Regional Circulations Flashcards
What intrinsic mechanisms regulate blood flow?
Myogenic
Endothelial mediated
Metabolic
What extrinsic mechanisms regulate blood flow?
Neural
Hormonal
What supplies the myocardium?
Right and left coronary arteries
Where does the innermost section of myocardium receive blood from?
Directly from cardiac chambers via specialised vessels
Where does blood from the left ventricle drain to?
Coronary sinus
Where does blood from the right ventricle drain to?
Anterior cardiac veins
What is the resting coronary blood flow?
~225 ml/min
~5% of totatl cardiac output
How much may cardiac output increase by during exercise?
6-9 fold
How much may coronary blood flow increase by during exercise?
3-4 fold
What is unique to cardiac muscle?
Very high requirements of O2
How much O2 is extracted from blood in cardiac capillaries during one passage?
Near maximum amount possible
How would one increase coronary blood flow?
Increasing O2 delivery to myocaridal cells
What does increased cardiac O2 cosnumption do?
Triggers corresponding increase in coronary blood flow
What is the primary controller of resistance of coronary vessels?
Metabolic regulation
What is the rate of coronary blood flow determined by?
Constriction/dilation of resistance vessels
What is released when levels of O2 in coronary muscles become insufficient?
Vasodilatory metabolites
What triggers active hyperaemia?
Inadequate coronary blood flow
Decreased arterial O2
Increased metabolic activity
What are believed to be the mediators of active hyperaemia?
Adenosine
Nitric oxide
What does a reduction in ATP concentration do?
Opening of Katp (potassium-sensitive ATP) channels and a hyperpolarisation
Relaxation of coronary VSM
What does stimulation of autonomic nerves to heart affect?
Coronary blood flow both directly and indirectly
What do the direct effects of stimulation of autonomic nerve affect?
Blood vessels themselves
What does the activation of sympathetic nerves do?
Triggers vasoconstriction via alpha adrenergic receptors on coronary VSM
What are beta adrenergic receptors associated with in regard to VSM?
Dilation
What does vagal nerve stimulation do to coronary resistance vessels?
Slightly dilates them
How do indirect effects in coronary blood flow arise?
Changes in coronary blood flow by changes in the activity of cardiac muscle
What does sympathetic nerve activity do to cardiac muscle?
Increased contractility
Increased tachycardia
What does increased cardiac activity produce?
Metabolic vasodilatory metabolites
What do metabolic vasodilatory metabolites do?
Have the effect of increasing coronary blood flow
What does sympathetic activity indirectly cause?
Vasodilation in the coronary circulation
What does vagal activity indirectly result in?
Vasodilation in the coronary circulation
What is highly important in coronary circulation?
Metabolic regulation
Is extravascular compression a pathological or physiological phenomenon?
Physiological
What affect does contraction of the left ventricle on left coronary arterial blood flow?
May slow, halt or temporarily reverse the flow
Why is the effect of extravascular compression less apparent on the right coronary artery than the left coronary artery?
As there is a lower pressure found in the right ventricle than the left ventricle
What is ischaemia?
Restriction in blood supply to a tissue
What causes myocardial ischaemia?
Acute occlusion of a coronary artery by atherosclerotic plaque
How may myocardial infarction prove fatal?
Decreased cardiac output
Pulmonary oedema
Fibrillation of heart
Rupture of heart
What is blood flow i skeletal muscle determined by?
Contractile activity of the muscle
What are the factors that regulate skeletal blood flow?
Neural and intrinsic factors
What is the resting blood flow in skeletal muscle?
3ml/min/100g
What is the exercise blood flow in skeletal muscle?
60ml/min/100g
What is the predominant regulator of skeletal blood flow at rest?
Neural regulation
What is the predominant regulator of skeletal blood flow during exercise?
Local factors
What does tonic sympathetic nerve activity do?
Maintain a degree of vasoconstriction
What does noradrenaline bind to?
