6. Control of the cardiovascular system 2 Flashcards
Design of the CVS
2 Circulations: Pulmonary and Systemic
Right heart- Lungs- Left heart- Body
What is venous volume distribution affected by?
Peripheral venous tone (how constricted the veins are)
Gravity
Skeletal muscle pump
Breathing
What is central venous pressure and what does it determine?
Mean pressure in the right atrium
Determines the amount of blood flowing back to the heart.
What does constriction of veins do?
Reduces compliance
Increases venous return
What does constriction of arteries determine?
Blood flow to downstream organs
Mean arterial blood pressure
The pattern of blood flow to organs
Local regulation of blood flow
Local mechanisms are intrinsic to the smooth muscle (or closely associated)
Important for reflex local blood flow regulation within an organ/ to an organ.
Systemic regulation of blood flow
Systemic mechanisms are extrinsic to the smooth muscle:
Circulating hormones e.g. those released by adrenal gland
Autonomic nervous system
What would lack of local mechanisms regulating blood flow result in?
If we drop perfusion pressure, there would be a decrease in flow (as it is primarily determined by pressure difference)
Autoregulation
Intrinsic capacity of the vessels to compensate for changes in perfusion pressure by changing vascular resistance
When perfusion pressure drops, resistance in vessels drops to allow flow to get back up to nearly normal (compensatory mechanism)
What are the 2 theories of why auto regulation occurs?
Myogenic theory: states that smooth muscle fibres respond to tension in the vessel wall (e.g. as pressure rises, fibres contract; stretch sensitive channels involved)
Metabolic theory: as blood flow decreases, metabolites accumulate downstream and vessels dilate; subsequent increased flow ‘washes’ metabolites away
Name 4 local (endothelium derived) hormones that affect blood flow and state how
Nitric oxide: Vasodilator
Prostacyclin: Vasodilator
Thromboxane A2: Vasoconstrictor
Endothelins: Vasoconstrictor (minor vasodilator effect)
Name 5 circulating (non-endothelium derived) hormones that affect blood flow and state how
Kinins: Vasodilator
Atrial natriuretic peptide (ANP): Vasodilator
Vasopressin (ADH): Vasoconstricor
Noradrenaline/Adrenaline: Vasoconstrictior
Angiotensin II: Vasoconstrictor
What do pre-ganglionic fibres use as their neurotransmitter?
ACh
Describe the Parasympathetic nervous system (PNS)
‘rest and digest’
PNS nerves tend to arise from Cranial and Sacral parts of spinal cord
PNS is important for controlling heart rate
Long pre-ganglionic fibres
Ganglia near target organ
Short post-ganglionic neurone
What neurotransmitter is used by the PNS?
ACh
Describe the Sympathetic nervous system (SNS)
‘fight or flight’
SNS nerves arise from thoracic and lumbar vertebrae
SNS is important for controlling the circulation and vessel radii
Short pre-ganglionic fibres
Long post-ganglionic neurone
What neurotransmitter is used by the SNS?
Noradrenaline
Sympathetic innervation to blood vessels
SNS fibers innervate the heart and ALL VESSELS except capillaries, precapillary sphincters and some metarterioles
SNS innervation is variable, different vascular beds have different degrees of innervation and different expression levels of receptors
Heavily innervated: kidneys, gut, spleen and skin
Poorly innervated: skeletal muscle and brain
What does noradrenaline bind to on blood vessels and what does this cause?
Binds to alpha-1 adrenoceptors to cause smooth muscle contraction and vasoconstriction
Where is the vasomotor centre?
VMC is located bilaterally in the reticular substance of the medulla and the lower 1/3 of the pons
What is the vasomotor centre composed of?
A vasoconstrictor (pressor) area A vasodilator (depressor) area A cardioregulatory inhibitory area
Where does the vasomotor centre transmit impulses?
Transmits impulses distally through the spinal cord to almost all blood vessels
What can exert excitatory/ inhibitory effects of the vasomotor centre?
Many higher centres of the brain e.g. hypothalamus
What do the lateral portions of the vasomotor centre control?
Heart activity by influencing heart rate and contractility
What does the medial portion of the vasomotor centre do?
Transmits signals via vagus nerve to heart that tend to decrease heart rate.
