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FM1020 Physiology > Physiology of Capillaries and Veins > Flashcards

Physiology of Capillaries and Veins Flashcards

1
Q

What do we mean by microcirculation?

A

Arterioles, capillaries, venules

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2
Q

How many capillaries are in peripheral circulation and what is their total surface area?

A

~10 billion
500 - 700 m^2

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3
Q

What is the greatest distance a capillary can be to a cell?

A

20-30 microns

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4
Q

What is the blood flow in capillaries?

A

Slow
0.5-1.0 mm/sec

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5
Q

What is the total thickness of the capillary wall?

A

0.5 microns

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6
Q

What are the internal diameters of capillaries?

A

4-9 microns

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7
Q

What permits passage of ions and small molecules in continuous capillaries?

A

Intercellular clefts

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8
Q

What are continuous capillaries not permeable to?

A

Proteins

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9
Q

Where are continuous capillaries found?

A

Muscle, lungs, adipose tissue

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10
Q

Are fenestrated capillaries permeable to proteins?

A

Yes, but only small ones

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11
Q

Where are fenestrated capillaries found?

A

Kidney, endocrine gland and intestine - high-exchange rate

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12
Q

What are discontinuous capillaries also known as?

A

Sinusoidal capillaries

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13
Q

What do discontinuous capillaries allow to pass?

A

RBCs, WBCs, various serum proteins

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14
Q

Where are sinusoidal capillaries found?

A

Liver, spleen and bone marrow

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15
Q

How many times does a nutrient artery branch when entering an artery?

A

6-8 times

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16
Q

How many times do arterioles branch?

A

2-5 times

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17
Q

What do arterioles supply blood to?

A

Terminal arterioles

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18
Q

How do metarterioles branch from terminal arterioles?

A

At right angles

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19
Q

What are the distal sections of metarterioles devoid of smooth muscle called?

A

Thoroughfare or preferential channels

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20
Q

Where do capillaries branch from?

A

Terminal arterioles
Metarterioles

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21
Q

What are true capillaries?

A

Smooth muscle fibre encircles the capillary at its origin - precapillary sphincter

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22
Q

What is the role of the precapillary sphincter?

A

Regulates entry of blood into capillary

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23
Q

What do true capillaries do?

A

Delivery of nutrients to/removal of wastes from cells and deliver blood into venules

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24
Q

What do capillaries deliver?

A

Nutritional flow

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25
Q

Where do thoroughfare channels deliver blood to?

A

Directly into venules

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26
Q

What structures are present in some tissues such as fingertips and ears that bypass capillaries?

A

Arteriovenous shunts

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27
Q

What name is given to blood in the tissue where arteriovenous shunts are present?

A

Non-nutritional flow

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28
Q

What is the name used for capillary blood pressure?

A

Hydrostatic pressure

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29
Q

How does hydrostatic pressure increase?

A

Dilation of arterioles by increasing inflow
Increased venous pressure reduces outflow

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30
Q

What does a larger volume of blood in capillaries do?

A

Increase transmural pressure

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31
Q

What does increased transmural pressure trigger?

A

Contraction of terminal arterioless

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32
Q

What does decreased transmural pressure trigger?

A

Relaxation of terminal arterioles

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33
Q

What overrides local vasomotion?

A

Extrinsic neural regulation of upstream vessels

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34
Q

How does transcapillary exchange occur?

A

Diffusion
Filtration
Pinocytosis

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35
Q

Where is interstitial fluid in the body?

A

Lies outside cells and the circulatory system directly bathing the cells

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36
Q

What is the rate of diffusion determined by?

A

Fick’s Law

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37
Q

Define Fick’s Law.

A

J=PS(Co-Ci)
J = rate of diffusion
P = capillary permeability
S = capillary surface area
Co = concentration of substance outside capillary
Ci = concentration of substance inside capillary

38
Q

What is a limitation of diffusion at capillaries?

A

Flow-limited transport
Diffusion limited transport

39
Q

What is meant by flow-limited transport?

A

Small molecules pass rapidly through pores and accumulate at arterial end of capillary if blood flow is slow, opposite if it is fast
Larger molecules diffuse at slower rate so will diffuse further down the capillary

40
Q

What would increase supply of diffusible solutes in capillaries?

A

Increase in blood flow
Increase in capillary density

41
Q

Explain diffusion limited transport.

A

Molecules > 60000 kDa cannot diffuse
If capillary density is low, or during oedema, increased distance between capillaries and tissue may limit efficient exchange

42
Q

Where in the capillaries, which side has a higher density of pores and increased permeability?

A

Venous end

43
Q

What may density of pores depend on?

A

Tissue type
Fenestrated and discontinuous capillaries have increased permeability

44
Q

What molecules may diffuse through endothelial cell walls without the presence of pores?

A

Lipid-soluble molecules

45
Q

What determines the direction and rate of movement of lipid-soluble molecules?

A

Relative values of PO2 and PCO2 in blood and interstitial fluid

46
Q

What is filtration at capillaries?