Alpha and beta 1 adrenergic receptors, little affinity for beta 2 receptors
What does the binding of noradrenaline do?
Exclusively triggers vasoconstriction
Are most skeletal capillary beds perfused at rest?
No
What is sympathetic nerve activity to skeletal muscle modulated by?
Baroreceptor reflex
What contributes greatly to total peripheral resistance?
Resistance in skeletal muscle blood vessels
What does carotid artery occlusion do?
Reduces baroreceptor firing
What reduces muscle blood flow in carotid artery occlusion?
Increased sympathetic nerve activity
What contributes to the large increase in mean arterial pressure?
Increased sympathetic nerve activity
What does releasing the carotid artery increase?
Baroreceptor firing
What causes vasodilation?
Reduced sympathetic nerve activity
What does vasodilation do?
Increases muscle blood flow
Reduces MAP
What is skeletal muscle blood flow closely coupled to?
Metabolic demand
In active muscle, what rapidly increases blood flow?
Vasodilatory metabolites
Name some vasodilatory metabolites.
Adenosine, K+, CO2, lactic acid
What does “unopposed” increased vasoconstriction in inactive muscles do?
Diverts blood flow to active muscles
What do beta 2 adrenergic receptors promote?
Vasodilation
What does adrenaline have a higher affiity for?
Beta 1 and beta 2 receptors
What do low concentrations of circulating adrenaline bind to?
Preferentially bind to beta 2 receptors triggering vasodilation
What do high concentrations of adrenaline bind to?
Alpha 1 adrenergic receptors?
What do alpha 1 adrenergic effects lead to?
Vasoconstrictor effects predominating
What nerves of the ANS release ACh?
Parasympathetic and sympathetic cholinergic nerves
What can ACh trigger from endothelial cells?
Nitric oxide release
What does acetylcholine activate?
Endothelial nitric oxide synthase
What is the function of nitric oxide?
Potent vasodilator
Where may ACh spillover from the NMJ diffuse to?
Local blood vessels
What does ACh spillover promote in local blood vessels?
Endothelial NO production
VSM relaxation
Further vasodilation
What induces dilation in the microcirculation?
Metabolites in actively contracting muscle
What does dilation in the microcirculation promote?
A pressure gradient with upstream feed arteries
What does the resulting increase in blood flow caused by a pressure gradient with upstream feed arteries do?
Elevates shear stress
Release of endothelial-derived NO
Further vasodilation
What is functional sympatholysis?
Increased activity of tissue blocks sympathetic effects on that tissue
What may nitric oxide inhibit?
Noradrenaline release from varicosities
Directly opposes alpha 2 mediated constriction of vascular smooth muscle
What may skeletal muscle fibers contain?
Neuronal NOS
What does contraction increase?
NO release
What do strong contractions of skeletal muscle do to blood vessels?
Compress them
What does compression do to the rate of venous return?
Increase it
What may cause blood flow to almost cease in muscle?
Tetanic contraction
What does tetanic contraction do to the contraction itself?
Rapid weakening of the strength of the muscle
What is venous occlusion plethysmography?
Technique for measuring limb blood flow
How does venous occlusion plethysmography work?
Venous return is interrupted by inflating cuff around upper arm at 40mmHg
Hand circulation is completely occluded with a wrist cuff higher than systolic pressure
Arterial inflow in unaltered and forearm volume increases
Changes in arm circumference is measured by strain-gauge plethysmogrpah
What does VOP do?
Provide a measure of arterial blood flow to that part of the forearm enclosed by the two cuffs
When is VOP used?
TO study the effect of vasoactive mediators or drugs in the forearm vascular bed in human clinical trials
What is the path of capillaries in cutaneous circulation?
Loop under epidermis then flows into a venous plexus, acting as a blood resevoir
What is responsible for colouration of skin?
Blood accumulation in venousn plexus
What is the role of arteriovenous anastomoses?
Specialised shunts that feed blood into plexuses directly from subcutaneous arteries
Where are AV anastomoses found?
