Microcirculation

Question 1

What is the overall aim of the cardiovascular system?

Answer 1

adequate blood flow through capillaries

Question 2

What is blood flow rate?

Answer 2

volume of blood passing through a vessel per unit of time

Question 3

What is Darcy’s Law / Fluid Circuit?

Answer 3

ΔP = Q x R
ΔP = change in pressure / pressure gradient
Q = flow rate
R = resistance

Question 4

How is Pressure gradient (ΔP) calculated?

Answer 4

Pressure gradient (ΔP) = Pressure (A) - Pressure (B)

Question 5

What is the result of increasing ΔP?

Answer 5

increases flow rate

Question 6

What is resistance (R)?

Answer 6

hindrance to blood flow due to friction between moving fluid + stationary vascular walls

Question 7

What is the equation for R (resistance)?

Answer 7

R = 8Lη/πr^4
• L = vessel length
• η = blood viscosity
• r = vessel radius

Question 8

What happens to resistance and blood flow rate when vessel radius is halved?

Answer 8

increases resistance by 16 times + decreases flow by 16 times

Question 9

How would increase in blood pressure affect pressure gradient?

Answer 9

increases - arriving blood will be higher in pressure

Question 10

How would increase in blood pressure affect flow rate?

Answer 10

increases - higher blood pressure = more flow

Question 11

How would arteriolar vasoconstriction affect resistance?

Answer 11

increases - resistance increases with vasoconstriction

Question 12

How would arteriolar vasoconstriction affect flow rate?

Answer 12

decreases - due to increase in resistance

Question 13

How do you calculate the flow rate of any organ?

Answer 13

F (organ) = ΔP (MAP) / R (organ)

Question 14

Why is pressure difference important in the arterioles?

Answer 14

without pressure difference, blood would not reach tissue capillary beds

Question 15

What happens to vessel radius when vasoconstriction occurs?

Answer 15

decreases

Question 16

What happens to vessel radius when vasodilation occurs?

Answer 16

increases

Question 17

What happens to resistance when vasoconstriction occurs?

Answer 17

increases

Question 18

What happens to resistance when vasodilation occurs?

Answer 18

decreases

Question 19

What happens to flow rate when vasoconstriction occurs?

Answer 19

decreases

Question 20

What happens to flow rate when vasodilation occurs?

Answer 20

increases

Question 21

What is vascular tone?

Answer 21

arteriolar smooth muscle normally displays a state of partial constriction

Question 22

For what 2 functions are the radii of the arterioles adjusted independently?

Answer 22

• match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs)
• help regulate systemic arterial BP

Question 23

How are the 2 functions of the arterioles regulated?

Answer 23

• local (intrinsic) controls + independent of nervous or endocrine stimulation
• extrinsic controls which travel via nerves or blood + are usually centrally coordinated

Question 24

How can blood flow meet metabolic needs of tissue chemically?

Answer 24

chemically driven by increasing metabolites + O2 usage - causes active hyperaemia

Question 25

What is active hyperaemia?

Answer 25

vasodilation of arterioles

Question 26

How can blood flow meet metabolic needs of tissue physically?

Answer 26

physically driven by decrease in blood temp + increases stretch (distention) due to increase blood pressure - caused myogenic autoregulation

Question 27

What is myogenic autoregulation?

Answer 27

vasoconstriction of arteries

Question 28

What is the equation for cardiac output (which is flow rate) involving BP and resistance?

Answer 28

F = ΔP (MAP) / R Q or F = Cardiac Output MAP = Blood Pressure R = Total Peripheral Resistance

Question 29

How is arterial blood pressure regulated neurally?

Answer 29

cardiovascular control centre in medulla

Question 30

How is arterial blood pressure regulated hormonally?

Answer 30

vasoconstriction caused by: • vasopressin / ADH secreted by pituitary • angiotensin II from lungs • adrenaline/noradrenaline from adrenals

Question 31

What is the purpose of capillary exchange?

Answer 31

delivery of metabolic substrates to the cells of the organism

Question 32

What are the general dimensions of a capillary?

Answer 32

7µM lumen diameter | 1µM cell width

Question 33

What are capillaries specially designed for?

Answer 33

Ideally suited to enhance diffusion by Fick's Law • minimise the diffusion distance • maximise the surface area and time for diffusion

Question 34

What tissues have denser capillary networks? Examples?

Answer 34

Highly metabolically active tissues: • Skeletal muscle = 100cm2/g • Myocardium/Brain = 500cm2/g • Lung = 3500cm2/g

Question 35

What is special about the capillaries in skeletal muscle?

Answer 35

huge capacity but limited flow at rest

Question 36

What are the 3 types of capillaries?

Answer 36

* continuous * fenestrated * sinusoid / discountinued

Question 37

What are features of continuous capillaries?

Answer 37

continuous basement layer + endothelial layer w/ intracellular clefts

Question 38

What are features of fenestrated capillaries?

Answer 38

fenestrations in the endothelial layer + continuous basement membrane - allows large molecules to pass through

Question 39

What are features of discontinued capillaries?

Answer 39

intracellular gaps + incomplete basement membrane - allows proteins and even blood cells to pass through

Question 40

What is bulk flow?

Answer 40

volume of protein-free plasma filters out fo the capillary, mixes with the surrounding interstitial fluid + is reabsorbed

Question 41

How does hydrostatic pressure affect capillaries?

Answer 41

pushing force - pushes plasma out of capillary

Question 42

How does oncotic pressure affect capillaries?

Answer 42

pulling force - pulls plasma to be reabsorbed into the capillary

Question 43

What was Starling's theory on capillary fluid movement?

Answer 43

hydrostatic pressure of the blood in the capillaries = | osmotic attraction of the blood for the surrounding fluids

Question 44

What causes Ultrafiltration in the capillaries?

Answer 44

pressure in capillaries > in interstitial fluid

Question 45

What causes Reabsorption in the capillaries?

Answer 45

inward driving pressures > outward pressure across capillary

Question 46

What is more effective; ultrafiltration or reabsorption?

Answer 46

ultrafiltration

Question 47

Why is it important that ultrafiltration is more effective?

Answer 47

any fluid lost is picked up by lymph vessels + returned to circulation to maintain BP

Question 48

What happens if there is more ultrafiltration than reabsorption?

Answer 48

fluid accumulates in IF

Question 49

Why is the lymphatic system important when ultrafiltration > reabsorption?

Answer 49

drains the fluid accumulated

Question 50

What happens if the lymphatic system fails?

Answer 50

rate of production of fluid > rate of drainage so OEDEMA can occur

Question 51

Where can continuous capillaries be found?

Answer 51

* fat * muscle * nervous system

Question 52

Where can fenestrated capillaries be found?

Answer 52

* intestinal villi * endocrine glands * kidney glomeruli

Question 53

Where can discontinuous capillaries be found?

Answer 53

* liver * bone marrow * spleen