Cardiovascular control 2 Flashcards
What determines venous return?
central venous pressure
What does venous return affect?
stroke volume and contractility by affecting the preload (Starling’s Law)
What does venoconstriction do?
reduce compliance, reduce venous return
What does arteriolar constriction do?
determines
- blood flow to downstream organs
- MAP
- pattern of blood flow to organs
How do intrinsic (local) mechanisms regulate blood flow?
closely related to smooth muscle
needed for REFLEX LOCAL BLOOD FLOW REGULATION IN ORGAN
What is autoregulation?
intrinsic capacity to compensate for changes in perfusion pressure by changing vascular resistance
WITHOUT: fall in perfusion pressure causes fall in flow rate with passive constriction leading to increased resistance
WITH: resistance in vessels falls to allow flow to increase
it is a compensatory mechanism
What is the myogenic theory of autoregulation?
some smooth muscle fibres have stretch sensitive receptors and channels that respond to tension in the vessel wall
as pressure decreases, vessels dilate as muscle relaxes
What is the metabolic theory of autoregulation?
blood flow decreases
metabolites accumulate and vessel dilate with the aim of increasing flow to wash these away
How does injury affect autoregulation?
serotonin released from platelets and causes constriction
What is hormonal control of autoregulation?
local endothelium derived hormones also have an influence
What are 2 examples of autoregulators?
VASODILATORS
NO
- from arginine
- diffuse into vascular smooth muscle cells
PGI2 (prostacyclin) - cardioprotective
- antiplatelet/coagulative
- made from PGH2 prostaglandin precursor
What are 2 examples of autoregulators?
VASOCONSTRICTORS
TXA2
- from PGH2
- made heavily in platelets to amplify their activation
ET (endothelins)
- made from nucleus of endothelial cells
- has minor vasodilator effects
How do extrinsic mechanisms regulate blood flow?
these are mechanisms extrinsic to smooth muscle
consist of hormone regulation and autonomic nervous system
What are examples of circulating (non endothelium derived) hormones?
produced by glands elsewhere
- kinins - vasodilator (from cells of atria of heart)
- bind to receptors on endothelial cells
- stimulate NO synthesis - Atrial natriuretic peptide - secreted from atria in response to stretch
- vasodilator
- reduce BP - Vasopressin/ADH - secreted from posterior pituitary with high blood osmolality
- vasoconstrictor
- bind to VI smooth muscle receptors - Nor/adrenaline - from adrenal gland
- vasoconstriction - Angiotensin II - from renin-angiotensin-aldosterone axis
- vasoconstrictor
- stimulate SNS activity and ADH secretion
- produced via ACE (targeted by ACE inhibitors to lower BP)
How does the ANS influence flow?
PNS
- emerge from cranial and sacral spinal cord regions
- control heart rate
SNS
- emerge from thoracic and lumbar vertebra
- control circulation
Describe sympathetic innervation in the body?
SNS fibres innervate heart and all vessels except capillaries, pre-capillary sphincters and some meta-arterioles
Innervation is variable
- more to kidney/gut/spleen/skin
- poor to skeletal muscle and brain
What does noradrenaline do?
binds to alpha 1 receptors
cause smooth muscle contraction –> vasoconstriction
Describe the structure of the vasomotor centre in the brain?
- located bilaterally in reticular substance of medulla, lower third of pons
- made of vasoconstrictor (pressor) and vasodilator (depressor) regions + cardioregulatory inhibitory area + higher centres (anticipatory response when thinking about activity)
What does the VMC do?
transmits impulses distally through the spinal cord to almost all blood vessels
spinal cord modulated processes of ANS
- lateral areas control HR and contractility
- medial portion transmits signals via vagus nerve to the heart (generally decrease HR)
Describe nervous control of blood vessel diameter?
- vessels receive post ganglionic sympathetic nerve innervation with noradrenaline that maintains BASELINE VASCULAR TONE
- increased tonic activity –> vasoconstriction
- decreased tonic activity –> vasodilation
- no PNS innervation to vasculature
What other factors control blood vessel diameter in addition to sympathetic vasoconstrictor nerves?
circulating hormones loca controls (O2, CO2, K, H, osmolality, metabolites)
How is heart rate controlled?
P/SNS both innervate heart - achieve balance between them
There is always some level of tone down nerves supplying the heart so if one of P/SNS cut, other becomes dominant - if both are cut, intrinsic HR takes over (above normal)
How can HR be increased?
increase SNS
decrease PNS
increase plasma adrenaline levels
How can force of contraction be controlled?
- SNS stimulation
- PNS has no effect
- Starling’s Law
