Microcirculation And Lymphatic System Flashcards Preview

Cardio-respiratory Physiology And Pharmacology > Microcirculation And Lymphatic System > Flashcards

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 ?

Lymphatic system


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 ?

Hydrostatic oedema
Low plasma oncotic pressure
Permeability oedema
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


What are the causes of impaired lymphatic drainage ?

Malignancy, surgical removal of lymph nodes or infestation by parasitic nematodes( elephantiasis)