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CVS > 5. Microcirculation > Flashcards

5. Microcirculation Flashcards

1
Q

What is the starting point of microcirculation?

A
  • Arterioles
  • Branch off the artery
  • Divert blood towards the organ
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2
Q

What is the ‘blood flow rate’ (F) and the equation (relating it with pressure and resistance)?

A
  • Volume of blood passing through a vessel per unit time
  • F = ΔP / R
  • directly proportional to pressure gradient
  • inversely proportional to vascular resistance
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3
Q

What is ‘resistance’?

A

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

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4
Q

What 3 factors affect resistance?

A
  • Blood viscosity
  • Vessel Radius (narrower => more resistance)
  • Vessel length
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5
Q

How does F, ΔP & R change with increasing BP and arteriolar vasoconstriction?

A
  • ↑BP = ↑F, ↑ΔP

* ↑ Arteriolar Vasoconstriction = ↑R, ↓F

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6
Q

How does the BP generated by the heart and the MAP have an influence on the microcirculation?

A
  • BP generated by heart - does not change much
  • Blood pressure in any artery around the body is around the MAP
  • MAP (going in) in arterioles but pressure in capillaries influenced by dilation
  • Allows blood to pass slowly to allow exchange of nutrients
  • End of capillaries - venous pressure, around 0 mmHg
  • Resistance of arterioles in the organ is the main determinant of blood flow in the body
  • Pressure difference allows blood to reach tissue capillary beds
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7
Q

What is the state of the arteriolar smooth muscle and why?

A
  • Partial constriction - vascular tone

* Allows space for changing the blood flow by constricting or dilating

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8
Q

What can the adjustment of the radii of arterioles be used to accomplish?

A
  • Match blood flow to the metabolic needs of specific tissue - local intrinsic control (independent of nerves and hormones)
  • Help regulate arterial blood pressure - extrinsic controls
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9
Q

What is active hyperaemia?

A
  • Muscle becomes more active (↑ metabolism and oxygen consumption)
  • Change detected in tissues and signal is sent to the arterioles to vasodilate - more blood flow
  • Response to local conditions
  • in other words: “an increase in organ blood flow that is associated with increased metabolic activity of an organ or tissue”
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10
Q

What happens in the microcirculation if blood temperature decreases?

A
  • Arteriolar smooth muscle contracts
  • Less blood reaches the surface - less heat radiated away
  • This is why cold is applied to a damaged area as it reduces blood flow => swelling
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11
Q

What is myogenic vasoconstriction?

A
  • Type of autoregulation
  • When BP increases (e.g. during exercise) more blood is needed in some places than others
  • Active hyperaemia in lungs, heart and muscles
  • Myogenic vasconstriction in tissues that don’t need as much blood (not all the blood can reach every tissue so an appropriate distribution is needed)
  • MV increases resistance to match the increase in pressure, so the flow rate returns to normal
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12
Q

How can F = ΔP / R be substituted into an equation describing cardiac output?

A

CO = MAP / TPR (total peripheral resistance)

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13
Q

Describe the neural regulation of arterial blood pressure (extrinsic)

A

• Regulated by the cardiovascular control centre in the medulla (of the brain)
• To increase BP - brain secretes noradrenaline via sympathetic nervous system
• Adrenoreceptors
- Alpha - constriction of arterioles
- Beta - increases heart rate
• Blood flow to organs is reduced
• Brain will always try to receive the normal amount of blood to survive - so, some organs may receive very little blood if blood loss occurs

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14
Q

Describe the hormonal regulation of arterial blood pressure (extrinsic)

A

• The brain can mimic the sympathetic nervous system to control the heart
- stimulates the production of adrenaline and noradrenaline
• Can act on arterioles
- stimulates the production of vasopressin and angiotensin II
• Vasopressin comes from the posterior pituitary
• Angiotensin II (mainly) comes from the lungs

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15
Q

What is the pressure entering and leaving the arterioles?

A
  • 93 mmHg entering

* 37 mmHg leaving

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16
Q

Describe the hormonal regulation of arterial blood pressure (extrinsic)

A

• The brain can mimic the sympathetic nervous system to control the heart
- stimulates the production of adrenaline and noradrenaline
• Can act on arterioles
- stimulates the production of vasopressin and angiotensin II
• Vasopressin comes from the posterior pituitary
• Angiotensin II (mainly) comes from the lungs

17
Q

What is the pressure entering and leaving the arterioles?

