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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
Describe the structure of a discontinuous capillary
Endothelial gaps over 100nm wide Disrupted basal lamina
26
What are discontinuous capillaries for?
Allow free movement of macromolecules and cells
27
Where are discontinuous capillaries found?
Liver Spleen Bone marrow
28
How do lipophilic molecules permeate the capillary wall?
Easily diffuse across endothelial cells
29
How do small lipophobic molecules permeate the capillary wall?
Fenestrae Intercellular gaps
30
How do large lipophobic molecules permeate the capillary wall?
Wide intercellular gaps formed during inflammation Transcytosis Trans-endothelial channels
31
What substances move across the capillary wall the fastest?
Gases
32
What percentage of water passes through intercellular gaps and water channels?
90% intercellular gaps 10% water channels
33
What is filtration?
Movement of water across capillary wall
34
How much plasma is pumped through capillaries each day?
4000L
35
What net volume of plasma leaves the circulation each day?
8L
36
How is the volume of plasma that leaves the circulation returned to the bloodstream?
Lymphatic system
37
What is the primary force driving plasma filtration?
Hydrostatic pressure gradient
38
In which direction does the hydrostatic pressure gradient tend to drive water?
Out of capillaries
39
How do you calculate the hydrostatic pressure gradient?
Pcap - Pint
40
What is the primary force retaining fluid within capillaries?
Oncotic osmotic pressure gradient
41
What is the oncotic osmotic pressure gradient caused by?
Large plasma proteins
42
How do you calculate the osmotic pressure gradient?
σ(πcap - πint)
43
What is σ in relation to the osmotic pressure gradient?
Reflection coefficient for plasma which corrects for endothelium as an imperfect barrier
44
What is the range of values for σ of plasma?
0.8-0.95
45
In which direction does the oncotic osmotic pressure gradient tend to drive water?
Into capillaries
46
What is Starling's equation for the net fluid of movement?
Jv ∝ (Pcap - Pint) - σ(πcap - πint)
47
If the Jv calculated is positive, which direction is water moving?
Out of capillaries
48
If the Jv calculated is negative, which direction is water moving?
Into capillaries
49
Which pressure gradient does not normally change along a capillary and why?
Oncotic osmotic Proteins cannot leave capillaries
50
What happens when you have been standing for a long time?
Increased Pcap in lower extremities due to gravity Filtration transiently increased Reflexes quickly cause arteriole constriction Pcap reduced
51
What happens in the splanchnic circulation during exercise? (same as following major tissue injury and blood loss)
Local arteriolar constriction to reduce Pcap Reduced filtration Local net fluid absorption
52
Describe the flow in lymphatic vessels
Unidirectional (larger and blunted vessels) Driven by periodic compression
53
Describe the structure of a lymphatic capillary
Continuous, overlapping endothelium Interrupted basal lamina Anchoring filaments including elastin
54
Why does fluid readily move into lymphatic capillaries?
Lots of protein in lymphatic vessels = high internal oncotic osmotic pressure
55
What prevents backflow in lymphatic capillaries?
Valves
56
Where in the circulatory system do the lymphatic ducts drain?
Where subclavian and internal jugular veins meet
57
When does oedema occur?
When amount of fluid leaving microcirculation increases
58
Why might the amount of tissue fluid formed increase?
Increased capillary hydrostatic pressure Decreased capillary oncotic osmotic pressure Blocked lymphatic system
59
What may cause an increased capillary hydrostatic pressure gradient and how?
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
What may cause a decreased oncotic osmotic pressure gradient and how?
Inflammation (histamine) Local vasodilatation and increased vascular permeability allows proteins to leave Increased πint so decreased oncotic osmotic pressure gradient
61
What may cause a blockage in the lymphatic system and how?
Elephantiasis: Nematode worm/filariasis blocks nodes

BMS10 (13 decks)

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