Microcirculation

Question 1

What comprise microcirculatory system?

Answer 1

Arterioles
Capillaries
Venules

Question 2

What is the overall aim of CVS?

Answer 2

Adequate blood flow through the capillaries

Question 3

What is the blood flow rate?

Answer 3

Volume of blood passing through a vessel per unit time

Question 4

What determine blood flow rate?

Answer 4

Flow rate = Pressure gradient/Resistance

Darcy’s law

Question 5

What is pressure gradient equal to?

Answer 5

Difference in pressure between Pressure A (arteriole) and Pressure B (capillary)

Question 6

Define resistance

Answer 6

‘Hindrance to blood flow due to friction between moving fluid and stationary vascular walls’

Question 7

What affects resistance

Answer 7

Vessel length
Vessel radius
Blood viscosity

Question 8

Which factor affecting resistance changes dynamically?

Answer 8

Vessel radius

The only factor that changes resistance dynamically

Question 9

What happens to pressure, resistance and flow rate when blood pressure increase?

Answer 9

delta P- increases
R- unchanged
F- increases

Question 10

What happens to pressure, resistance and flow rate during arteriolar vasoconstriction?

Answer 10

delta P- unchanged
R- increases
F- decreases

Question 11

What has the biggest influence on resistance?

Answer 11

Arterioles
Lots of muscle
Lots of capacity to constrict and dilate
Biggest change in pressure seen between two ends of an arteriole (50mmHg drop)
Greatest resistance in the CVS

Question 12

What is blood flow to different tissues dependent on?

Answer 12

Resistance
Pressure gradient is the same across all tissues
Resistance determines how much blood flow each tissue gets

Question 13

How can arterioles control resistance?

Answer 13

Vasoconstriction:

• radius increases
• resistance increases
• flow decreases

Vasodilation:

• relaxation
• radius increases
• resistance decreases
• flow increases

Question 14

Why are arterioles partially constricted at rest?

Answer 14

Aterirole smooth muscle normally displays a state of partial constriction- this is called vascular tone

At rest arterioles must be partially constricted to allow them to both dilate and further constrict

Question 15

What are the two functions of adjusting arteriole radii?

Answer 15

Match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs)

Help regulate systemic arterial blood pressure

Question 16

How is matching blood flow regulated?

Answer 16

Regulated by local (intrinsic) controls and independent of nervous or endocrine stimulation

Question 17

How is relating systemic arterial blood pressure regulated?

Answer 17

Regulated by extrinsic controls which travel via nerves or blood and are usually centrally coordinated

Question 18

What is matching blood flow chemically driven by?

Answer 18

Chemically driven by:
Increase in metabolites
Increase in O2 usage
Will lead to vasodilation

Question 19

What is the increase in blood flow in response to increase demand called?

Answer 19

Active hyperaemia

Question 20

What is matching blood flow physically driven by?

Answer 20

Decrease in blood temperature
Increase in stretch (distension) due to increased blood pressure
Leads to vasoconstriction

Question 21

What is the decrease in blood flow in response to factors?

Answer 21

Myogenic autoregulation

Question 22

What would occur skeletal muscle and small intestine arterioles in response to exercise?

Answer 22

SM- active hyperaemia

SI- myogenic vasoconstriction

Question 23

How do arterioles help regulate arterial blood pressure? (Neural)

Answer 23

Cardiovascular control centre in the medulla
Causes vasoconstriction
Decreases blood flow the specific organs

Question 24

How do arterioles help regulate arterial blood pressure? (Hormonal)

Answer 24

Arginine vasopressin (ADH)
Angiotensin II
Adrenaline/Noradrenaline

Stimulate vasoconstriction

Question 25

What is the purpose of capillary exchange?

Answer 25

delivery of metabolic substrates to the cells of the organism [which is the ultimate function of the CVS]

Question 26

Why is capillary density important?

Answer 26

Depends on how metabolically active the tissue is Ideally suited to enhance diffusion: Fick's law Specially designed to minimise diffusion distance Maximise the SA and time for diffusion

Question 27

How does capillary density vary within the body?

Answer 27

Highly metabolically active tissues have denser capillary networks Skeletal muscle = 100cm2/g Myocardium/brain = 500cm2/g Lung = 3500cm2/g (not related to metabolic activity bur rather gas exchange)

Question 28

Why is capillary structure important?

Answer 28

For exchange Thin walls Small H2O filled gap junctions Proteins embedded in capillary

Question 29

What are fenestrations?

Answer 29

Circular holes that allow for slightly larger molecules to pass through the capillary

Question 30

What is discontinuous capillary?

Answer 30

Need to allow for much larger substances between the blood and tissue e.g. in bone marrow when white cells need to be transported and liver

Question 31

What is bulk flow?

Answer 31

A volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid (IF) and is reabsorbed

Question 32

What draws fluid back into he blood?

Answer 32

Oncotic pulling force | protein gradient related

Question 33

What pushes fluid out into tissues?

Answer 33

Hydrostatic pushing force

Question 34

What did Starling hypothesise?

Answer 34

"...there must be a balance between the hydrostatic pressure of the blood in the capillaries and the osmotic attraction of the blood for the surrounding fluids. " " ...and whereas capillary pressure determines transudation, the osmotic pressure of the proteins of the serum determines absorption."

Question 35

When would ultrafiltration take place?

Answer 35

If pressure inside the capillary > in the IF

Question 36

When would reabsorption take place?

Answer 36

If inward driving pressures > outward pressures across the capillary

Question 37

What is the significance of the fact that ultrafiltration is more effective than reabsorption?

Answer 37

Overall net loss | Role of the lymphatic system is to return fluid back to the blood