Special Circulations Flashcards

1
Q

What is the format of the systemic circulation

A

,a number of circulations in parallel (cerebral, coronary, skeletal, cutaneous, others)

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2
Q

Describe the blood supply to the lungs

A

• The lungs have two circulations
• Bronchial circulation
– part of systemic circulation
– meets the metabolic requirements of the lungs that are not easily accessible by pulmonary circulation eg trachea

• Pulmonary circulation
– blood supply to alveoli
– required for gas exchange

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3
Q

Is the pulmonary circulation in series or parallel with systemic

A

Series , Pulmonary circulation has to accept entire cardiac output

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4
Q

What is cardiac output at rest and maximum

A
• Cardiac output at rest
~ 5 l/min
• Maximum cardiac
output ~ 20 -25 l/min
(non athlete)
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5
Q

Describe the pressure and resistance of the pulmonary circulation

A

Low pressure and low resistance in order to accept the full output
Pressure of pulmonary artery in systole - same as RV in systole - but slightly higher diastolic bc of elastic recoil
Pressure in arteries doesnt drop low in diastole bc of recoil

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6
Q

Describe the pressure in the pulmonary circulation

A

Low pressure
– mean arterial pressure  12-15mmHg
– mean capillary pressure  9-12mmHg
– mean venous pressure  5mmHg

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7
Q

Why does the pulmonary cirulation have low resistance

A

Low resistance
– short, wide vessels
– lots of capillaries (many parallel elements) - reduces resistance
– arterioles have relatively little smooth muscle

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8
Q

What are teh adaptations of the pulmonary circulation for gas exchange

A

• Very high density of capillaries in
alveolar wall
– large capillary surface area • Short diffusion distance
– very thin layer of tissue separating gas phase from plasma
- mobiles Endo and epithelium thickness is 0.3um
• Large surface area and short
diffusion distance produce high
O2 and CO2 transport capacity

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9
Q

What is the V/Q ratio

A

Ventilation – Perfusion ratio (V/Q ratio)
• For efficient oxygenation - need to match ventilation of alveoli with perfusion of alveoli
• Optimal V/Q ratio = 0.8 (ventilatare of 4l/m, output of 5 l/m)
• Maintaining this means diverting blood from alveoli which are not well ventilated
Ventilation – Perfusion ratio (V/Q ratio)
• For efficient oxygenation - need to match
ventilation of alveoli with perfusion of alveoli
• Optimal V/Q ratio = 0.8
• Maintaining this means diverting blood from
alveoli which are not well ventilated

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10
Q

What is hypoxia pulmonary vasoconstriction

A

Hypoxic pulmonary vasoconstriction ensures optimal ventilation/perfusion ratio
• Most important mechanism regulating pulmonary
vascular tone
• Alveolar hypoxia results in vasoconstriction of pulmonary
vessels - opposite to in systemic
• Ensures that perfusion matches ventilation
• Poorly ventilated alveoli are less well perfused
• Helps to optimise gas exchange

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11
Q

What is the downside to chronic hypoxia vasoconstriction

A

Chronic hypoxia can occur at altitude or as a
consequence of lung disease such as emphysema.
– chronic increase in vascular resistance
- chronic pulmonary hypertension
– high afterload on right ventricle - can lead to right ventricular heart failure
Chronic disease such as emphysema - increased vascular resistance - chronic pulmonary hypertension - RV not work as hard to pump around pul mreistance - increase resistance - RV has to work harder- hypertrophy of RV - lad to right sided heart failure - rewatch - consequence of long term hypertension - it rarely occurs on its own (usually a consequence of left - on its own w/ lung disease that increases resistcance)

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12
Q

How is the pressure affected by. Gravity

A

Low pressure pulmonary vessels are strongly influenced by gravity
• In the upright position (orthostasis) there is greater hydrostatic
pressure on vessels in the lower part of the lung
Apex of lung - vessels collapse during diastole vice versa systole
Level of heart - vessels continuously patent
Base - vessels distended (increased hydrostatic pressure)

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13
Q

What is the effect of excercise on pulmonary blood flow?

