Special Circulations Flashcards
What is the format of the systemic circulation
,a number of circulations in parallel (cerebral, coronary, skeletal, cutaneous, others)
Describe the blood supply to the lungs
• 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
Is the pulmonary circulation in series or parallel with systemic
Series , Pulmonary circulation has to accept entire cardiac output
What is cardiac output at rest and maximum
• Cardiac output at rest ~ 5 l/min • Maximum cardiac output ~ 20 -25 l/min (non athlete)
Describe the pressure and resistance of the pulmonary circulation
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
Describe the pressure in the pulmonary circulation
Low pressure
– mean arterial pressure 12-15mmHg
– mean capillary pressure 9-12mmHg
– mean venous pressure 5mmHg
Why does the pulmonary cirulation have low resistance
Low resistance
– short, wide vessels
– lots of capillaries (many parallel elements) - reduces resistance
– arterioles have relatively little smooth muscle
What are teh adaptations of the pulmonary circulation for gas exchange
• 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
What is the V/Q ratio
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
What is hypoxia pulmonary vasoconstriction
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
What is the downside to chronic hypoxia vasoconstriction
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)
How is the pressure affected by. Gravity
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)
What is the effect of excercise on pulmonary blood flow?
• 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
How is tissue fluid formed
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)
What minimises the formation of lung lymph
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
What happens if capillary prssure is increased
More fluid filter out - pulmonary oesdema
Lungs are more sensitive to increases in art and venous pressure when it comes to oedema )both)
How does capillary pressure affect oedema
• 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
How can pulmonary oedema affect gas exchange
• 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
Describe the o2 consumption of th brain
• 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
How does the cerebral circulation meet the high demand for O2?
• 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
Why is a secure o2 supply to the brain vital
• 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
How is a secure blood supply ensured?
• Structurally
– anastomoses between basilar and internal carotid arteries
• Functionally
– myogenic autoregulation maintains perfusion during
hypotension
– metabolic factors control blood flow
– brainstem regulates other circulations
What is myogenic autoregualion
• 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
Describe metabolic regulation of cerebral blood flow
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
How does regional activity prince local increase in blood flow
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
What is cushings reflex?
• 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
What is the function of the coronary circulation
• 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
When does flow in the left coronary artery mainly occur
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
Describe the coronary circulation ad compare with skeletal muscle
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
How does coronary blood low change with myocardial oxygen demand?
- 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
Why are coronary arteries prone to atheroma s?
• 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
Describe teh skeletal muscle circulation
• 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
What arevasodilator and vasoconstrictormetabolites
• 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
Describe the cutaneous circulation
• 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
What ate atrerovenous anastomoses
Acral (apical) skin has specialised structures called artereovenous anastomoses (AVAs)
How do Ava’s regulate heat loss from apical skin?
• 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
