CVS 9 - Regulation of the CVS Flashcards
Recall the key equations for SV, CO, MBP, flow (Q), Resistance
SV = EDV - ESV CO = SV x HR MBP = CO x TPR Q = difference in P / R R proportional to 1/r^4
The volume of blood in our veins depends on:
Peripheral venous tone
Gravity
Skeletal muscle pump
Breathing (respiratory pump)
NOTE
Venous return determines SV (Starling’s law)
How do we alter flow? How is this done (3) ?
Alter flow by changing vessel radius
This is done in 3 ways:
LOCAL MECHANISMS – intrinsic to the smooth muscle
SYSTEMIC REGULATION – due to circulating hormones
AUTONOMIC NERVOUS SYSTEM
Describe local mechanism to regulate blood flow
Autoregulation: the intrinsic capacity to compensate for changes in perfusion pressure by changing vascular resistance.
This means that if Perfusion Pressure drops,, local mechanisms cause resistance to decrease for blood to flow better. (Q=P/R)
What are the 2 theories of autoregulation?
MYOGENIC THEORY
The smooth muscles themselves are sensitive to stretch.
Say pressure drops the tension (or stretch) of the vessels fall.
The smooth muscles senses this and vasodilates as a result to INCREASE FLOW
METABOLIC THEORY
When pressure drops and flow reduces the blood accumulates in the vessels
In the blood are a bunch of metabolites which act on the vascular smooth muscle
Thus, causing vasodilation
When the flow increases again those metabolites are taken away by the blood
State two molecules produced in the endothelium that cause vasodilation.
Nitric Oxide and Prostacyclin
State two molecules produced in the endothelium that cause vasoconstriction.
Thromboxane A2
Endothelin-1
State five hormones that regulate blood flow.
Vasodilators
Atrial Natriuretic Peptide (ANP) (from atria as a response to RA stretch)
Kinins (e.g. bradykinin) - oppose ANG-II
Vasoconstrictors:
Vasopressin (ADH) from posterior pituitary
Angiotensin II
Noradrenaline released from adrenal medulla (not ANS)
What neurotransmitter do all parasympathetic neurones release?
ACh
What neurotransmitters could sympathetic neurones release?
Noradrenaline, adrenaline or acetylcholine (sweat glands)
Which vessels do not have sympathetic innervation?
Capillaries, pre-capillary sphincters and metarterioles
sympathetic innervates all the others including large veins
Which particular organ also has sympathetic innervation?
The heart
NOTE
Distribution to other organs is variable.
To which receptors does NA bind to?
Noradrenaline binds to α1-adrenoceptors to cause smooth muscle contraction and vasoconstriction.
To which receptors does A bind to?
To beta2 receptors on smooth muscle to cause vasodilation.
HOWEVER at HIGH CONC. A binds to aplha1 and alpha2 receptors which overrides vasodilator effect of Beta2 and causes vasoconstriction.
The distribution of sympathetic nerve fibres is not even. Where do you find more sympathetic nerve fibres?
There are more sympathetic nerve fibres in the spleen, gut, kidneys and skin because there is more potential to divert blood away from these organs without causing damage. There are fewer sympathetic fibres in the skeletal muscle and brain.
Where is the vasomotor centre located?
VMC is located bilaterally within medulla & lower ⅓ of pons 3 areas: - vasodilator (depressor) - vasoconstrictor (pressor) - cardioregulatory inhibitory area
The role of the VMC is to transmit signal to almost all vessels.
The lateral portions control HR and contractility.
Medial portions transmit signals via the vagus nerve to the heart -> decrease HR
Describe nervous control of blood vessel diameter
There is always some level of tonic activity to the vasculature
Activating or decreasing this tonic baseline leads to constriction/dilation
Generally there is NO PARASYMPATHETIC innervation to vascular system
Transmitter = NA
What effects does the sympathetic nervous system have on the cardiovascular system?
Increased sympathetic activity increases heart rate, increases force of contraction and decreases the vessel radius (increases resistance)
What effect does the parasympathetic nervous system have on the cardiovascular system?
It decreases heart rate
Causes vasoconstriction of the coronary arteries
Other than decreased sympathetic activity, increased sympathetic activity, what increases HR?
Increased plasma adrenaline
Describe the mechanism of action of noradrenaline on the heart.
Sympathetic nerves can increase the heart’s force of contraction. They release NA.
Noradrenaline binds to the beta-1 adrenergic receptor on myocyte, which causes activation of adenylate cyclase and the upregulation of cAMP leading to the activation of PKA. PKA phosphorylates, hence activates, the L-type calcium channel, Sarco/endoplasmic calcium ATPase (SERCA), SR calcium release channel,
Therefore,
Calcium influx increases
Calcium uptake into intracellular stores
What are the three ways in which stroke volume can be increased?
Starling’s Law (increased EDV by increased venous return to the heart + increased respiratory movements which decrease introthoracic pressure which assists in increased EDV)
Increased plasma adrenaline
Increased sympathetic innervation
How do respiratory movements affect the cardiac output?
Inspiring decreases the intrathoracic pressure meaning that the pressure on the blood vessels is decreased so the blood flows more easily into the heart and hence it increases the stretch on the heart (preload) so EDV increases so cardiac output increases (Starling’s Law)
What nerves carry information from the two baroreceptors to the vasomotor centre?
Carotid Bodies - Glossopharyngeal nerves
Aortic arch- Vagus nerve
What are the 2 baroreceptors?
Aortic arch
One on each carotid sinus
What is the consequence of increased baroreceptor activity?
Parasympathetic activity mirrors baroreceptor activity. Increases baroreceptor activity leads to increased parasympathetic activity and decreased sympathetic activity. The increase in parasympathetic activity leads to a reduction in heart rate so a decreased in BP (as increased baroreceptor activity = increased BP)
What are the effects of a decrease in sympathetic activity?
Decrease in sympathetic activity decreases heart rate and decreases force of contraction of the heart. This also increases the blood vessel radius and decreases resistance.
What is the point in this whole response?
To maintain blood pressure