Skin circulation of hands, ears, nose and lips
What controls the tone of the thick VSM in AV anastomoses?
Sympathetic neural control
What is the intrinsic regulation of AV anastomoses?
N/A
What is the main purpose of skin circulation?
Regulation of body temperature
What regulates cutaneous circulation?
SN activity - alpha adrenergic recepetors
When is cutaneous circulation subject to a high degree of adrenergic tone?
Normal temperatures - high degree of vasoconstriction
What does vasoconstriction of arterioles and AV anastomoses result in?
Reduction/blocking of flow into skin capillaries
what is blushing/blanching caused by?
Inhibition/stimulation of SN in blood to skin of face
What has a low degree of autoregulation in cutaneous circulation?
Resistance arterioles - produce vasodilatory metabolites
What is reactive hyperaemia?
Increased blood flow following ischaemia
What happens to resistance vessels when a region of the body is exposed to cold?
Causes vasoconstriction to capacitance/resistance vessels at that region (local axon reflex) and other extremities
Where do temperature receptors send signals to?
Temperature regulating centre of the hypothalamus
What happens to resistance/capacitance vessels when there is sustained exposure to cold?
Localised cold vasodilation
What happens if the cold keeps persisting?
Alternating vasoconstriction and vasodilation caused by local axon reflexes
What does an application of heat cause at an affected region and at other regions of the body?
Vasodilation of vessels
What is the purpose of widespread vasodilation when excessive heat is present?
Increase flow of warm blood to sub-epidermal circulation which is cooled by proximity to external environment and perspiration
What kind of nerves innervate sweat glands?
Cholinergic sympathetic nerves
What does sweat production increase the production of in skin?
Bradykinin, a potent vasodilator, which further stimulates the formation of nitric oxide
What does exercise do to cutaneous circulation?
Overall increase in sympathetic outflow resulting in vasoconstriction
What allows counter-current heat exchange?
Close proximity of major arteries and veins
What is counter-current heat exchange essential for?
Protecting the body core from extremes of heat
Which arteries are responsible for cerebral circulation?
Internal carotid and vertebral arteries which supply the basilar artery
What does the basilar artery supply and where is this structure found?
Circle of Willis which loops around the brain stem
How does blood travel from the circle of Willis into brain tissue?
Cerebral arteries
Which is higher: intercranial pressure and central venous pressure?
Intercranial pressure (0-10mmHg)
What is affected by changes in intracranial pressure?
Cerebral perfusion
What does elevated ICP (over 20mmHg) cause?
Compression of venous vessels and reduced blood flow by reducing cerebral perfusion pressure
What is the equation for cerebral perfusion pressure?
Mean arterial pressure - intracranial pressure
How can we describe the blood supply to the brain?
Remains constant i.e., the rate of inflow via arterioles = rate of outflow via venules resulting in a constant value of blood and tissue fluid
What do neuronal cells have a high requirement for?
O2 and glucose
How much O2 does the brain consume at rest and how much of the cardiac output does it receive?
consumes 20% of resting O2
Receives ~14% of cardiac output
What is brain tissue very sensitive to?
Ischaemia
How long must ischaemia last in brain tissue to cause unconsciousness and irreversible brain damage?
~5 sec for unconsciousness
~4 min for irreversible brain damage
What is the driving force for brain perfusion?
MAP
When does cerebral circulation exhibit autoregulation?
MAP between 60 and 160 mmHg
What happens when there is an MAP below 60 mmHg?
Syncope
What happens when there is an MAP above 160 mmHg?
Damage to blood brain barrier resulting in cerebral oedema
What does increased blood flow in certain neuronal regions correlate with?
C14 glucose uptake in these regions
What is paramount in regulating cerebral blood flow?
Local factors - metabolic regulation
What effect does maximum SN activity have on cerebral resistance?
May increase it by ~20%
What does metabolic regulation exert in cerebral blood flow?
Functional sympatholysis
What doesn’t really effect cerebral blood flow?
