S8 Special Circulations Flashcards
List 5 of the special circulations.
- Pulmonary
- Coronary
- Cerebral
- Skeletal
- Cutaneous
What are the two circulations in the lungs?
- bronchial circulation
* pulmonary circulation
What is the bronchial circulation?
- part of the systemic circulation
* meets the metabolic requirements of the lungs
What is the pulmonary circulation?
- supplies blood to the alveoli
* required for gas exchange
What must the pulmonary circulation accept the whole of?
Whole cardiac output
What is normal CO at rest? What is the maximum CO of a non-athlete?
- 5L/min
* 20-25L/min
Is the pulmonary circulation low or high pressure and resistance?
- low pressure
* low resistance
Why is diastole in the right ventricle the same/lower than diastole in the right atrium?
If it was higher than the right atrial diastolic pressure then blood wouldn’t flow as wouldn’t be down a concentration gradient
Why is the pressure in the atria only diastolic pressure?
The atria don’t undergo systole like the ventricles do
How is low resistance maintained in the pulmonary circulation?
- short, wide vessels
- lots of capillaries
- arterioles with little smooth muscle
What are the mean arterial, capillary and venous pressures in the pulmonary circulation?
Arterial - 12-15mmHg
Capillary - 9-12mmHg
Venous - 5mmHg
How is efficient gas exchange promotes in the pulmonary circulation?
- high density of capillaries in alveolar wall - large SA
- short diffusion distance
Means high oxygen and carbon dioxide transport capacity produced
What is the optimal ventilation-perfusion ratio (V/Q)?
0.8
What is needed for efficient oxygenation in terms of V/Q ratio? How is this maintained? When might this happen?
Need to match ventilation of alveoli with perfusion of alveoli
Maintained by diverting blood away from alveoli that aren’t well ventilated
Hypoxia
What is hypoxic pulmonary vasoconstriction?
Important mechanism in regulating pulmonary vascular tone.
When alveolar hypoxia occurs, vasoconstriction of pulmonary vessels occurs to ensure perfusion matches ventilation (poorly ventilated alveoli are less well perfumed) - to optimise gas exchange
This is the opposite to what happens in systemic circulation
What is a disadvantage of hypoxic vasoconstriction?
Can lead to right ventricular heart failure
- chronic hypoxia can occur at altitude or in diseases like hypoxia
- the chronic increase in vascular resistance leads to chronic pulmonary hypertension
- a high afterload on the right ventricle (due to pulmonary hypertension) leads to right ventricular heart failure
What are the low pressure pulmonary vessels strongly influenced by?
Gravity
How does gravity effect the pulmonary vessels?
In the upright position (orthostasis) there’s increase hydrostatic pressure on the blood vessels in the lower part of the lung
- at the apex - vessels collapse during diastole
- at the level of heart - vessels are continuously open
- at the base - vessels are ‘swollen’/distended
What is the effect of exercise on pulmonary blood flow?
- increases CO
- there’s a small increase in pulmonary arterial pressure
- this open apical capillaries (improves the V/Q ratio)
- there’s increased oxygen uptake by lungs
- as blood flow increases, capillary transit time decreases (can fall to 0.3s from 1s without compromising gas exchange)
How is tissue fluid formed?
Due to Starling forces
- hydrostatic pressure of blood in capillaries - pushes fluid out
- oncotic pressure - draws fluid in
In systemic circulation, does arterial or venous pressure influence capillary hydrostatic pressure more?
Venous pressure
How is lung lymph formation kept to a minimum?
Due to the low capillary pressure
Is oncotic pressure of tissue fluid higher in the lungs or periphery?
Lungs
Is capillary hydrostatic pressure higher in the lungs or systemic capillaries?
Systemic capillaries
Is plasma oncotic pressure higher in lungs or systemic circulation?
Equal
What does increased capillary pressure cause?
Causes more fluid to filter out and this leads to oedema
What prevents pulmonary oedema?
Low capillary pressure
When can you get pulmonary oedema?
When capillary pressure increases e.g. if left atrial pressure rises to 20-25mmHg
- mitral valve stenosis
- left ventricular failure
What does pulmonary oedema impair? What is it affected by?
Impairs gas exchange
Affected by posture (changes in hydrostatic pressure due to gravity) - when upright, oedema forms at base of lungs and when lying down, oedema forms throughout lungs
How do you manage/treat pulmonary oedema?
- use diuretics to relieve symptoms
* treat the underlying cause e.g. mitral valve stenosis, left ventricular heart failure
How much of the CO does the brain receive?
15%
How does the cerebral circulation meet the high demand for oxygen?
- high capillary density (large SA and reduced diffusion distance)
- high basal flow rate
- high oxygen extraction
What are neurones sensitive to, why is a secure oxygen supply to the brain needed?
Sensitive to hypoxia
Leads to loss of consciousness within few seconds of cerebral ishcaemia and damage is irreversible after 4 mins
How is a secure blood supply to the brain ensured?
