Lecture 19: Microcirculation

Question 1

What is transmural pressure?

Answer 1

The pressure across a vessel wall

P = Pi-Po

Question 2

What is transmural pressure important for?

Answer 2

Determining the tension of the blood vessel wall

Question 3

What is wall tension dependant on?

Answer 3

Laplace

• Radius
• Pressure
• 1/wall thickness

Question 4

What is the implication of the laplace law?

Answer 4

The larger the vessel radius, the greater the wall tension required to withstand an internal pressure

Question 5

What is an aneurysm and what does it lead to?

Answer 5

• Thinning of vessel wall
• Increased lumen radius
  = Wall tension increases

Question 6

What is compliance?

Answer 6

The stretchability at various points along the P/V curve i.e the local change in volume for a given pressure

Reflects the elasticity of the vessel. (veins are more complicated than art.)

Question 7

What is capacitance of blood vessels?

Answer 7

The measure of the volume to pressure relationship over the entire P/V curve. Reflects the storage capacity of the vessels.

i.e Change in volume / change in pressure

Question 8

Describe the structures of capillary beds;

Answer 8

Terminal arteriole
- Branching true capillaries with pre-capillary sphincters

Post-capillary venule

Question 9

Describe the precapillary spincters

Answer 9

Cuffs of smooth muscle (pre-capillary sphincters)

open = blood to true capillarys

closed = blood flows through meta-arteriole throughfare channels

Question 10

What acts as the precapillary resistance vessels?

Answer 10

Arterioles, metarterioles, precapillary sphincters act together as the pre-capillary resistance vessels

i.e parrallel

Question 11

Whats the width generally of a capillary?

Answer 11

1 RBC

Question 12

What is the permeability of cap like?

Answer 12

Varying depending on tissue (Covered elsewhere)

i.e liver - highly permable

muscle, skin, heart - low

Question 13

What are the intrisinc mechanisms of microcirculation flow control?

Answer 13

Metabolic control

• pH
• O2
• CO2
• K
• NO

Myogenic control

• Enothelins
• Stretch

Question 14

Describe the metabolic autoregualtion of blood flow:

Answer 14

Factors promote dilation include:

• Reduced tissue oxygen demand
• Inflammatory molecules i.e histamine
• NO
• K or H+ from intersitial fluid
• Lactic acid or others
• Increased CO2

Question 15

What is reactive hyperemia and what is it driven by?

Answer 15

Restoration of blood flow after brief occlusion results in flow exceeding pre-occlusion levels for a period of time proportional to the duration of occlusion

Driven by metabolites and entirely localised

Question 16

What is the definition of autoregulation?

Answer 16

Intrinsic capacity to maintain constant blood flow despite changes in perfusion pressure

In many tissues (I.e brain) blood flow is near constant over a wide range of pressures), less important in other tissues i.e skin

Question 17

What is myogenic autoregulation?

Answer 17

• Myogenic mechanisms are intrinsic to the smooth muscle bed

- Stretch b/c inc BP = Stretch sensitive Ca channels opening and depolarisation of cells (vasoconstrictio)

Question 18

What are the three cap. transport mechanisms?

Answer 18

1. Diffusion, through membrane, dependant on [Conc]
2. Filtration, depnd on pressure gradient
3. Large molecule movement i.e pinocytosis or via fenestrations

Question 19

What is diffusion dependant on?

Answer 19

1. Concentration gradient (flow dependant) and ability to cross membrane i.e lipid soluble or via endothelial process

Described by Ficks law.

• Membrane thickness
• Diffusable area
• Free diffusion co-eficient

Question 20

Describe filtration:

Answer 20

The bulk flow of fluid across membrane
- Pressure dependant (hydrostatic and osmotic)

Starlings law of ultrafiltration

Question 21

Describe starlings law of ultrafiltration:

Answer 21

Fluid movement (FM) = K [ (Pc-Pi) - Colloid osmotic]

k = permeability
Pc = Capillary hydrostatic pressure
Pi = Interstitial hydrostatic pressure
Colloid osmotic pressure (some proteins cant leave blood and exert pressure)

Question 22

What influences cap. pressure?

Answer 22

Inc. cap pressure = more filtration
- Increased art. or venous pressure increases Pc

Increasing resistance

• Increasing upstream resistance decreases Cap pressure
• Increasing downstream resistance increases Cap pressure.

Question 23

How does vasodilation and venous pressure influence filtration?

Answer 23

Vasodilation = Increased Cap hydrostatic pressure = more filtration

Increased venous pressure i.e heart failure = Results in an increase in cap hydrostatic pressure at venous end = reduced reabsorption

Question 24

What happens in heart failure at a microcirculation level?

Answer 24

Reduced CO
- Poor peripheral perfusion = neural and hormonal adaptations
- Fluid retention
- Increased EDP
- Increased central venous pressure
- Increased Cap resistance
- Increased net filtration
= Oedema

Question 25

Increased oncotic pressure leads to;

Answer 25

Reduced filtration

Question 26

How does increased oncotic pressure decrease filtration?

Answer 26

Decreasing fluid content or increasing large protein content of blood will increase the colloid osmotic (oncotic) pressure within the cap., sucking fluid back into the capilliary.

i.e in dehydration

Question 27

What is the function of lymphatic vessels?

Answer 27

1. Return blood components to circulation
2. Absorption from gut
3. Removal of RBC’s from tissue
4. Immunological - Removal/isolation of bacteria

Question 28

Describe lymph flow:

Answer 28

Terminal lymphatics = Endothelium + highly permeable basement membrane

• Vlaves to prevent backflow
• Vasomotion pushes lymph
• Fluid flow into primary lymph vessels requires interstitial hydrostatic pressure to exceed the lymph pressure.

Localised swelling indicates that rate of lymph flow is at its max.