CVS > 5. Microcirculation > Flashcards

5. Microcirculation Flashcards

(38 cards)

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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)

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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?

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A
  • 93 mmHg entering

* 37 mmHg leaving

18
Q

When is capillary density important?

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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?

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

When is capillary density important?

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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?

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A

Nothing that the adipose tissue does that needs a rapid response

22
Q

What is special about the perfusion of the skeletal muscle?

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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?

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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?

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A
  • Large holes in the capillary between the endothelial cells

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

25
What are the features of fenestrated capillaries?
* Leakier capillaries * Slightly bigger holes - fenestrae (around 80nM) * Allow larger substances to pass e.g. glomerulus
26
What type of capillary do you mainly find as part of the blood-brain barrier and why?
(• 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
What happens to lipid soluble molecules in the blood-brain barrier?
* Lipid soluble molecules can diffuse through the lipid bilayer in endothelial cells * Specific carrier proteins remove them
28
What happens to lipid soluble molecules in the blood-brain barrier?
* Lipid soluble molecules can diffuse through the lipid bilayer in endothelial cells * Specific carrier proteins remove them
29
What is bulk flow?
A volume of protein-free plasma that filters out of the capillary, mixes with the surrounding interstitial fluid (IF) and is reabsorbed
30
What are starling forces?
• The 2 main pressure affecting the movement of fluid in and out of the capillary: - Hydrostatic pressure - Oncotic pressure
31
How does the pressure change across the capillaries and why?
• 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
How does the pressure change across the capillaries and why?
* Higher pressure at the resistance vessels (arteriolar end) | * Pressure drops considerably at the venular end
33
Where do you find lymphatic capillaries?
By normal blood capillaries
34
Does the circulation of blood and the lymphatic system form a closed loop?
* Circulation of blood forms a closed loop | * Lymphatic system doesn't - they are blind ended
35
How does fluid move through the lymphatic vessels?
* Lymphatic pressure (skeletal muscles, respiratory pump) | * Valves ensure flow in one direction
36
At which vessels does the lymph all come together and drain into?
Thoracic duct => right lymphatic duct => right subclavian vein => left subclavian vein
37
How much fluid is returned to the circulatory system by the lymphatic system per day?
3L
38
What can the parasitic blockage of lymph nodes lead to?
* Oedema * rate of production of fluid > rate of removal of fluid * e.g. filariasis => elephantiasis
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