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MRCEM Primary > Peripheral vascular physiology > Flashcards

Peripheral vascular physiology Flashcards

1
Q

What is the rate of cerebral blood flow?

A

750ml per minute

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2
Q

What percentage of the cardiac output does the brain receive per minute?

A

15-20%

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3
Q

In what 3 ways is the cerebral circulation adapted for it’s specialised nature?

A

Arterial anastamoses
BBB
Autoregulatory mechanisms

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4
Q

Why is the circle of Willis important?

A

Because this anastomosis provides collateral blood supply which is protective against cerebral ischaemia

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5
Q

Which 3 paired vessels are the main constituents of the circle of willis?

A

Anterior referral arteries
Internal carotid arteries
Posterior cerebral arteries

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6
Q

Which vessels connect the 3 paired vessels together to complete the circle of willis?

A

Anterior and posterior communicating arteries

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7
Q

What is the blood brain barrier?

A

A high selective permeable barrier that separates blood from the brain ECF in the CNS

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8
Q

Which 2 properties of the brain endothelium allow the BBB to be effective?

A

Tight junctions that prevent paracellular movement of molecules

AND

No transendothelial pathways (i.e. intracellular vesicles)

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9
Q

What is the BB permeable to?

A

Water
Non-ionised lipid soluble substances (barbiturates, ethanol, caffeine)

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10
Q

What is the BBB impermeable to?

A

Proteins
Protein-bound substances (drugs, hormones)
Strongly hydrophilic substances (Na/ K)
Most bacteria, antibodies and antibiotics (too large to cross BBB)

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11
Q

What allows molecules with low lipid solubility to move across the BBB?

A

Carrier-mediated transport.

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12
Q

How does glucose cross the BBB and enter the brain?

A

Enters the brain via the GLUT-1 transport protein.

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13
Q

What are the main auto regulatory mechanisms that help to regulate cerebral blood flow?

A

Myogenic and metabolic autoregulation

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14
Q

What is myogenic auto regulation?

A

A mechanism that regulates local blood flow to the brain by allowing changes in the diameter of blood vessels in response to changes in BP.

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15
Q

What does a fall in blood pressure do to vessels in the brain?

A

Compensatory vasodilation of blood vessels = increased blood flow

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16
Q

At what blood pressure does myogenic auto regulation fail?

A

50mmHg (blood vessels cannot dilate any further at this point)

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17
Q

What is metabolic auto regulation?

A

A mechanism that helps to maintain local blood flow to the brain by allowing compensatory changes in blood vessel diameter in response to changes in the partial pressure of arterial CO2.

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18
Q

What does metabolic auto regulation respond to?

A

CO2

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19
Q

What does myogenic auto regulation respond to?

A

Blood pressure

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20
Q

What does hypercapnia cause to happen to vessels in the brain?

A

Vasodilation

**Hypercapnia is indicative of high metabolic demand = increased blood flow needed

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21
Q

What does hypocapnia cause to happen to vessels in the brain?

A

Vasoconstriction

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22
Q

An increase in which ion causes vasodilation in the cerebral circulation?

A

Potassium

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23
Q

How is the pulmonary circulation adapted to facilitate consistent gas exchange for high volumes of blood?

A

Anatomical adaptations
Short diffusion distance
Hypoxic pulmonary vasoconstriction

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24
Q

Give the 3 anatomical adaptations of pulmonary circulation that allow for effective gas exchange.

A

Low pressure system
Low resistance system
Large surface area for gas exchange

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25
Q

Why are pressures within pulmonary circulation less than pressures within systemic circulation?

A

Because pulmonary arteries have thin vascular walls and high compliance

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26
Q

What is the MAP within pulmonary circulation?

A

5-15mmHg

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27
Q

Give the 3 reasons as to why pulmonary circulation has low resistance.

A

Pulmonary vessels are shorter and wider
Pulmonary capillaries run in parallel, not in series
Relatively little smooth muscles in arterials = reduced arterial tone

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28
Q

What allows for there to be a large surface area for gas exchange in the lungs?

