small radius even with high P
= law of laplace

Question 7

How does wall stress of capillary compare with that of aorta?

Answer 7

6x greater wall stress in aorta

Question 8

What are two main mech of gas/nutrient movement across capillaries?

Answer 8

• diffusion

- filtration

Question 9

Where is most O2 transfer in capillary?

Answer 9

at beginning of capillary

Question 10

What are 3 things that mainly transfer out of capillary by diffusion

Answer 10

• O2
• CO2
• glucose

Question 11

How does velocity in capillary allow effect?

Answer 11

slow velocity through capillaries allows diffusion

Question 12

What are normal values of hydrostatic and oncotic pressures?

Answer 12

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

Question 13

What is πc? Where is it high? Value?

Answer 13

• πc is oncotic pressure in capillary created by plasma proteins [albumin]
• πc = 25 mmHg
• high through capillary since plasma proteins don’t exit capillary

Question 14

What is Pc? Where is it high/low? Values?

Answer 14

• Pc = hydrostatic pressure in capillary
• high at beginning of capillary [35 mmHg]
• low at end of capillary [15 mmHg]

Question 15

What is normal value of πi?

Answer 15

close to zero since no plasma proteins in interstitial space

Question 16

What is normal value of Pi?

Answer 16

close to zero

Question 17

What is the starling equation?

Answer 17

Q = k[(Pc - Pi) - (πc - πi)]

Q = fluid movement [mL/min]
k = filtration constant

Question 18

What does positive Q mean in starling equation?

Answer 18

movement out of capillary

Question 19

What does negative Q mean in starling equation?

Answer 19

movement into capillary

Question 20

What part of capillary has filtration and what has absorption?

Answer 20

• filtration on arterial end

- absorption on venous end

Question 21

What is NFP?

Answer 21

• net filtration pressure

- NFP = Pc - πi

Question 22

What causes increase in albumin in interstitial space? What effect?

Answer 22

cause: damage to capillary may allow filtration of protein from capillary to interstitium
effect: increased πi, decreased absorption, edema

Question 23

What causes increase in capillary hydrostatic pressure? What effect?

Answer 23

cause: increased pressure in veins, propagates back to capillaries and increases Pc
effect: increased Pc, increased filtration, edema

Question 24

What effect of decrease in plasma protein?

Answer 24

decreased πc, reduced absorption, edema

Question 25

Does increased arteriolar resistance alter Pc?

Answer 25

No!

Question 26

What happens to Pc and pressure drop across arterioles when you constrict arterioles?

Answer 26

• same Pc
• bigger pressure drop across arterioles
• increased MAP

Question 27

What is autoregulation of blood flow? where does it occur?

Answer 27

• 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

Question 28

What are 3 ways of local regulation of blood flow?

Answer 28

• autoregulation
• active hyperemia
• endothelial-vascular communication

Question 29

What is active hyperemia? Where does it occur?

Answer 29

• usually in skeletal muscle

- metabolic activity –> releases agents that cause vasodilation –> increased blood flow

Question 30

What are a few important dilating factors in active hyperemia?

Answer 30

• lactic acid
• intravascular O2
• adenosine
• CO2
• H+
• extracellular K+

Question 31

What is endothelial-vascular communication?

Answer 31

• 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

Question 32

laminar flow vs turbulent flow

Answer 32

• laminar flow is straight through
• turbulent flow in all directions
• most blood flow is laminar

Question 33

Is most blood flow laminar or turbulent?

Answer 33

laminar

Question 34

What 3 things encourage turbulent flow?

Answer 34

• high velocities [v = Q/A]
• low fluid viscosity [decreased hematocrit]
• narrow blood vessels [bc increases velocity]

Question 35

What is path of function of NO [EDRF]

Answer 35

turbulent flow –> increased endothelial shear stress –> production endothelial NO –> vasodilation –> decreased endothelial shear stress

Question 36

What is ultimate fate of fluid filtered in capillaries?

Answer 36

• returns to blood via lymphatic system, leads to edema