Special Circulations Flashcards
What is the route of Coronary Circulation?
Most coronary venous blood drains via coronary sinus into the right atrium
What are the special adaptations of the Coronary Circulation?
Can only be supplied by increasing coronary blood flow
High Capillary Density
High Basal Blood Flow
High Oxygen Extraction
How is Coronary blood flow controlled?
Extrinsic and Intrinsic Mechanisms
What are the intrinsic mechanisms to control coronary blood flow?
decreased Po2 causes vasodilatation of the coronary arterioles
Metabolic hyperaemia matches flow to demand
Adenosine (from ATP) is a potent vasodilator
What are the extrinsic mechanisms to control coronary blood flow?
Coronary arterioles supplied by sympathetic vasoconstrictor nerves but are over-ridden by metabolic hyperaemia as a result of increased heart rate and stroke volume
So sympathetic stimulation of the heart results in coronary vasodilatation despite direct vasoconstrictor effect (functional sympatholysis)
Circulating adrenaline activates 2 adrenergic receptors, which cause vasodilatation
When does most of the coronary blood flow occur and why?
diastole - when the subendocardial vessels from the left coronary artery are not compressed
How is the Cerebral system supplied?
Brain is supplied by internal carotids and vertebral arteries
What happens to gray matter after 3 minutes without oxygen?
consciousness lost after few seconds of ischaemia
irreversible cell damage
Where do major cerebral arteries arise from?
Circle of Willis
What two arteries astamose to form the Circle of Willis?
BASILAR (formed by two vertebral arteries)
CAROTID arteries
Why is Cerebral Circulation Autoregulated?
guards against changes in cerebral blood flow if mean arterial blood pressure changes within a range (~ 60 - 160mmHg)
How does autoregulation work?
If MABP rises, resistance vessels automatically constrict to limit blood flow
If MABP falls, resistance vessels automatically dilate to maintain blood flow
When would autroregulation fail? and what is the result?
MABP below 60mmHg (cerebral blood flow falls)
MABP rise above 160mmHg (cerebral blood flow rises)
What does MABP below 50 result in?
confusion, fainting, and brain damage
What is a normal intercranial pressure in the skull?
8-13 mm Hg
What does Cerebral Perfusion Pressure =?
Mean Arterial Pressure (MAP) - Intercranial Pressure (ICP)
What can cause an increase in intercranial pressure?
brain tumour, head injury
What does increased ICP lead to?
decreased cerebral perfusion pressure and cerebral blood flow
What is the blood brain barrier?
tight intercellular junctions that the cerebral capillaries have
How does glucose cross the blood brain barrier?
facilitated diffusion using specific carrier molecules
What is the blood brain barrier impermeable to?
And why is this advantageous?
- hydrophilic substances such as ions, catecholamines, proteins
- helps protect brain neurones from fluctuating levels of ions etc in blood
Describe the Pulmonary Circulation
Entire cardiac output flows from right ventricle into pulmonary circulation
What is a normal pulmonary artery BP?
20-25/6-16 mmHg
What are the special adaptations of the pulmonary circulation?
Pulmonary capillary pressure is low (~ 8-11 mmHg) compared to systemic capillary pressure (~ 17-25 mmHg)
Absorptive forces exceed filtration forces - protects against pulmonary oedema
Hypoxia causes vasoconstriction of pulmonary arterioles which helps divert blood from poorly ventilated areas of lung
Describe the Skeletal muscle circulation
Resistance of skeletal muscle vascular bed has large impact on blood pressure
Resting blood flow is low because of sympathetic vasoconstrictor tone
How does skeletal muscle blood flow change during exercise?
increases
How does skeletal muscle blood flow increase during exercise?
- local Metabolic hyperaemia overcomes sympathetic vasoconstrictor activity
- Circulating adrenaline causes vasodilatation (B2 adrenergic receptors)
- increased cardiac output during exercise, this could increase skeletal muscle blood flow many folds
What does contraction of muscles aid?
Venous return
What does the skeletal muscle pump reduce?
the chance for postural hypotension & fainting
What is the skeletal muscle pump?
One-way venous valves allow blood to move forward towards the heart
What are Varicose Veins?
Blood pools in lower limb veins if venous valves become incompetent
