Special Circulations Flashcards
Special circulations
All these -
Cerebral Coronary Skeletal Cutaneous Others
flow in parallel so the blood supply splits up
Blood supply to the lungs
The lungs have 2 circulations
Bronchial circulation - part of systemic circulation - meets the metabolic requirements of the lungs
Pulmonary circulation - blood supply to the alveoli - Required for gas exchange
The pulmonary circulation must accept the entire cardiac output
RA pressure < LA pressure
RV < LV pressure
Pulmonary artery < aorta pressure (elastic recoil of arteries maintains pressure in artery therefore higher diastolic pressure)
Features of pulmonary circulation
its under low pressure
the mean arterial pressure is: 12-15mmHg
the mean capillary pressure is: 9-12mmHg
the mean venous pressure is: 5mmHg
It’s under low resistance - it has short wide vessels, there are lots of capillaries - arterioles have relatively little smooth muscle
Adaptations to promote efficient gas exchange
There is a very high density of capillaries in alveolar wall - so there’s a larger capillary SA
There’s a short diffusion distance - Due to the very thin layer of tissue separating gas phase from plasma - Also the combined endothelium and epithelium thickness is 0.3um
Large surface area and short diffusion produce high O2 and CO2 capacity
Ventilation – Perfusion ratio (V/Q ratio)
For efficient oxygenation - we 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
Hypoxic pulmonary vasoconstriction ensures optimal ventilation/perfusion ratio - Hypoxic pulmonary vasoconstriction is the most important mechanism regulating pulmonary vascular tone
Alveolar hypoxia results in vasoconstriction of pulmonary vessels
to Ensure that perfusion matches ventilation - so Poorly ventilated alveoli are less well perfused, which aids optimising gas exchange
Chronic hypoxic vasoconstriction can cause right ventricular failure - Chronic hypoxia can occur at altitude or as a consequence of lung disease such as emphysema.
This leads to chronic increase in vascular resistance, which leads to chronic pulmonary hypertension
This then affects the right side of the heart due to the high afterload on right ventricle, which can lead to right ventricular heart hypertrophy and eventual failure
Low pressure pulmonary vessels are strongly influenced by gravity - In the upright position, there is a greater hydrostatic pressure on vessels in the lower part of the lung - therefore vessels here are distended and wide
Effect of exercise on pulmonary blood flow - Increased cardiac output
Small increase in pulmonary arterial pressure , which opens apical capillaries
Increased O2 uptake by lungs - As blood flow increases capillary transit time is reduced – at rest transit time ~ 1s
– can fall to ~ 0.3s without compromising gas exchange
Formation of tissue fluid
Starling forces determine fluid formation
Hydrostatic pressure of blood within the capillary - the force that pushes fluid out of the capillary
Oncotic pressure (colloid osmotic pressure) – pressure exerted by large molecules such as plasma proteins – the force that draws fluid back into the capillary
Capillary hydrostatic pressure is influenced more by venous pressure in the systemic circulation
Hypertension does not usually result in peripheral oedema
Low capillary pressure minimises the formation of lung lymph
Increased capillary pressure causes more fluid to filter out into the lungs leading to oedema
Pulmonary oedema
Low capillary pressure prevents pulmonary 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 20-25mmHg, which can be caused by Mitral valve stenosis (causes pressure to back up into lungs) or Left ventricular failure
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, this decreases hydrostatic blood pressure, therfore decreasing capillary pressure
Treat underlying cause if possible - e.g. if mitral valve is stenosed then need to treat that
Cerebral circulation
The brain has a high O2 demand
Receives about 15% of cardiac output
– but only accounts for 2% of body mass
O2 consumption of grey matter accounts for ~ 20% of total body consumption at rest - Therefore we must provide a secure O2 supply
How does the cerebral circulation meet the high demand for O2?
