Microcirculation And Lymphatic System

Question 0

Where are capillary beds most dense ?

Answer 0

In the most metabolically active tissues

Question 1

What is the microcirculation?

Answer 1

Flow through the smallest vessels-smallest arterioles, capillaries and smallest venules

Question 2

What blood vessels confer the greatest resistance to flow ?

Answer 2

Arterioles

Question 3

Why is the microcirculation so important ?

Answer 3

Site of solute exchange - mainly in the capillary beds

Question 4

What is autoregulation ?

Answer 4

It is an intrinsic adjustment of blood flow to the tissues so that the flow to the tissue meets the tissues demand

Question 5

How do changes in local blood flow occur ?

Answer 5

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

Question 6

What is the relationship between metabolism and rate of flow ?

Answer 6

As rate of metabolism increases the rate of blood flow increases
- this occurs without autonomic inputs so it is an intrinsic property

Question 7

How is oxygen a factor in metabolic autoregulation ?

Answer 7

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

Question 8

Other than oxygen what other metabolites are involved in autoregulation ?

Answer 8

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

Question 9

What is the mechanism of action of nitric oxide ?

Answer 9

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

Question 10

The blood flow through a vascular bed is kept constant over a range of perfusion pressures. What happens when the pressure gradient is increased ?

Answer 10

This is met by an increase in resistance to maintain the constant flow

Question 11

What is the major function of myogenic control on the capillaries ?

Answer 11

It ensures a constant flow by increasing the resistance alongside an increase in pressure

Question 12

What is the mechanism behind the myogenic response ?

Answer 12

Increased pressure increases the stretch of smooth muscle
Stretch triggers contraction of the smooth muscle
Causing reduction in lumen size which increases resistance

Question 13

When does long term autoregulation develop ?

Answer 13

When the nutritional and oxygen demands of a tissue chronically exceed delivery
- these develop over weeks/months

Question 14

What are the long term autoregulation adaptations ?

Answer 14

Increase in the number of microcirculatory vessels supplying the tissue
Enlargement of the existing vessels

Question 15

How do hydrophilic solutes cross the lipid bilayer ?

Answer 15

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

Question 16

What are the 2 main forces which act on the movement of fluid at the capillaries ?

Answer 16

Hydrostatic and osmotic pressure

Question 17

What is hydrostatic pressure ?

Answer 17

Capillary blood flow that forces fluid out

- opposed by interstitial fluid which forces fluid in (this pressure is low though)

Question 18

Where is capillary hydrostatic pressure greatest ?

Answer 18

At the arterial end favouring filtration

Question 19

What is osmotic pressure ?

Answer 19

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

Question 20

Explain the net fluid balance through the capillary …

Answer 20

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

Question 21

What system is responsible for returning the net fluid loss from the capillaries back into circulation ?

Answer 21

Lymphatic system

Question 22

What are the 3 functions of the lymphatic System ?

Answer 22

• transports interstitial fluid back into vascular system
• transports fat from small intestine to the blood
• cells contribute to immune system

Question 23

Describe the lymphatic capillaries..

Answer 23

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

Question 24

What are the larger lymphatic vessels like ?

Answer 24

Their walls are similar to veins and they contain valves to prevent backflow
Fluid is pushed through by peristaltic contractions - assisted by skeletal pump

Question 25

Where does the systemic lymphatic vessels drain into ?

Answer 25

Thoracic duct which drains into left subclavian vein

Question 26

Where does the pulmonary lymphatic system drain into ?

Answer 26

Right lymphatic duct and then into right subclavian veins

Question 27

What is the relationship between interstitial fluid pressure and lymph flow ?

Answer 27

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

Question 28

When does oedema occur ?

Answer 28

When the rate of fluid filtration out of the capillaries exceeds the lymphatic drainage

Question 29

What are the 4 major causes of oedema ?

Answer 29

Hydrostatic oedema
Low plasma oncotic pressure
Permeability oedema
Impaired lymphatic drainage

Question 30

What is hydrostatic oedema ?

Answer 30

Caused by venous occlusion or congestion - increase in venous pressure increases capillary pressure so more fluid leaves the capillaries accumulating in interstitial space

Question 31

What is low plasma oncotic pressure?

Answer 31

Reduced plasma protein levels reduces plasma oncotic pressure promoting excessive release from capillaries
Causes : severe liver disease, malnutrition and renal disease

Question 32

What is permeability oedema ?

Answer 32

Increased capillary permeability promotes excessive filtration
Causes : burns, allergic reaction and inflammation

Question 33

What are the causes of impaired lymphatic drainage ?

Answer 33

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