A

• Increased cardiac output • Small increase in pulmonary arterial pressure
• Opens apical capillaries
• Increased O2 uptakeby lungs
• As blood flow increases capillary transit time is reduced
– at rest transit time ~ 1s
– can fall to ~ 0.3s without compromising gas exchange

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14
Q

How is tissue fluid formed

A

Starling forces - at arterial end hydrostatic pressure greater than plasma oncotic pressure - fluid pushed out
Increases in venous pressure tend to increase the hydrostatic pressure
Increases in art rssure in systemic dont have much affect on capillary hydrostatic pressure
Heart failure - peripheral oeadema (increased venous pressure)

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15
Q

What minimises the formation of lung lymph

A

Low capillary pressure
Oncotic pressure of tissue fluid in lungs > than in periphery
Capillary hydrostatic pressure in lung < than systemic capillaries
Plasma oncotic pressure is the same

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16
Q

What happens if capillary prssure is increased

A

More fluid filter out - pulmonary oesdema

Lungs are more sensitive to increases in art and venous pressure when it comes to oedema )both)

17
Q

How does capillary pressure affect oedema

A

• Pulmonary capillary pressure is normally low (9 - 12mmHg)
– only a small amount of fluid leaves the capillaries (lung lymph)
• Can get pulmonary oedema if capillary pressure increases
– if the left atrial pressure rises to 20 - 25 mmHg
• Mitral valve stenosis - harder for blood to flow from la to LV - increase in la pressure - pvd hve higher pressure - that causes pulmonary oedema
• Left ventricular failure

18
Q

How can pulmonary oedema affect gas exchange

A

• Pulmonary oedema impairs gas exchange
– Affected by posture (changes in hydrostatic pressure due to gravity)
– Forms mainly at bases when upright
– Forms throughout lung when lying down
• Use diuretics to relieve symptoms
• Treat underlying cause if possible
When patient lies Down - similar distension throughout - oedema throughout - also more of a return from systemic - less blood pooled in veins - overall increase in pressure - often have many pillows - diuretic reduce blood volume

19
Q

Describe the o2 consumption of th brain

A
• The brain has a high O2 demand  
• Receives about 15% of cardiac outpud
– but only accounts for 2% of body mass
• O2 consumption of grey matter accounts for ~20% of total body consumption at rest 
• Must provide a secure O2
supply
20
Q

How does the cerebral circulation meet the high demand for O2?

A
• high capillary density
– large surface area for gas exchange 
– reduced diffusion distance (<10μm)
• high basal flow rate
– X10 average for whole body 
• high O2 extraction
– 35% above average
21
Q

Why is a secure o2 supply to the brain vital

A

• Neurones are very sensitive to hypoxia
• Loss of consciousness after a few seconds of cerebral
ischaemia
• Begin to get irreversible damage to neurones in ~ 4 minutes
• Interruption to blood supply e.g. stroke causes neuronal death

22
Q

How is a secure blood supply ensured?

A

• Structurally
– anastomoses between basilar and internal carotid arteries
• Functionally
– myogenic autoregulation maintains perfusion during
hypotension
– metabolic factors control blood flow
– brainstem regulates other circulations

23
Q

What is myogenic autoregualion

A

• Cerebral resistance vessels have a well developed myogenic (from smooth muscle cells) response respond to changes in transmural pressure
- increase pressure causes vasoconstriction to maintain blood flow
- if blood pressure decreases vessels ill vasodilation to maintain blood flow
• Serves to maintain cerebral blood flow when BP changes
• Fails below 50mmHg
- cerebral blood flow doesnt change much with map bc of myogenic response - see graph

24
Q

Describe metabolic regulation of cerebral blood flow

A

Hyper apnea - is pco2 high - vasodilation
Hypocapnia - low pco2 - vasoconstriction
Cerebral vessels very sensitive to changes in arterial pco2
Panic hyperventilation can cause hypocapnia and
cerebral vasoconstriction leading to dizziness or fainting

25
Q

How does regional activity prince local increase in blood flow

A

Areas with increased neuronal activity have increased blood flow.
Increased PCO2, increased [K+], increased adenosine, decreased in pO2 - all lead to vasodilation
Adenosine is a powerful vasodilator of cerebral arterioles
Different regions of the brain can have differet blood flowed pending on activity

26
Q

What is cushings reflex?