A

Bulk movement of fluid through the pores in capillaries into interstitial area

47
Q

Why does a tissue gel form in the interstitial area?

A

Due to the proteoglycan content of extracellular environment

48
Q

What determines direction and magnitude of movement of fluid between capillaries and interstitium?

A

Hydrostatic forces
Oncotic forces

49
Q

What raises hydrostatic pressure?

A

Vasodilation of arterioles
Constriction of venules

50
Q

What is capillary blood pressure dependent on?

A

Arterial and venous pressures

51
Q

What is the average capillary hydrostatic pressure at both arterial and venous end of capillaries?

A

arterial - 32mmHg
venous - 15 mmHg

52
Q

What is the negligible force that opposes filtration?

A

Interstitial hydrostatic force

53
Q

What is the capillary wall semi-permeable to?

A

Proteins

54
Q

What determines the rate of water movement?

A

Osmotic pressure in mmHg

55
Q

Is the protein concertation of tissue fluid or plasma greater?

A

Plasma

56
Q

What is the name of the osmotic pressure that promotes movement of water out of tissue fluid into capillaries?

A

Colloid osmotic pressure or oncotic pressure

57
Q

What is the colloid osmotic pressure of plasma?

A

25 - 28 mmHg

58
Q

What does oncotic pressure do?

A

Oppose hydrostatic pressure

59
Q

What are opposing hydrostatic and oncotic pressures collectively known as?

A

Starling forces

60
Q

What is the direction of fluid movement determined by?

A

Starling equation

61
Q

Where does filtration and absorption occur at an idealised capillary and why?

A

Arterial end - filtration
Venous end - absorption
Due to drop in hydrostatic pressure along capillary

62
Q

Why do hydrostatic and oncotic pressures usually equilibrate quickly in most capillaries?

A

Due to rapid movement of water

63
Q

In the entire capillary circulation does net filtration or net reabsorption occur?

A

Net filtration

64
Q

In what form is net filtration returned to vasculature?

A

Lymph fluid

65
Q

Name a tissue where only filtration occurs.

A

Renal glomerulus

66
Q

Name a tissue where only reabsorption occurs.

A

Intestinal

67
Q

What is the determining factor for Starling forces?

A

Magnitude of Pc (capillary pressure)

68
Q

What is pinocytosis?

A

Movement of fluid across endothelial cells by vesicles

69
Q

What is pinocytosis mostly responsible for?

A

Exchange of large molecules

70
Q

Where do lymph ducts drain?

A

Left and right subclavian veins

71
Q

How does interstitial fluid and suspended particles enter the lymphatic system?

A

One-way valves created by adjoining endothelial cells

72
Q

What is the rate of total lymph production?

A

120ml/hr

73
Q

Where is two thirds of all lymph derived from?

A

Liver and intestinal capillaries - leaky sinusoidal capillaries

74
Q

What is oedema?

A

Excess fluid accumulation in extracellular spaces

75
Q

What underlying causes give rise to oedema?

A

Excessive capillary filtration and/or poor lymphatic drainage

76
Q

How does oedema occur in the Starling equation?

A

Increased capillary filtration coefficient
Increased capillary hydrostatic pressure - NB
Decreased plasma oncotic pressure

77
Q

What clinical causes lead to oedema?

A

Increased blood pressure or venous obstruction
Increased tissue protein concentration
Decreased plasma protein concentration
Obstruction of lymphatic vessels

78
Q

What is a common form of oedema?

A

Pulmonary oedema

79
Q

When is pulmonary oedema usually caused?

A

Left-ventricular failure/congenitive heart failure

80
Q

How does congenitive heart failure lead to pulmonary oedema?

A

Build-up of blood in pulmonary circulation as left ventricle backs up blood on the left side of the ehart

81
Q

Is pulmonary oedema serious?

A

Yes, as excess fluid interferes with gas exchange

82
Q

When would blood be transferred from veins to arterial circulation?

A

Exercise
Haemorrhage

83
Q

What is central venous pressure?

A

Pressure of blood in right atrium

84
Q

What does a reduction in CVP do?

A

Increase venous return

85
Q

What increases overall venous pressure?

A

Increase in blood volume
Reduction in compliance

86
Q

What is the drop in arterial pressure that causes a sense of dizziness when going from a lying down to a standing position?

A

Orthostatic hypotension

87
Q

What immediately corrects orthostatic hypotension?

A

Baroreceptor reflex

88
Q

What causes the venous pump?

A

Muscular activity in legs that compresses veins (reducing compliance)

89
Q

What happens in individuals with prolonged increases in venous pressure stretch and damage venous valves?

A

Varicose veins

90
Q

What happens on standing and even following muscular activity of individuals with varicose veins?

A

Downward movement of blood into ankles
Venous and capillary pressures become very high
Leakage of fluid from capillaries causes constant oedema and swelling in the feet

FM1020 Physiology (12 decks)

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