A

The branching structure of the tracheobronchial tree

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29
Q

What is the combined thickness of the alveolar and capillary endothelium?

A

0.3 micrometres

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30
Q

What is the main intrinsic regulator of pulmonary blood flow?

A

The local partial pressure of alveolar oxygen

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31
Q

What does a low alveolar partial pressure of O2 cause?

A

Vasoconstriction of arterioles

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32
Q

What is the hypoxic pulmonary vasoconstriction (HPV) reflex?

A

Vasoconstriction of arterioles at low alveoli partial pressures of oxygen

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33
Q

What does the HPV reflex facilitate?

A

Diversion of blood flow away from poorly ventilated alveoli and towards well-ventilated alveoli to maximise gaseous exchange

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34
Q

When does maximal hypoxic vasoconstriction occur?

A

At 70mmHg (at a normal pH)

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35
Q

What percentage of total body weight does skeletal muscle make up?

A

50%

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36
Q

What percentage of cardiac output at rest is directed towards skeletal muscle?

A

15-20%

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37
Q

What percentage of cardiac output can skeletal muscle receive during exercise?

A

Can rise to over 80%

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38
Q

Blood flow is tightly matched to what in skeletal muscle?

A

Metabolic rate

39
Q

What causes most skeletal capillaries to be closed off and therefore not perfused during rest?

A

Contraction of pre-capillary sphincters

40
Q

What does contraction of pre-capillary sphincters and lack of capillary perfusion cause?

A

Higher vascular tone and constriction of vessels

41
Q

What are capillaries recruited by during exercise?

A

Metabolic hyperaemia

42
Q

What is metabolic hyperaemia caused by?

A

Release of K+, CO2 and adenosine

43
Q

Which type of muscle plays an important part in helping venous blood return to the heart?

A

Skeletal muscle

44
Q

What structural aspect of veins helps in return of blood to the heart?

A

One-way valves that act as pumps during muscle activity

**This forces blood that would otherwise pool in lower limbs back to the heart

45
Q

What is cutaneous circulation involved in?

A

Bloods supply of the skin

46
Q

What is the primary function of the cutaneous circulation?

A

Thermoregulation

47
Q

Which aspect of cutaneous circulation is involved in temperature regulation?

A

Arteriovenous anastomoses (AVAs)

48
Q

What are arteriovenous anastomoses?

A

Short vessels that connect small arteries and veins.

49
Q

What happens when AVAs are open?

A

They provide a low resistance connection between arteries and veins = blood is shunted into superficial venous complexes of limbs

50
Q

What are AVAs innervated by?

A

Adrenergic fibres from the hypothalamus (temperature regulation centre)

51
Q

What happens to AVAs at normal core temperatures?

A

High sympathetic output = vasoconstriction of AVAs = blood flows through capillary networks and deep plexuses

52
Q

What happens to AVAs at raised core temperatures?

A

Reduced sympathetic output = AVA vasodilation = shunting of blood to superficial venous plexus = loss of heat via radiation to the environment

53
Q

Where are AVAs found in the body?

A

In high numbers in peripheral regions:

Fingers
Palms
Soles
Lips
Pinna of the ear

54
Q

The exchange of fluids across the capillary membrane is mainly governed by what?

A

Oncotic and hydrostatic pressures

55
Q

What are the 4 pressure componenet stat effect exchange of fluids across the capillary membrane?

A

Interstitial hydrostatic pressure
Capillary hydrostatic pressure
Interstitial osmotic pressure
Capillary osmotic pressure

56
Q

What happens to osmotic pressures along the course of the capillary?

A

They are consistent

57
Q

What is the overall effect of oncotic pressure along the capillary?

A

It drives water into the capillary

58
Q

What happens to hydrostatic pressure along the course of the capillary?

A

Direction of new flow changes due to the change in capillary hydrostatic pressure from arterials to venous end.

59
Q

What is interstitial hydrostatic pressure?

