3 Control of Blood vessels: Blood flow regulation Flashcards
Describe the regulation of blood flow to the Brain
Autoregulation - receive constant blood supply
- receives ~14% of Cardiac Output (at rest)
- around 50ml/100g/min
Describe the neural control of blood flow to the brain (cerebral)
Relatively minor neural control (vasoconstriction)
Describe the hormonal control of blood flow to the brain (cerebral)
Minor importance
Describe the local control of blood flow to the brain (cerebral)
There is good autoregulation of flow across a wide pressure range (that resets in hypertension) 60-160mmHg, abolished by hypercapnia
- important metabolic control during metal activity (regional)
- H+, K+, adenosine, hypercapnia (Too much CO2), hypoxia ALL cause vasodilation
- Endothelin may be an important (potent) vasoconstrictor in pathological states e.g. subarachnoid haemorrhage
Describe the mechanical control of blood flow to the brain (cerebral)
As the brain is constrained in the rigid cranium
- this is influenced by Cerebrospinal Fluid pressure (CSF)
e.g. space-occupying lesions can increase the ICP (intracranial pressure) + reduce Cerebral Blood flow (CBF)
Describe some special features of blood flow control of the brain (cerebral)
Medullary ischaemic reflex (Cushing)
- e.g. tumour-induced reduction in Cerebral Blood flow (CBF) causes medullary ischaemia
- which stimulates an increase in BP in an attempt to restore CBF
How much blood flow is directed to the heart (coronary)?
The heart receives around 4% of Cardiac Output (CO)
Describe the neural control of blood flow to the heart (coronary)
Minor direct influence (vasoconstriction), but the secondary effect on blood flow due to changes in cardiac function and hence metabolism
- Sympathetic stimulation causes a B-mediated increase in Heart Rate
- and supraventricular Tachycardia, which increases O2 consumption
Describe the local control of blood flow to the heart (coronary)
Metabolites have a major influence:
- Hypoxia, hypercapnia, adenosine ALL cause vasodilation
Describe the hormonal control of blood flow to the heart (coronary)
Adrenaline - vasodilator
- and also stimulates metabolism
Describe the mechanical control of blood flow to the heart (coronary)
A major influence on flow during the cardiac cycle
- peak flow in early diastole
- 0 or -ve flow on the onset of systole
(heart contracts - blood vessels are compressed against myocardium)
Describe some special features of blood flow control of the heart (coronary)
There is a parallelism between metabolism and flow
How much blood flow is directed to the skin?
The skin receives around 4% of cardiac output
- at rest in a thermoneutral environment
Describe the neural control of blood flow to the skin
Arteries can have a relatively weak innervation (a vasoconstriction)
AV anastomoses have a dense innervation (a vasoconstriction)
- increase in core temperature causes AVAs to dilate, increasing skin blood flow and hence heat loss
Describe the local control of blood flow to the skin
Arterioles show some degree of myogenic autoregulation
AV anastomoses show no autoregulation and no reactive hyperaemia
- Endothelin may be involved in pathological states (Raynaud’s)
Describe the mechanical control of blood flow to the skin
Minimal influence
Describe some special features of blood flow control of the skin
The primary function is thermoregulation
- sweat glands have cholinergic innervation (sudomotor)
- which can cause vasodilation via the release of Bradykinin
How much blood flow is directed to Skeletal Muscles?
Skeletal muscles receive around 15% of Cardiac output
- at rest
- it can vary between 3 and 60ml/100g/min
Describe the neural control of blood flow to skeletal muscles
At rest
- important a vasoconstriction, some B vasodilation, maybe cholinergic vasodilation
- involved in systemic BP regulation.
