Flashcards in Microcirculation And Lymphatic System Deck (34)
What is the microcirculation?
Flow through the smallest vessels-smallest arterioles, capillaries and smallest venules
Where are capillary beds most dense ?
In the most metabolically active tissues
What blood vessels confer the greatest resistance to flow ?
Why is the microcirculation so important ?
Site of solute exchange - mainly in the capillary beds
What is autoregulation ?
It is an intrinsic adjustment of blood flow to the tissues so that the flow to the tissue meets the tissues demand
How do changes in local blood flow occur ?
Changing the arterioles diameter that is supplying the capillary bed
Altering the degree of contraction of the precapillary sphincter
- they are intrinsic mechanisms which are short term changes
What is the relationship between metabolism and rate of flow ?
As rate of metabolism increases the rate of blood flow increases
- this occurs without autonomic inputs so it is an intrinsic property
How is oxygen a factor in metabolic autoregulation ?
Because when metabolic rate is increased oxygen consumption is increased which causes a decrease in the oxygen concentration
- if consumptions exceeds delivery hypoxia occurs - this stimulates smooth muscle to relax to dilate vessels to increase blood flow
- this increases the oxygen concentration and acts as a negative feedback loop so the relaxation stops
Other than oxygen what other metabolites are involved in autoregulation ?
Waste products of metabolism such as carbon dioxide, protons, potassium and adenosine
- these trigger dilation of the blood vessels which increases blood flow so increases the reoval of the waste products
Prostacyclin and nitric oxide also cause vasodilation of vessels
What is the mechanism of action of nitric oxide ?
Diffuses into muscle cells because it is a gas
- converts guanyl Cyclase into cGMP
- this causes relaxation
GTN increases the bioavailability of NO so utilises its properties
The blood flow through a vascular bed is kept constant over a range of perfusion pressures. What happens when the pressure gradient is increased ?
This is met by an increase in resistance to maintain the constant flow
What is the major function of myogenic control on the capillaries ?
It ensures a constant flow by increasing the resistance alongside an increase in pressure
What is the mechanism behind the myogenic response ?
Increased pressure increases the stretch of smooth muscle
Stretch triggers contraction of the smooth muscle
Causing reduction in lumen size which increases resistance
When does long term autoregulation develop ?
When the nutritional and oxygen demands of a tissue chronically exceed delivery
- these develop over weeks/months
What are the long term autoregulation adaptations ?
Increase in the number of microcirculatory vessels supplying the tissue
Enlargement of the existing vessels
How do hydrophilic solutes cross the lipid bilayer ?
They cross through the intercellular clefts which have a diameter of about 60 A - allowing passage of water, ions and small organic solutes but not albumin and other plasma proteins
What are the 2 main forces which act on the movement of fluid at the capillaries ?
Hydrostatic and osmotic pressure
What is hydrostatic pressure ?
Capillary blood flow that forces fluid out
- opposed by interstitial fluid which forces fluid in (this pressure is low though)
Where is capillary hydrostatic pressure greatest ?
At the arterial end favouring filtration
What is osmotic pressure ?
Caused by the non diffusable plasma proteins
This pressure favours absorption
Doesn't vary much over the length of the capillary
Encourages fluid to leave the interstitial fluid and enter the capillary
Explain the net fluid balance through the capillary ...
Hydrostatic and osmotic pressure gradients cause a net movement of fluid out of circulation at the arterial end and a net movement of fluid into the circulation at the venous end
- causes a net loss of fluid from the circulation
What system is responsible for returning the net fluid loss from the capillaries back into circulation ?
What are the 3 functions of the lymphatic System ?
- transports interstitial fluid back into vascular system
- transports fat from small intestine to the blood
- cells contribute to immune system
Describe the lymphatic capillaries..
Microscopic closed ended capillaries that intertwine around the blood capillaries
Walls are made of endothelial cells with porous junctions so fluid, proteins and microorganism can pass from interstitial space into them
What are the larger lymphatic vessels like ?
Their walls are similar to veins and they contain valves to prevent backflow
Fluid is pushed through by peristaltic contractions - assisted by skeletal pump
Where does the systemic lymphatic vessels drain into ?
Thoracic duct which drains into left subclavian vein
Where does the pulmonary lymphatic system drain into ?
Right lymphatic duct and then into right subclavian veins
What is the relationship between interstitial fluid pressure and lymph flow ?
Pressure increases flow up to a point
- breaks down if accumulation of interstitial fluid leads of abnormally high pressures - drainage is insufficient and edema occurs
When does oedema occur ?
When the rate of fluid filtration out of the capillaries exceeds the lymphatic drainage
What are the 4 major causes of oedema ?
Low plasma oncotic pressure
Impaired lymphatic drainage
What is hydrostatic oedema ?
Caused by venous occlusion or congestion - increase in venous pressure increases capillary pressure so more fluid leaves the capillaries accumulating in interstitial space
What is low plasma oncotic pressure?
Reduced plasma protein levels reduces plasma oncotic pressure promoting excessive release from capillaries
Causes : severe liver disease, malnutrition and renal disease
What is permeability oedema ?
Increased capillary permeability promotes excessive filtration
Causes : burns, allergic reaction and inflammation