A

Negligible (close to 0)

60
Q

What is capillary hydrostatic pressure at the arterial end?

A

35mmHg

61
Q

What is capillary hydrostatic pressure at the venous end?

A

15mmHg

62
Q

At the arterial end of a capillary, what is the net movement of fluid?

A

Movement of water out of the capillary

63
Q

At the venous end of a capillary, what is the net movement of fluid?

A

Absorption of fluid into capillary

64
Q

In what direction does water flow under oncotic pressure?

A

Low to high

65
Q

In what direction does water flow under hydrostatic pressure?

A

High to low

66
Q

Which protein mainly creates oncotic pressure?

A

Albumin

67
Q

Why does oncotic pressure cause water to move down the gradient?

A

Because proteins displace water, creating a relative water deficit, drawing water towards them

68
Q

What is hydrostatic pressure?

A

Pressure of blood against vessel walls.

69
Q

How does fluid filtered by microcirculation return to the blood circulation?

A

Via lymphatics

70
Q

What are lymphatic capillaries?

A

Blind ending bulbous tubes that permit entry of fluid, proteins and bacteria but do not allow their exit

71
Q

How is movement of lymph facilitated in vessels?

A

Via valves that prevent back flow

AND

Via smooth muscle that contracts to push lymph through the lymphatic system

72
Q

How is fluid returned to circulation from lymph nodes?

A

Via efferent vessels and then the thoracic duct.

73
Q

What process happens in microcirculation?

A

Exchange between blood and tissues

74
Q

How is flow through microcirculation regulated?

A

By vasoconstriction of arterioles

75
Q

Which molecules are able to pass through capillary walls easily along their diffusion gradient?

A

Lipophillic molecules (oxygen, CO2)

76
Q

What is the least permeable type of capillary?

A

Continuous

77
Q

Where are continuous capillaries found?

A

Skin
Fat
Muscle
Nervous tissue (incl. BBB)

78
Q

Where are fenestrated capillaries found?

A

GI tract
Joints
Kidneys

79
Q

What are the most permeable type of capillaries?

A

Sinusoidal (discontinuous)

80
Q

Where are sinusoidal (discontinuous) capillaries found?

A

Bone marrow
Liver
Spleen

**locations where red cells need to cross the capillary wall

81
Q

Most vasoconstrictors bind to what protein?

A

G-protein

82
Q

What is G-protein?

A

An important mediator in vasoconstriction

83
Q

What does the G-protein mediator cause?

A

A rise in intracellular calcium ions = smooth muscle contraction.

84
Q

What does a rise in intracellular calcium ions cause?

A

Smooth muscle contraction

85
Q

Name 3 important vasoconstrictors.

A

Endothelin
NAD
Angiotensin II

86
Q

Name the 2 endothelium derived vasodilators.

A

NO
Prostacyclin

87
Q

Name the 2 endothelium derived vasoconstrictors.

A

Endothelin-1
Thromboxane A2

88
Q

Give the 5 mechanisms by which endothelial release of NO is triggered.

A

Bradykinin
Histamine
Serotonin
Substance P
Increased blood flow (shear stress)

89
Q

How do vasodilators work?

A

By lowering concentrations of intracellular calcium ions

90
Q

How is lowering intracellular calcium levels normally achieved?

A

Stimulation of SERCA and PMCA

AND

Hyperpolarisation the cell membrane to inhibit L-type Ca2+ channels

91
Q

How does the smooth endoplasmic reticulum Ca2+ ATPase low intracellular calcium levels?

A

Sequesters calcium ions into the smooth ER

92
Q

How does the plasma membrane CA2+ ATPase lower intracellular calcium levels?

A

Pumps calcium out of the cell

93
Q

How are SERCA and PMCA stimulated?

A

Via secondary chemical messengers such as cGMP and cAMP.

94
Q

Endothelin-1 release from the endothelium can be stimulated by which 4 factors?

A

Angiotensin II
ADH
NAD
Hypoxia

MRCEM Primary (39 decks)

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