A
  • 93 mmHg entering

* 37 mmHg leaving

18
Q

When is capillary density important?

A
  • More metabolically demanding tissue - greater capillary density
  • e.g. skeletal muscle, myocardium, brain, lungs
  • Myocardium and brain are vulnerable to hypoxia so this is important
  • Lung has this to maximise gas exchange, not metabolic activity
19
Q

What 3 things do the capillaries aim to achieve with their design?

A
  • Minimise diffusion distance
  • Maximise surface area
  • Maximise diffusion time
20
Q

When is capillary density important?

A
  • More metabolically demanding tissue - greater capillary density
  • e.g. skeletal muscle, myocardium, brain, lungs
  • (Myocardium and brain are vulnerable to hypoxia so this is important)
21
Q

Why is adipose tissue poorly perfused?

A

Nothing that the adipose tissue does that needs a rapid response

22
Q

What is special about the perfusion of the skeletal muscle?

A
  • High capillary density
  • Large number of these are shut off at rest
  • Precapillary sphincter shuts off arterioles/capillaries
23
Q

What are the features of fenestrated capillaries?

A
  • Leakier capillaries
  • Slightly bigger holes - fenestrae (around 80nM) in cells
  • Allow larger substances to pass e.g. glomerulus
24
Q

What are the features of discontinuous capillaries?

A
  • Large holes in the capillary between the endothelial cells

* Important in the bone marrow - white cells have to get into blood

25
Q

What are the features of fenestrated capillaries?

A
  • Leakier capillaries
  • Slightly bigger holes - fenestrae (around 80nM)
  • Allow larger substances to pass e.g. glomerulus
26
Q

What type of capillary do you mainly find as part of the blood-brain barrier and why?

A

(• continuous)
• No water-filled gap junctions between endothelial cells
• Really tight gap junctions instead
• Substances have to diffuse across endothelial cells - access to the brain tightly regulated and it is more protected
• Brain has the fewest blood borne agents

27
Q

What happens to lipid soluble molecules in the blood-brain barrier?

A
  • Lipid soluble molecules can diffuse through the lipid bilayer in endothelial cells
  • Specific carrier proteins remove them
28
Q

What happens to lipid soluble molecules in the blood-brain barrier?

A
  • Lipid soluble molecules can diffuse through the lipid bilayer in endothelial cells
  • Specific carrier proteins remove them
29
Q

What is bulk flow?

A

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

30
Q

What are starling forces?

A

• The 2 main pressure affecting the movement of fluid in and out of the capillary:

  • Hydrostatic pressure
  • Oncotic pressure
31
Q

How does the pressure change across the capillaries and why?

A

• Higher pressure at the resistance vessels (arteriolar end)
• Pressure drops considerably at the venular end
• Oncotic pressure doesn’t change
• Balance of Starling forces determine movement in and out:
- Arteriolar end: hydrostatic > oncotic - ultrafiltration
- Venular end: oncotic > hydrostatic - reabsorption
• Some fluid might not return to the capillary (e.g. 9 mmHg leaves, 8 mmHg returns) and is drained by the lymphatic system

32
Q

How does the pressure change across the capillaries and why?

A
  • Higher pressure at the resistance vessels (arteriolar end)

* Pressure drops considerably at the venular end

33
Q

Where do you find lymphatic capillaries?

A

By normal blood capillaries

34
Q

Does the circulation of blood and the lymphatic system form a closed loop?

A
  • Circulation of blood forms a closed loop

* Lymphatic system doesn’t - they are blind ended

35
Q

How does fluid move through the lymphatic vessels?

A
  • Lymphatic pressure (skeletal muscles, respiratory pump)

* Valves ensure flow in one direction

36
Q

At which vessels does the lymph all come together and drain into?

A

Thoracic duct => right lymphatic duct => right subclavian vein => left subclavian vein

37
Q

How much fluid is returned to the circulatory system by the lymphatic system per day?

A

3L

38
Q

What can the parasitic blockage of lymph nodes lead to?

A
  • Oedema
  • rate of production of fluid > rate of removal of fluid
  • e.g. filariasis => elephantiasis

CVS (21 decks)

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