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BMS10 > Microcirculation & Capillary Filtration > Flashcards

Microcirculation & Capillary Filtration Flashcards

1
Q

What vessels make up the microcirculation?

A

Terminal arterioles

Capillaries

Post-capillary venules

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2
Q

What occurs in the microcirculation?

A

Exchange of gases, fluids, nutrients and waste materials

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3
Q

What do terminal arterioles do?

A

Control flow through capillaries

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4
Q

What stimulates the smooth muscle in terminal arterioles?

A

Local factors

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5
Q

In which vessels does exchange occur in the microcirculation?

A

Capillaries

Post-capillary venules

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6
Q

How long is a typical capillary?

A

500 - 1000 um (<1mm)

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7
Q

How wide is a typical capillary?

A

4 - 8 um

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8
Q

Where is the smooth muscle in the microcirculation?

A

Terminal arterioles

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9
Q

What can alter the lumen diameter of continuous capillaries?

A

Pericytes

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10
Q

What do lymphatic vessels do?

A

Absorb fluid and protein and return them to the blood

Take up and transport micro-organisms to lymph nodes

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11
Q

What is vasomotion?

A

Terminal arterioles constrict and relax periodically due to transient and intermittent depolarisation of smooth muscle cells

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12
Q

What does heterogeneity in perfusion mean?

A

Blood flow is not uniform in all vessels in a bed

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13
Q

How can uniformity in blood flow through a bed be increased?

A

Decreased arterial tone/vasodilatation

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14
Q

How does vasodilatation affect transit time?

A

Decreased

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15
Q

How is smooth muscle arranged around the vessel lumen?

A

Concentrically

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16
Q

How are endothelial cells arranged in capillaries?

A

Aligned with direction of flow due to shear stress/laminar flow

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17
Q

Why is the arrangement of endothelial cells in capillaries beneficial?

A

Helps stimulate production of local mediators (for regulation)

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18
Q

Describe the structure of a continuous capillary

A

Continuous thin layer of 1-3 endothelial cells surrounded by a basement membrane

Gap and tight junctions

Pericytes

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19
Q

Why is exchange efficient in capillaries?

A

Short transcapillary diffusion distance ~0.3um

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20
Q

Where are continuous capillaries found? (6)

A

Lungs

Skeletal muscle

Myocardium

Skin

Connective tissue

Fat

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21
Q

Describe the structure of a fenestra

A

Basal lamina and basal membrane of endothelial cells come together to form fenestrae

Bridged by a fenestrae diaphragm due to continuous basal lamina

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22
Q

How large are fenestrae?

A

50-60nm

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23
Q

What are fenestrated capillaries for?

A

Rapid movement of macromolecules and fluid

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24
Q

Where fenestrated capillaries found?

A

Kidneys

Intestinal mucosa

Some endocrine glands

Joints

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25
Q

Describe the structure of a discontinuous capillary

A

Endothelial gaps over 100nm wide

Disrupted basal lamina

26
Q

What are discontinuous capillaries for?

A

Allow free movement of macromolecules and cells

27
Q

Where are discontinuous capillaries found?

A

Liver

Spleen

Bone marrow

28
Q

How do lipophilic molecules permeate the capillary wall?

A

Easily diffuse across endothelial cells

29
Q

How do small lipophobic molecules permeate the capillary wall?

A

Fenestrae

Intercellular gaps

30
Q

How do large lipophobic molecules permeate the capillary wall?

A

Wide intercellular gaps formed during inflammation

Transcytosis

Trans-endothelial channels

31
Q

What substances move across the capillary wall the fastest?

A

Gases

32
Q

What percentage of water passes through intercellular gaps and water channels?

A

90% intercellular gaps

10% water channels

33
Q

What is filtration?

A

Movement of water across capillary wall

34
Q

How much plasma is pumped through capillaries each day?

A

4000L

35
Q

What net volume of plasma leaves the circulation each day?

A

8L

36
Q

How is the volume of plasma that leaves the circulation returned to the bloodstream?

A

Lymphatic system

37
Q

What is the primary force driving plasma filtration?

A

Hydrostatic pressure gradient

38
Q

In which direction does the hydrostatic pressure gradient tend to drive water?

A

Out of capillaries

39
Q

How do you calculate the hydrostatic pressure gradient?

A

Pcap - Pint

40
Q

What is the primary force retaining fluid within capillaries?

A

Oncotic osmotic pressure gradient

41
Q

What is the oncotic osmotic pressure gradient caused by?

A

Large plasma proteins

42
Q

How do you calculate the osmotic pressure gradient?

A

σ(πcap - πint)

43
Q

What is σ in relation to the osmotic pressure gradient?

A

Reflection coefficient for plasma which corrects for endothelium as an imperfect barrier

44
Q

What is the range of values for σ of plasma?

A

0.8-0.95

45
Q

In which direction does the oncotic osmotic pressure gradient tend to drive water?

A

Into capillaries

46
Q

What is Starling’s equation for the net fluid of movement?

A

Jv ∝ (Pcap - Pint) - σ(πcap - πint)

47
Q

If the Jv calculated is positive, which direction is water moving?

A

Out of capillaries

48
Q

If the Jv calculated is negative, which direction is water moving?

A

Into capillaries

49
Q

Which pressure gradient does not normally change along a capillary and why?

A

Oncotic osmotic

Proteins cannot leave capillaries

50
Q

What happens when you have been standing for a long time?

A

Increased Pcap in lower extremities due to gravity

Filtration transiently increased

Reflexes quickly cause arteriole constriction

Pcap reduced

51
Q

What happens in the splanchnic circulation during exercise? (same as following major tissue injury and blood loss)

A

Local arteriolar constriction to reduce Pcap

Reduced filtration

Local net fluid absorption

52
Q

Describe the flow in lymphatic vessels

A

Unidirectional (larger and blunted vessels)

Driven by periodic compression

53
Q

Describe the structure of a lymphatic capillary

A

Continuous, overlapping endothelium

Interrupted basal lamina

Anchoring filaments including elastin

54
Q

Why does fluid readily move into lymphatic capillaries?

A

Lots of protein in lymphatic vessels = high internal oncotic osmotic pressure

55
Q

What prevents backflow in lymphatic capillaries?

A

Valves

56
Q

Where in the circulatory system do the lymphatic ducts drain?

A

Where subclavian and internal jugular veins meet

57
Q

When does oedema occur?

A

When amount of fluid leaving microcirculation increases

58
Q

Why might the amount of tissue fluid formed increase?

A

Increased capillary hydrostatic pressure

Decreased capillary oncotic osmotic pressure

Blocked lymphatic system

59
Q

What may cause an increased capillary hydrostatic pressure gradient and how?

A

Congestive heart failure

Increased CVP due to decreased cardiac output and/or fluid retention

Systemic and pulmonary Pcap increases

Increased filtration overwhelms lymphatic clearance

60
Q

What may cause a decreased oncotic osmotic pressure gradient and how?

A

Inflammation (histamine)

Local vasodilatation and increased vascular permeability allows proteins to leave

Increased πint so decreased oncotic osmotic pressure gradient

61
Q

What may cause a blockage in the lymphatic system and how?

A

Elephantiasis:

Nematode worm/filariasis blocks nodes

BMS10 (13 decks)

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