Baroreceptor reflex
How may cerebral blood flow be regulated?
Vasodilatory metabolites by inducing active hyperaemia
What is the most important vasodilatory factor in cerebral blood flow?
Increased production of carbon dioxide by neuronal cells
What can CO2 do in cerebral blood flow?
Diffuse across blood-brain barrier and reduce the pH of CSF triggering vasodilation of cerebral arterioles
What reduces washout of CO2 from brain?
Reduction in CPP
What can trigger vasodilation and vasoconstriction in cerebral blood flow?
Increased PCO2 (hypercapnia) and decreased PCO2 (hypocapnia) respectively
What mediates hypercapnia and hypocapnia in cerebral blood flow?
Changes in pH of CSF
What causes a strong vasodilation of vessels to maintain O2 supply to brain?
PO2 < 50mmHg
What may cause an elevated ICP?
Brain trauma
Cerebral oedema
What happens if ICP>MAP?
Severe cerebral ischaemia
What may happen as a result of an elevated ICP?
Shift in brain tissue within skull aka herniation
What does the cushing reflex do?
Activates the SNS causing peripheral vasoconstriction and increased cardiac output thereby increasing MAP
Increased MAP stimulates baroreceptors in carotid bodies resulting in bradycardia
What is Cushing’s triad and what are they symptoms of?
Hypertension, bradycardia, irregular respirations - brain trauma and increased ICP
What does splanchnic circulation refer to?
Blood supply to visceral organs:
GI tract
Spleen
Pancreas
Liver
Pancreas
What feature is found in the circulation of villi?
Counter-current exchange of O2 directly form arterioles to venules
What happens to blood at the base of microvilli?
Blood is shunted directly form arterioles to venules reducing O2 supply to mucosal cells at the tip - necrosis of villi may follow
How is regulation of intestinal blood flow mediated?
Both by SN and intrinsic mechanisms
When is vasoconstriction of mesenteric arterioles and capacitance vessels commonly seen?
During exercise
What is intestinal blood flow increased by?
Functional hyperaemia
What do all aspects of GI tract function have?
Vasodilatory properties
Name a digestive hormone with vasodilatory properties.
Gastrin
What increases metabolic activity of tissue in the GI?
Absorption of biomolecules
What vasodilator is produced locally?
NO
What do parasympathetic nerves innervate in the GI tract and why?
Intestinal smooth muscle and glands to increase motility and secretions - indirectly to blood flow by stimulating metabolic activity and by producing bradykinin and NO
How much of cardiac output goes to hepatic circulation?
25%
How does blood flow enter the liver?
Portal vein from GI tract, spleen and pancreas
What are the functions of the liver?
Processing and storage of GI-derived nutrients
Re-entry of GI-derived nutrients into the general circulation
Clears blood of drugs and toxins
Blood reservoir
What is O2 needed for in the hepatic artery?
Metabolic activity so the liver may perform its functions
What may impact hepatic blood flow?
GIT, splenic and pancreatic blood flow
How does an acinus work?
Portal venules and hepatic arterioles enter the centre of an acinus and deliver blood to sinusoids
Sinusoids are very leaky, permitting rapid exchange
These sinusoids radiate to the periphery of an acinus and connect with hepatic venules, hepatic veins and IVC
Is there low or high oncotic pressure of plasma in hepatic circulation?
Relatively low
What greatly affects filtration, lymph production and substance exchange in hepatic circulation?
Hydrostatic pressure in sinusoidal capillaries
How are rates of blood flow in portal venules and hepatic arterioles regulated?
Reciprocally
What may rapidly return half of liver blood into heart?
SNS-induced vasoconstriction of capacitance vessels
What does heart failure increase?
Increase in CVP
What does increased CVP cause?
Increases in hepatic venous and hepatic sinusoidal pressure leading to hepatic oedema
What does hepatic oedema lead to?
Accumulation of fluid in the abdominal cavity - ascites
When may ascites occur?
Heart failure
Liver cirrhosis as it causes portal hypertension