> Skeletal muscle - 40% of the mass, hence vasoconstriction has a large influence on total peripheral resistance
Excercise
- very little neural influence
- some B vasodilation
Describe the hormonal control of blood flow to skeletal muscles
Adrenaline at low concentrations
- will Vasodilate (B receptors)
Describe the mechanical control of blood flow to skeletal muscles
Muscle pumping
Describe some special features of blood flow control of skeletal muscles
Capacity to increase flow in exercise (20-fold) - Active hyperaemia
A large increase in blood flow post-occlusion - Reactive hyperaemia (increased blood flow)
- Reactive hyperaemia - transient increase in organ blood flow - that occurs following a brief period of ischaemia (arterial occlusion)
How much blood flow is directed to the splanchnic circulation (GI tract, pancreas, liver etc)
Superior mesenteric: 10% of cardiac output
Hepatic: around 25% of cardiac output
- 70-75% is via Hepatic portal vein (low [O2], low pressure
- 25-30% is via the hepatic artery (high [O2], high pressure)
Describe the neural control of blood flow to the splanchnic circulation (GI tract, pancreas, liver etc)
Intestinal: moderate (a vasocontriction)
Hepatic: important (a vasoconstriction)
The liver stores around 15% of total blood volume
- and hepatic vasoconstriction can expel around 50% of hepatic blood volume into the systemic circulation
Describe the local control of blood flow to the splanchnic circulation (GI tract, pancreas, liver etc)
Intestinal: poor autoregulation, but importantly influenced by local peptides
- (Ang II - vasoconstrictor)
Hepatic:
- portal vein - no autoregulation
- hepatic artery - good autoregulation
Describe the hormonal control of blood flow to the splanchnic circulation (GI tract, pancreas, liver etc)
- GI hormones (gastrin, cholecystokinin) vasodilate
Vasopressin + Angiotensin vasoconstrict potently
Describe the mechanical control of blood flow to the splanchnic circulation (GI tract, pancreas, liver etc)
Minimal influence
Describe some special features of blood flow control of the splanchnic circulation (GI tract, pancreas, liver etc)
Intestinal circulation exhibits functional hyperaemia following feeding
Intense vasoconstriction can lead to damage + release of toxins
- Vasoconstriction (neurohormonal) beneficial in baroreflex, but can be detrimental in haemorrhage/septic shock
How much blood flow is directed to the Kidneys (renal)?
The kidneys receive around 25% of the Cardiac Output
Describe the neural control of blood flow to the Kidneys (renal)
Important a vasoconstriction
Some B vasodilation
Renin secreting cells have sympathetic innervation (B adrenoceptors)
Describe the local control of blood flow to the Kidneys (renal)
Good autoregulation of flow over a wide pressure range
Describe the hormonal control of blood flow to the Kidneys (renal)
Noradrenalin, adrenaline, angiotensin
- ALL cause Vasoconstriction
Vasopressin may cause vasodilation
- via prostaglandin/NO release
Dopamine - vasodilation
Describe the mechanical control of blood flow to the Kidneys (renal)
The renal capsule may restrict flow in pathological states
Describe some special features of blood flow control of the Kidneys (renal)
- Excretory function of the kidney depends on well-maintained flow (autoregulation)
- Vascular connection provides for the capacity to regulate afferent/efferent resistances
How much blood flow is directed to the Lungs (pulmonary)?
The lungs receive 100% of cardiac output
Describe the neural control of blood flow to the Lungs (pulmonary)
Relatively minor neural influence
- a vasoconstriction
Describe the local control of blood flow to the Lungs (pulmonary)
Unlike elsewhere, hypoxia causes vasoconstriction
- which is augmented by hypercapnia
(possibly mediated by endothelin)
NO causes vasodilation
(therapeutic uses)
- Pulmonary hypertension - possible therapeutic strategies include endothelin receptor antagonism and NO inhalation
Describe the mechanical control of blood flow to the Lungs (pulmonary)
Flow is affected by changes in:
- Alveolar pressure
- Lung volume
An increase in blood flow (cardiac output) is associated with recruitment and distension of microvessels, and a decrease in vascular resistance
- If alveolar pressure > intravascular pressure, blood flow is reduced
Lung inflation
- reduces resistance in extra/alveolar vessels (traction)
- and increases resistance in intra-alveolar vessels (compression)
