Physiology: Special Circulations (Coronary, Cerebral, Pulmonary, MSK) Flashcards
List the special adaptations of the coronary circulation that allow it to adequately supply the heart
- high capillary density
- high basal flow rate
- high oxygen extraction from blood
Describe why the only way to to increase heart oxygenation is to increase coronary blood flow
(i.e. why can’t O2 extraction increase)
Because at resting conditions oxygen extraction is very high (~75%) (normal tissues have ~25%)
This means O2 extraction can’t be increases further, thus can only be increased by increasing flow
List the intrinsic factors that increase coronary blood flow
PO2 decrease –> vasodilatation
Metabolic hyperaemia (due to metabolites) –> matches flow with tissue demand
Adenosine (from ATP) –> vasodilatation
Define metabolic hyperaemia
The increased blood flow that occurs when tissue is active
Due to:
- Increased cardiac output
- Metabolites
List the extrinsic factors that control coronary blood flow
Sympathetic stimulation of vessels –> Vasoconstriction
Metabolic hyperaemia (due to increased heart rate and stroke volume)
Adrenaline (in blood) –> Vasodilatation
Give examples of metabolites that contribute to metabolic hyperaemia
- CO2
- K+
- H+
Define functional sympatholysis
Opposition to sympathetic stimulation
Is there a period of peak flow in the right coronary artery?
Why?
No, not really
During systole the subendocardial vessels in the right coronary artery are not compressed much
Is there a period of peak flow in the left coronary artery?
When?
Why?
Yes, during diastole
Due to:
During systole the subendocardial vessels in the left coronary artery are compressed
Describe why tachycardia decreases coronary blood flow
Tachycardia --> Diastole duration decreases --> Flow decreases
(more important in the left coronary artery)
Why does the brain need a very secure supply of oxygen?
Grey matter is very sensitive to hypoxia
List the special adaptations of the cerebral circulation that allow it to adequately supply the brain
- The Circle of Willis
- Autoregulation of cerebral blood flow
- Regional hyperaemia
- Response to PCO2 levels
Describe the structure of the circle of Willis
What is its function?
A ring formed by the anastomosis of the Basilar artery and carotid arteries
It provides a collateral circulation for major cerebral arteries
Describe the autoregulation of cerebral blood flow
Give the autoregulatory response to an increase/decrease in MAP
Autoregulation of cerebral blood flow guards against changes in MAP
It ensures constant cerebral blood flow
MAP increase = Vasoconstriction
MAP decrease = Vasodilation
Describe the participation of the cerebral vasculature in the baroreceptor reflex
Why is this?
Negligible
To maintain a constant cerebral blood flow
Describe the effect of sympathetic stimulation on overall cerebral blood flow
Direct sympathetic stimulation has very little effect in overall cerebral blood flow
Minor vasoconstrictor effects
Give the MAP range in which the autoregulation of cerebral blood flow works
60 - 160 mmHg
Describe the effects of an increase/decrease in local PCO2 on cerebral arterioles
Pco2 increase:
- cerebral vasodilatation
- prevents CO2 build up
Pco2 decrease:
- cerebral vasoconstriction
This is why hyperventilation could lead to fainting
Define cerebral regional hyperaemia
Give a possible mechanism for its action
Blood flow increases to active parts of the brain
Mechanism unknown:
- Due to rise in extracellular [K+] as a result of K+ efflux from repetitively active neurons
What is the normal intracranial pressure (ICP)?
8 - 13 mmHg
What 2 factors determine the cerebral perfusion pressure?
Give the equation linking them
MAP and ICP
Cerebral Perfusion Pressure (CPP) = MAP - ICP
Describe the effect of an increased intracranial pressure (e.g. due to trauma)
Decreases cerebral perfusion pressure
–>
Cerebral perfusion decreases
What is the blood brain barrier?
The barrier between cerebral capillaries and brain tissue
Due to cerebral capillaries’ very tight intercellular junctions
What can cross the blood brain barrier?
- oxygen
- carbon dioxide
- glucose (facilitated diffusion)
What cannot cross the blood brain barrier?
Hydrophilic substances:
- ions
- catecholamines
- proteins
How much of the cardiac output flows through the pulmonary circulation?
The entire CO
What circulation meets the metabolic needs of the pulmonary tissue?
Systemic bronchial circulation
What is the average pressure in the pulmonary artery?
Describe the average pressure in the pulmonary capillaries
Low
(20/6 - 25/12 mmHg)
Low
(8-11 mmHg)
Describe the special adaptation of the pulmonary circulation:
- Low capillary Bp
- Absorptive forces exceed filtration forces (protects against pulmonary oedema)
- Hypoxia causes vasoconstriction of pulmonary arterioles
Why does hypoxia causes vasoconstriction of pulmonary arterioles?
Diverts blood from poorly ventilated areas of lung to better ventilated areas
This increases efficiency
What factors increase blood flow to skeletal muscle during exercise?
- regional metabolic hyperaemia
- Adrenaline in blood
- CO increases, increasing blood flow
On what receptor does adrenaline act?
β2 adrenergic receptors
Describe the mechanism of the skeletal muscle pump
Skeletal muscle contraction aids in venous return from deep veins
Describe varicose veins
Blood pools in lower limb veins if venous valves become incompetent
They don’t lead to a drop in CO as it develops slowly