Nervous control of vessel diameter
Blood vessels receive SNS post-ganglionic innervation (noradrenaline (NA))
Always some level of tonic activity (producing a baseline constriction) which can be:
Increased: causing vasoconstriction
Decreased: causing vasodilation
Generally, no PNS innervation to blood vessels
How can heart rate be increased?
Increased SN activity
Increase in circulating adrenaline or noradrenaline
Decreased PN activity
How does the SNS influence force of contraction?
NA binds to Beta-1 receptor on heart cells
Increases cAMP
Increases PKA activity
Phosphorlyates Ca2+ handling proteins
Increases influx of Ca2+
Increases amount pumped up to stores and increases amount delivered to myofilaments
Increases force of contraction
How can stroke volume be controlled extrinsically?
Increase SNS activity to heart: increases force of contraction
Increase plasma adrenaline: binds to Beta-1 receptor on heart: increases force of contraction
How can stroke volume be controlled intrinsically?
Starling’s law:
Increase venous return: increase end diastolic volume
Increase stretch and increase force of contraction
What 3 mechanisms increase cardiac output in a Fight or flight response?
Increase in plasma adrenaline: (stroke volume and heart rate)
Increased SNS efferents to heart: (stroke volume and heart rate)
Increased respiratory movements: (stroke volume)
Describe the feedback mechanism involved in regulating blood pressure
Cardiovascular control centre sends signals to SNS and to evoke release of angiotensin II/ adrenaline/ ADH systemically
Causes vasodilation/ vasoconstriction to produce a new BP
New BP detected by Baroreceptors
Baroreceptors send signals back to cardiovascular control centre
Where are baroreceptors found? What do they do?
Aortic arch
Carotid bodies
Send pulses back to vasomotor centre
Baroreceptors in the carotid bodies send afferent PNS pulses along which nerve?
Glossopharyngeal nerve
Baroreceptors in the aortic arch send afferent PNS pulses along which nerve?
Vagus nerve
Vagus nerve also has efference to SAN
How do baroreceptors respond to stretch?
More stretch: Fire more
Less stretch: Fire less
What type of receptors are baroreceptors?
Mechanoreceptors
What pressures do the carotid sinus baroreceptors respond to? How?
Changes in arterial pressure
Pressures between 60 and 180 mmHg
Fire more when pressure builds up as there is greater stretch in the vessels
At which pressures are the baroreceptor reflex most sensitive?
90-100 mmHg
Describe reciprocal innervation
Increased pressure detected by baroreceptors, increase afferent input:
Increases PNS activity to heart (decreases HR)
Simultaneously, it inhibits SNS activity
Inhibitory interneurons can inhibit tonic activity, decreasing activity of SNS, decreasing innervation to heart, arterioles and veins
=Decrease HR at SAN
What does decreased sympathetic stimulation of blood vessels cause?
Vasodilation
How does the PNS respond to increased BP?
Increases traffic from baroreceptors fed to vasomotor centre
Nerve traffic in vagus nerve closely mirrors traffic coming from baroreceptors
Increases nerve activity in vagus nerve and PNS
Stimulates SAN to slow HR
How does the SNS respond to increased BP?
Tonic activity decreased
In sympathetic cardiac nerves, due to recipricol innervation, it does the opposite e.g.
High baroreceptor nerve activity = Low sympathetic cardiac nerve activity, Slows HR
In sympathetic vasoconstrictor nerves there is also little nerve activity = Vasodilation
How can venous return be increased?
Increasing blood volume returning to right atrium
Increasing SNS activation of veins: Constricts veins and pushes more blood back to heart, decreasing capacitance of veins and venules
Increasing skeletal pump activity and respiratory movements
What are the compensatory mechanisms of cardiovascular system when large blood loss occurs e.g. Haemorrhage?
Haemorrhage causes decrease in blood volume, which decreases venous pressure and venous return
This decreases EDV and as there is less stretch, SV decreases and CO decreases = Fall in BP
Baroreceptors detect fall in BP
Increases SNS discharge to veins: constricts capacitance vessels to push blood back to heart
What reflexes are in place to maintain arterial pressure?
Decrease PNS discharge to heart and increase in SNS discharge to heart: Increases contractility: Increases SV: CO
Increase SNS discharge to veins, increases venous tone, increases venous pressure, increases venous return, increases EDV: Increases contraction
Increase SNS discharge to arterioles, increases arteriolar vasoconstriction, increases total peripheral resistance
