CVS control mechanisms Flashcards
SNS; RAAS; Baroreceptors; Cardiac calculations; Cardiovascular stress Blood pressure measurement from practical cardiovascular physiology
What is the organisation of the sympathetic nervous system?
Branch of autonomic nervous system Fight or flight response Needed for controlling circulation via thoracic and lumbar vertebra T1 to L2 Short pre-ganglionic fibres Long post-ganglionic fibres
What are the neurotransmitters that act within the sympathetic nervous system?
Acetylcholine in a nicotinic receptor
Causes release of noradrenaline
What is renin and when is it released?
Enzyme released to cleave angiotensinogen to Ang I
Released from juxtaglomerular cells that line the afferent arterioles in kidneys when:
BP decreases - detected by macula densa cells in DCT that reabsorb more Na+ when low pressure which releases prostaglandins to stimulate renin production and release
Stimulated by SNS
What is the role of angiotensin II?
Cause vascular smooth muscle contraction
Causes PPG to secrete ADH
Causes adrenal gland to secrete aldosterone that increases reabsorption of water in kidneys =increased stroke volume
What is the synthesis pathway of angiotensin II?
Angiotensinogen from liver cleaved by renin from kidneys to Ang I
Blood enters lungs where ACE converts Ang I to Ang II
Ang II enters zona glomerulosa of adrenal cortex to stimulate aldosterone production
Explain the anatomy of baroreceptors
Specialised cells that can detect BP
Firing mirrors BP
Define cardiac output
Stroke volume x HR
Volume of blood pumped by heart per minute
(approx 5L)
Define stroke volume
EDV-ESV
Volume of blood pumped by heart per beat
approx 70 ml
Define mean arterial pressure
Cardiac output x TPR
or
DBP + 1/3Pulse pressure
(pulse pressure = SBP-DBP)
Explain the response of the cvs to standing
Standing causes increased hydrostatic pressure
Blood transiently pools in veins due to high compliance
Causes decrease in venous return which must be compensated otherwise reduction in CO and BP
Explain the response of the cvs to exercise
Increased exercise = increased venous return due to skeletal muscle pump work in legs
this increases EDV ∴ increases sympathetic activity of heart
∴increase in contractility and SV
∴Increased CO
Where does noradrenaline bind in the heart and what does it cause?
Binds to β1 adrenoreceptors on the heart
Increases force of contraction and HR ∴ larger stroke volume and increased contractility
Where is noradrenaline released from, what does it bind to and what does this cause?
Released from long post-ganglionic fibres of the SNS
Binds to α1-adrenoceptors
Causes smooth muscle contraction and vasoconstriction
What does the SNS innervate?
Heart and all vessels except capillaries
Innervation is variable with kidneys, gut and spleen being highly innervated and brain and muscle less so
Where is the vasomotor centre located and what are its main features?
Bilateral structure within medulla and lower 1/3 of pons in brain
3 main areas:
-Vasoconstrictor area
-Vasodilator area
-Cardioregulatory inhibitory area
Influenced by higher brain centres e.g. hypothalamus
What is the purpose of the vasomotor centre?
Excitatory or inhibitory
Transmits impulses to blood vessels
Lateral portions influence HR and contractility
Medial portion transmits via vagus nerve to heart to lovwer HR
Explain nervous control of blood vessel diameter.
Always some level of tonic activity to the vasculature from the SNS
Activating or decreasing baseline leads to constriction or dilation
Generally no PNS innervation to the vascular system
How can venous return be controlled?
Increased by skeletal muscle pumping/respiratory movements
Increased blood volume
Increase SNS vein activation
How does noradrenaline cause increased force of contraction?
Upregulates cAMP which causes increased activation of L type Ca2+ channels ∴ increased Ca2+ uptake and influx
Where do carotid sinus baroreceptors send impulses?
Down glossopharyngeal PNS afferent to the VasoMotor centre
Where do aortic arch baroreceptors send impulses?
Down vagus nerve PNS afferent to VasoMotor centre
What causes increased rate of baroreceptor firing and what happens when baroreceptor firing rate increases?
If BP increases, baroreceptor firing increases
Increases PNS efferent firing down vagus nerve to increase PNS activity at the heart to slow the rate by increasing length of a sinoatrial node action potential
This increases inhibitory interneuron firing ∴ inhibiting tonic SNS activity ∴ decreasing SNS activity to slow HR and cause vasodilation
What is the response of the nervous system to haemorrhage?
Decreased stroke volume = decreased baroreceptor firing
Reduced PNS&increased SNS firing to increase contractility and ∴ stroke volume
Increased SNS discharge to veins to increase tone and pressure/return
Increased SNS discharge to arterioles to increase vasoconstriction
Increased Mean Arterial Pressure due to increased CO by stroke volume and EDV from increased venous return and increased TPR by arteriolar constriction
Where is blood pressure measured?
In brachial artery on the upper arm
What is used to measure blood pressure?
Cuff, sphygmomanometer and stethoscope
How is blood pressure measured?
Cuff is inflated to obstruct blood flow by increasing pressure until the cuff exceeds arterial pressure
Release of cuff slowly whilst listening for turbulent flow - first korotkoff sound indicates SBP
Eventually artery no longer occluded and flow returns to normal laminar flow - sound disappearing = DBP
How can values obtained from manual blood pressure measurement be used to calculate mean arterial pressure?
DBP + 1/3Pulse pressure
pulse pressure = SBP-DBP
