Control of blood flow in differing vascular beds Flashcards
What is the coronary reserve?
Flow to cardiac muscle can increase 4-5x when cardiac output increases
What must be maintained?
A secure O2 supply to coronary muscle
What must an increased demand for O2 be met with?
Large increases in flow
How does the cardiac muscle extract the maximum amount of O2?
Very large a-vO2 difference
What are the main blood vessels of the heart?
Right coronary artery Left coronary artery Circumflex artery Left anterior descending branch Posterior descending artery
What must be the case for blood to flow through through a coronary vessel?
Arterial pressure > venous pressure
Pressure inside > Pressure outside
When does most blood flow to the left myocardium?
During diastole
What determines flow
Aortic pressure during diastole
What is the effect of high heart rate on perfusion?
Reduces time
Diastole shortened
When is right coronary flow highest?
During systole
How is coronary flow controlled?
Coronary arteries exhibit myogenic autoregulation in pressure range 60-180mmHg
Some sympathetic control but overridden by local control
Metabolic hyperaemia is the dominant form of regulation
What is angina?
Decreased coronary flow due to narrowed coronary arteries
Results in pain during exercise
What are the consequences of not enough oxygen reaching the brain?
Loss of consciousness
Neuronal damage
What is myogenic autoregulation?
Ensuring O2 supply to brain tissue is maintained
What is the circle of Willis?
The name given to the connected arteries of the brain
What makes up the circle of Willis?
Vertebral arteries Basilar artery Posterior cerebral artery Posterior communicating artery Carotid arteries Middle cerebral artery Anterior communicating artery Anterior cerebral artery
How are the blood vessels of the brain adapted?
Short arterioles
Dense capillary network
Relatively high vascular resistance
Cerebral perfusion maintained if carotid artery obstructed
How many capillaries are in grey matter?
3000-4000/mm2
What is the blood brain barrier?
Cerebral capillaries form tight junctions so there is no bulk flow
No vesicular transport
Protects neurones
Maintains environment
Responsible for long lasting effects of heroine
What is basal flow?
15% of cardiac output
How do we protect cerebral circulation?
Regulating other organs
Peripheral vasoconstriction
What is autoregulation?
A change in blood pressure is met by a change in resistance to maintain perfusion
What is the autoregulatory range?
~60 - ~170 mmHg
What are cerebral vessels responsive to?
CO2
What is hypercapnia?
High blood CO2 concentration
Why does vasodilation occur in hypercapnia?
Epithelial nitrous oxide
Fall in myocyte pH
What happens in hypocapnia?
Low CO2
Dizziness
Cerebral arteries contract
Why does hypoxia lead to vasodilation?
Adenosine
K+
Nitrous oxide
What dose systemic hypoxia cause?
Hyperventilation leading to hypocapnia
What factors couple tissue metabolism and local flow?
CO2
Increased interstitial K+ due to increased K+ permeability
Adenosine
Nitric oxide
What is the maximum effect of the sympathetic nervous system on cerebral flow?
20-30%
What is a cause of increased intracranial pressure?
Intracranial bleeding
Cerebral oedema
Tumour
What is the result of increased intracranial pressure?
Collapsed veins
Decreased effective cerebral perfusion pressure reduces blood flow
How do you calculate cerebral perfusion pressure?
Cerebral perfusion pressure = mean ABP - intracranial pressure
What is an ischemic stroke?
Area deprived of blood
What is a haemorrhagic stroke?
Area of bleeding
What are the symptoms of vascular dilation in the brain?
Headaches
Migraines
What must the cutaneous circulation be able to do?
Regulate body temperature
Respond to trauma
What % of cardiac output does the skin receive?
10%
Where do the capillaries sit?
In the dermis
What is the structure of blood vessels in the skin?
Capillaries Vein Venous plexus AV anastomosis Arteries
What is the response to increased core temperature?
Increased core temperature
Removal of a-adrenoceptor mediated sympathetic tone
Cutaneous vasodilation
Heat loss
What are AV anastomoses?
Found in hands, feet and face (ears, nose and lips)
50micrometre diameter with thick smooth muscle
Shunt blood from arterioles-venules
a1-adrenoceptors
What happens to AV anastomoses when core temperature rises?
Dilation
Increased blood flow to venous plexus
Heat loss
What is the result of sweating on heart rate?
Sweating leads to release of bradykinin a potent vasodilator
This decreases total peripheral resistance and increases heart rate
Cardiac output goes up
What happens when core temperature falls?
Stimulation of a-adrenergic receptors
Vasoconstriction
Heat gain
What is counter current exchange?
Radiation from warm arterial to cold venous blood flowing in opposite directions
What is Raynaud’s disease?
Overreactive skin vessels
Cold/emotional stimuli leads to vasoconstriction
Skin appears white, blue, then red
Numbness, pain and burning sensation
What is the triple response?
When a pointed object is drawn over skin, or a small burn
- White reaction - blanching due to mechanical stimulation
- Red reaction - local vasodilation and histamine
- Flare - wider intense vasodilation
- Wheals/ local oedema
What nerves convey information about injury to the skin?
C fibre nociceptor