high capillary density– therfore there’s a large surface area for gas exchange, theres also a reduced diffusion distance (<10μm)
high basal flow rate – x10 average for whole body - + high O2 extraction– 35%
Secure O2 supply to the brain is vital - Neurones are very sensitive to hypoxia
E.g. we get loss of consciousness after a few seconds of cerebral ischaemia
We 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, means that there are many routes for the blood to cross the brain - block in 1 route shouldn’t compromise blood flow
Functionally – myogenic autoregulation maintains perfusion during hypotension
metabolic factors control blood flow
Brain stem regulates others circulations
Myogenic autoregulation
Cerebral resistance vessels have a well developed myogenic response response to changes in trans mural pressure
Serves to maintain cerebral blood flow when BP changes - Falls below 50mmHg
Metabolic regulation - Cerebral vessels are very sensitive to changes in arterial PCO2
Panic hyperventilation can cause hypocapria and cerebral vasoconstriction leading to dizziness or fainting due to lack of blood
Areas with increased neuronal activity have increased blood flow
An increase in PCO2, K+ conc and adenosine, leads to vasodilation
A decrease in PO2 leads to vasodilation
Adenosine is a powerful vasodilation of cerebral arterioles
Cushing’s Reflex - Rigid cranium protects the brain – but does not allow for volume expansion
Increases in intracranial pressure impair cerebral blood flow
Eg cerebral tumour or haemorrhage - as brain has nowhere to expand too so increases pressure in cerebral vessels, and they also become narrower
Impaired blood flow to vasomotor control regions of the brainstem leads to an increase sympathetic vasomotor activity - therefore further increases in arterial BP
– helps maintain cerebral blood flow - but in situation of bleed - exacerbates the problem
Coronary circulation
Must deliver O2 at a high basal rate
must meet increased demand – work rate can increase five fold
Flow in the left coronary artery occurs mainly during diastole - during systole the contraction limits the blood supply to the left coronary blood flow
Cardiac muscle compared to skeletal muscle - the muscles fibers are a lot smaller (18um) compared to skeletal muscle (50um)
However this means that there is space for a lot of capillaries - cardiac muscle capillary density = 3000/mm2 compared to 400/mm2 for skeletal muscle
Therefore for cardiac muscle the capillaries are constantly perfused (for skeletal muscle not all capillaries are perfused at rest) and higher SA for gas exchange
High capillary density facilitates efficient O2 delivery
Diffusion distance < 9μm
Continuous production of NO by coronary endothelium maintains a high basal flow - due to its vasodilator properties
Coronary blood flow increases with myocardial O2 demand
Extra O2 required at high work load is 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, an increase in K+ conc and a decrease in pH
Coronary arteries are functional end arteries - 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
Skeletal muscle circulation
Must increase O2 and nutrient delivery and removal of metabolites during exercise.
Important role in helping to regulate arterial blood pressure – 40% of adult body mass
Resistance vessels have rich innervation by sympathetic vasoconstrictor fibres
– Baroreceptor reflex maintains blood pressure
Capillary density depends on muscle type
Postural muscles have higher capillary density
Very high vascular tone
Therefore permits lots of dilatation
And flow can increase more than 20 times in active muscle
At rest only ~ 1⁄2 of capillaries are perfused at any one time – allows for increased recruitment
Opening of precapillary sphincters allows more capillaries to be perfused - This increases blood flow and reduces diffusion distance
Increased flow due to metabolic hyperaemia - Various agents are thought to act as vasodilators
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 - e.g. Arterovenous anastomoses
Skin also has role in maintaining blood pressure - vasoconstriction in cutaneous circulation to maintain BP
Artereovenous anastomoses (AVAs) Acral (apical) skin has specialised structures called artereovenous anastomoses (AVAs)
AVAs 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
A decrease core temperature increases sympathetic tone in AVAs
which decreases blood flow to them and the apical skin
When there is increased core temperature, there is a decrease in sympathetic tone this opens AVAs and decreases sympathetic tone, so larger surface area nearer surface of skin so more blood flow, so more heat lost
Reduced vasomotor (sympathetic) drive to AVA’s allows them to dilate – Diverts blood to veins near surface
