Lecture 19: Microcirculation Flashcards
What is transmural pressure?
The pressure across a vessel wall
P = Pi-Po
What is transmural pressure important for?
Determining the tension of the blood vessel wall
What is wall tension dependant on?
Laplace
- Radius
- Pressure
- 1/wall thickness
What is the implication of the laplace law?
The larger the vessel radius, the greater the wall tension required to withstand an internal pressure
What is an aneurysm and what does it lead to?
- Thinning of vessel wall
- Increased lumen radius
= Wall tension increases
What is compliance?
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.)
What is capacitance of blood vessels?
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
Describe the structures of capillary beds;
Terminal arteriole
- Branching true capillaries with pre-capillary sphincters
Post-capillary venule
Describe the precapillary spincters
Cuffs of smooth muscle (pre-capillary sphincters)
open = blood to true capillarys
closed = blood flows through meta-arteriole throughfare channels
What acts as the precapillary resistance vessels?
Arterioles, metarterioles, precapillary sphincters act together as the pre-capillary resistance vessels
i.e parrallel
Whats the width generally of a capillary?
1 RBC
What is the permeability of cap like?
Varying depending on tissue (Covered elsewhere)
i.e liver - highly permable
muscle, skin, heart - low
What are the intrisinc mechanisms of microcirculation flow control?
Metabolic control
- pH
- O2
- CO2
- K
- NO
Myogenic control
- Enothelins
- Stretch
Describe the metabolic autoregualtion of blood flow:
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
What is reactive hyperemia and what is it driven by?
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
What is the definition of autoregulation?
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
What is myogenic autoregulation?
- Myogenic mechanisms are intrinsic to the smooth muscle bed
- Stretch b/c inc BP = Stretch sensitive Ca channels opening and depolarisation of cells (vasoconstrictio)
What are the three cap. transport mechanisms?
- Diffusion, through membrane, dependant on [Conc]
- Filtration, depnd on pressure gradient
- Large molecule movement i.e pinocytosis or via fenestrations
What is diffusion dependant on?
- 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
Describe filtration:
The bulk flow of fluid across membrane
- Pressure dependant (hydrostatic and osmotic)
Starlings law of ultrafiltration
Describe starlings law of ultrafiltration:
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)
What influences cap. pressure?
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.
How does vasodilation and venous pressure influence filtration?
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
What happens in heart failure at a microcirculation level?
Reduced CO - Poor peripheral perfusion = neural and hormonal adaptations - Fluid retention - Increased EDP - Increased central venous pressure - Increased Cap resistance - Increased net filtration = Oedema
Increased oncotic pressure leads to;
Reduced filtration
How does increased oncotic pressure decrease filtration?
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
What is the function of lymphatic vessels?
- Return blood components to circulation
- Absorption from gut
- Removal of RBC’s from tissue
- Immunological - Removal/isolation of bacteria
Describe lymph flow:
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.
