9 Special Circulation Flashcards
What is the blood flow like to the heart and brain compared to skin and muscle?
much higher
how much of the available oxygen goes to the heart?
1/3-2/3
What does a large a-vO2 difference suggest?
the vascular bed has a high oxygen consumption
What parts of the heart have a high blood supply?
the ventricles, particularly the left ventricle
What do coronary veins drain into?
the coronary sinus, and then into the right atrium
some is drained via thesbian veins
When might there be an increased demand for oxygen in the heart?
How is this met?
any increase in heart rate or contracitlity
this is met by an increase in flow
What is the distribution of blood vessels like in cardiac muscles compared to skeletal muscle?
much much denser in cardiac muscle, 3000/mm2 compared to 400/mm2 in skeletal muscle
How does myoglobin help cardiac muscle?
it has a much higher affinity to oxygen than haemoglobin, even if it can only bind to one molecule
the oxygen is passed along a chain of myoglobin, to maintin the diffusion of oxygen to mitochondria
How would you increase coronary flow?
vasodilating coronary vessels
What is the coronary flow reserve?
the difference between the resting level of flow, and the maximum flow that can be made by dilating the vessels
What does the coronary flow reserve allow?
blood flow to increase by up to 5x than resting autoregulated levels when cardiac output is increased (this ability is reduced in some cardiac conditions)
What would stimulate the coronary release of adenosine?
what does adenosine do?
increased metabolic activity
fall in coronary blood flow
fall in myocardial PO2
vasodilation - increase flow
What effect does extracellular potassium have on flow?
it contributes to the inital increase in coronary perfusion, although it is unlikely to contribute much to the sustained rises in coronary flow
How can acidosis alter vascular resistance?
changes in O2 and CO2 may cause acidosis, which can act to lower vascular resistance and increase oxygen supply
Where does stenosis usually occur?
how significant does it need to be to have effect?
large epicardial arteries, needs to exceed 60-70% reduction in lumen diameter to have a significant impact on flow
What happens in coronary artery disease?
affects artery lumen and causes endothelial damage and dysfunction
falls in NO and prostacyclin, which act as vasodilators
How might you treat unstable angina?
balloon angioplasty
implanting a stent
coronary bypass graft
Does grey or white matter receive more blood flow?
grey matter, despite the fact that there is less of it
What is the effect of grey matter being sensitive to hypoxia?
fainting occurs within seconds of hypoxia
hypoxia for over 4 minutes leads to neuronal damage
where do peel arteries go?
run over the brain surface
What is the benefit to the grey matter having a dense network of capillaries?
large surface area
low diffusion distance
Where are the fenestrations in cerebral blood vessels?
there are none, they are completely sealed by tight junctions
How can substances pass into the brain?
over the 2 cell membranes and cytoplasm of the endothelial cells
How do O2, Co2, alcohol, nicotine, and caffeine pass over the blood endothelium into the brain?
they are lipophilic, so passive diffusion
How do glucose and amino acids pass into the brain?
transport proteins
What happens to excess K+ in the brain?
what adaptation facilitates this?
it is pumped out via Na+/K+ ATPase
endothelial cells in the brain have 5x more mitochondria than muscle endothelium
how is high cerebral basal flow maintained?
peripheral vasoconstriction sacrifices perfusion to the peripheries, but not the heart
What is the autoregulatory range in the brain?
60-150mmHg
What happens in extreme cerebral hypoxia?
mental confusion
syncopy
What does cerebral hypercapnia cause?
How?
vasodilation, especially in small peel arteries
endothelial NO
fall in myocyte pH
What does cerebral hypocapnia cause?
what effect will an arterial CO2 of 2kPa have on cerebral flow?
vasoconstriction
this will halve total cerebral flow
How can cerebral flow be measured?
retinal flow, as the retina is an embryological derivative of the nervous system
What does severe hypoxia in the cerebrum stimulate?
vasodilation
hyperventilation via stimulation of carotid chemoreceptors
What is the effect of hyperventilation in response to systemic hypoxia?
systemic hypoxia evokes hyperventilation so hypoxic dilation is often masked by hypocapnic vasocontriction
What is the effect of systemic hypoxia on blood flow?
what might have a different effect?
only a minor effect, it has to be really really hypoxic before an increase in cerebral flow is observed
it is much more susceptible to changes in CO2 though
what can astrocytes do?
they surround the synapses and may also be stimulated, signalling to smooth muscle to control vessel diameter
What factors are important in coupling tissue metabolism to local flow?
increased interstitial [K+] adenosine neuronal NO metabolites released from astrocytes during increased activity CO2 (not as significant as the others)
What might ICP be increased by?
Intracranial Pressure
intracranial bleeding
cerebral edema
tumour
What might the effects of increased ICP be?
venous collapsing
decreased effective CPP, causing a reduced blood flow
How would we calculate CPP?
Central Perfusion Pressure = mean ABP - ICP
When might postural hypotention lead to syncopy?
only really if the baroreceptor reflex is impaired
What is a TIA?
transcient ischaemic attack
temporary reduction in flow (minutes to hours)
what is a stroke?
total interruption in blood flow
What is the range of blood flow exhibited by the skin?
1-200 ml/min/100g
What is the structure of cutaneous blood supply?
small arteries give rise to arterioles penetrating into the dermis
these give rise to capillaries looping under the epidermis
venules return the blood to the venous plexus
the venous plexus could also receive blood directly from subcutaneous arteries via atriovenous anastomoses
What do atriovenous anastomoses allow?
the blood to bypass cutaneous circulation
How much of cutaneous blood volume is in the venous plexus?
most of it
can be up to 1.5L
What affects changes in AVAs?
sympathetic activity, alpha adrenoreceptors
they do not show functional hyperaemia or autoregulation
What happens when temperature rises?
sympathetic nervous system stimulates vasodilation and sweat release
What stimulates sweat release and what effect does it have?
hypothalamic stimulation of sympathetic cholinergic fibres innervating the sweat glands
evaporates to remove heat
has an enzyme telling tissue to release bradykinin, a vasodilator acting on smooth muscle
How does an increased temperature affect baroreceptor afferent activity to CNS?
vasodilatoin of arterioles and AVAs
decreases TPR
decreases ABO
decreases baroreceptor afferent activity to CNS
What effect does decreasing baroreceptor afferent activity to CNS have?
decreased vagal activity to SA node
increased sympathetic activity to SA node
What happens to cutaneous circulation when temperature drops?
increase in sympathetic activity and noradrenaline release causes vasoconstriction of cutaneous arterioles
increased sympathetic activity to arteriovenous anastomoses causes constriction to prevent flow to veins
this increases resistance to blood flow, diverting flow to the interior deeper veins in fat
What happens in prolonged exposure to cold?
vasoconstriction in cutaneous circulation changes to a paradoxical cold circulation
What causes paradoxical cold circulation?
paralysis of noradrenergic neurotransmission in response to cold, and the release of vasodilators like prostacyclin
redness caused by increased affinity of O2 for haemoglobin
(left shift of O2 dissociation curve)
Why is it useful to have arteries and veins next to each other?
counter-current heat exchange preserves body temperature
What happens in Raynaud’s disease?
cold / emotional stimuli lead to vasoconstriction
white, blue, then red
numbness, pain, burning sensation