- anastomoses between basilar and internal carotid arteries (so even if one blokes, can still get blood to brain)
- myotonic autoregulation (maintains perfusion during hypotension)
- metabolic factors control blood flow
- the brainstem regulates other circulations
What is myogenic autoregulation?
Maintains cerebral blood flow when blood pressure changes (but fails if BP falls below 50mmHg)
- if BP increases, vasoconstriction occurs
- if BP decreases, vasodilation occurs
How is cerebral circulation controlled by metabolic regulation?
Cerebral vessels are very sensitive to changes in arterial carbon dioxide pressure
- hypercapnia (increased carbon dioxide) leads to vasodilation
- hypocapnia (decreased carbon dioxide) leads to vasoconstriction
What can panic hyperventilation cause?
Hypocapnia and so cerebral vasoconstriction which leads to dizziness and fainting
What does regional activity in the brain (neuronal activity) lead to?
Increased local blood flow
Which aspects/molecules of neuronal activity lead to vasodilation?
- increased carbon dioxide pressure
- increased K+ concentration
- increased adenosine - important vasodilator of cerebral arteries
- decreased oxygen pressure
What is Cushing’s Reflex?
- a rigid cranium protects the brain and doesn’t allow for volume expansion
- so increases in intracranial pressure (cerebral tumour/haemorrhage) impair cerebral blood flow
- this impaired blood flow to vasomotor control regions of brainstem leads to increased vasomotor activity (bradycardia occurs)
- arterial BP increases - this helps maintain arterial flow
But need to relieve this pressure, or will die
Where do the right and left coronary arteries arise from?
Arise from right and left aortic sinuses
When does most of the flow to the left coronary artery occur, during systole or diastole? What is a disadvantage of this?
Diastole
If HR increases, diastole is shortened, this mean less blood is able to flow to the L coronary artery. Becomes a problem when narrowing of coronary arteries occurs.
How is efficient oxygen delivery to heart facilitated?
- high capillary density and they are constantly perfused
- short diffusion distance
- a continuous production of NO by the coronary endothelium meant aims a high basal flow - NO causes vasodilation, decreased vascular resistance and inhibition of platelet aggregation
How does coronary blood flow increase with myocardial oxygen demand?
- extra oxygen is required if workload is high
- this is supplied by increased blood flow - vasodilation occurs due to hyperaemia
- almost linear relationship until v high oxygen demand
What are some vasodilators involved in increased blood flow for coronary circulation?
- adenosine
- increased K+ concentration
- decreased pH
What are coronary arteries prone to due to having few arterio-arterial anastomoses (having many end arteries)?
Atheromas
What ‘symptom’ do narrowed coronary arteries lead to on exercise? What conditions also lead to sympathetic vasoconstriction and this symptom?
Angina
- due to the increased oxygen demand
- diastole is reduced as HR increases, but most of blood flow in L coronary artery is during diastole
Stress and cold
Sudden obstruction of a coronary artery by a thrombus leads to what?
Myocardial infarction
When must oxygen and nutrient delivery and removal of metabolites increase in skeletal muscle circulation?
During exercise
What do skeletal muscle play an important role in regulating?
Arterial blood pressure
What does capillary density in the skeletal muscle circulation depend on?
Depends on muscle type
Some vessels in the skeletal muscle circulation are innervated by the sympathetic nervous system. What happens in these vessels, vasoconstriction or vasodilation? What maintains the blood pressure?
Vasoconstriction which increases resistance
Baroreceptors
What does the high vascular tone of skeletal muscle vessels permit?
Permits lots of vasodilation so flow can increased massively in active muscles
At rest, how many of the skeletal muscle capillaries are perfused? What is the advantage of this?
About half
Means when need perfusion of capillaries due to contraction/metabolic demand/etc the effect is bigger (increased recruitment)
What opens to allow more capillaries to be perfused in the skeletal muscle circulation?
Precapillary sphincters
What agents act as vasodilators?
- increased K+ concentration
- increased osmolarity
- inorganic phosphates
- adenosine
- increased H+ concentration
What does adrenaline act as at arterioles in skeletal muscle? What is the opposite effect?
Acts as a vasodilator
* acts through beta-2 adrenoreceptors
Noradrenaline acting on alpha-1 adrenoreceptors causes vasoconstriction
What does cutaneous circulation have a role in regulating?
Temperature (and blood pressure - via vasoconstriction)
What structures does the apical/acral skin have? What is their role in temperature regulation in terms of the cutaneous circulation?
Artereovenous anastomoses (AVAs)
They direct blood flow to superficial/deep vessels dependent on whether wanting to lower or increase body temperature
What type of control are AVAs under?
Neural control - sympathetic vasoconstrictor fibres
When are AVAs opened? When are they closed?
When there’s an increase in core temperature (decreased sympathetic tone) leads to dilation of AVAs and so diverts blood to superficial veins
Opposite if decreased core temperature - increased sympathetic done so vasocontraction and blood diverted to deep veins