A

• Rigid cranium protects the brain
– but does not allow for volume expansion
• Increases in intracranial pressure impair cerebral blood flow
– cerebral tumour or haemorrhage
• Impaired blood flow to vasomotor control regions of the brainstem increase sympathetic vasomotor activity.
– increases arterial BP
– helps maintain cerebral blood flow

27
Q

What is the function of the coronary circulation

A

• must deliver O2 at a high basal rate
• must meet increased demand
– work rate can increase five-fold
reminder: right and left coronary arteries arise from the right and left aortic sinuses

28
Q

When does flow in the left coronary artery mainly occur

A

Diastole
Thick wall of LV hard to perfuse when contracting
Similar but not as great effect in right side
If anything limited blood flow during execise eg obstruction, diastole shortens - less time for blood flow to take place

29
Q

Describe the coronary circulation ad compare with skeletal muscle

A
Cardiac 
• Fibre diameter 18μm 
• Capillary density 3000/mm2 
• Capillaries continuously perfused 
Skeletal 
Fibre diameter 50μm 
• Capillary density 400/mm2 
• Not all capillaries perfused at rest

Coronary circulation
• High capillary density facilitates efficient O2 delivery
• Diffusion distance < 9μm
• Continuous production of NO by coronary endothelium maintains a high basal flow

30
Q

How does coronary blood low change with myocardial oxygen demand?

A
  • Extra O2 requires at high work load os supplied mainly by increased blood flow
  • Almost linear relationship until very high O2 demand
  • Small increase in amount of O2 extracted
  • Vasodilation due to metabolic hyperaemia
  • Vasodilators - adenosine, high [K+], low pH
31
Q

Why are coronary arteries prone to atheroma s?

A

• Few aterio-arterial anastomoses
• Prone to atheromas
• Narrowed coronary arteries leads to angina on exercise (increased
O2 demand)
– blood flow mostly during diastole
• diastole is reduced as heart rate increases
– Stress and cold can also cause sympathetic coronary vasoconstriction and angina
• Sudden obstruction by thrombus causes myocardial infarction

32
Q

Describe teh skeletal muscle circulation

A

• Capillary density depends on muscle type
– Postural muscles have higher capillary density
• Very high vascular tone
– Permits lots of dilatation
– Flow can increase > 20 times in active muscle
• At rest only ~ ½ of capillaries are perfused at any one time
– allows for increased recruitment
• Opening of precapillary sphincters allows more capillaries to be perfused.
– increases blood flow and reduces diffusion distance

33
Q

What arevasodilator and vasoconstrictormetabolites

A

• Various agents are thought to act as vasodilators
– ↑[K+]
– ↑ osmolarity
– Inorganic phosphates
– Adenosine
– ↑[H+]
• Adrenaline also acts as a vasodilator at arterioles in skeletal muscle
– Acts through β2 receptors
– Vasoconstrictor response via NA on α1 receptors

34
Q

Describe the cutaneous circulation

A

• Special role in temperature regulation
• Core temperature is normally maintained around 37oC
– Balance of heat production and heat loss
• Skin is the main heat dissipating surface
– This is regulated by cutaneous blood flow
– also has role in maintaining blood pressure
– vasoconstriction in cutaneous circulation to maintain BP
– see lecture on shock

35
Q

What ate atrerovenous anastomoses

A

Acral (apical) skin has specialised structures called artereovenous anastomoses (AVAs)

36
Q

How do Ava’s regulate heat loss from apical skin?

A

• Apical (acral) skin has a high surface area to volume ratio
• AVAs are under neural control
– sympathetic vasoconstrictor fibres
• Not regulated by local metabolites
• Decrease core temperature increases sympathetic tone in AVAs
– decreases blood flow to apical skin
• Increased core temperature opens AVAs
• Reduced vasomotor drive to AVA’s allows them to dilate – diverts
blood to veins near surface