Physiology 3120 Lecture Notes - Lecture 36: Baroreflex, External Carotid Artery, Carotid Sinus

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Lecture 36 Baroreceptor Reflex and CV Control Centre
CV regulatory mechanisms
Mechanisms for the control of circulation (REGUALTING BLOOD FLOW):
- 1. Local mechanisms (autoregulation) regulating blood flow at a normal level
o Myogenic theory
o Metabolic theory
- 2. Humoral mechanisms hormones
o Vasoconstrictors (increase MAP)
o Vasodilators (decrease MAP)
- 3. Neural mechanisms (ANS)
o Cholinergic decreases MAP
o Adrenergic increases MAP
- PSYN: influences the heart predominantly and does NOT have a direct influence on b.v
o Changes the conductance of the ions in the SA nodal cells decreases slope of the
pacemaker potential and hyperpolarizes the cells so it takes longer to reach
threshold
o Activated PNS = shutting off SNS which causes a general vasoconstriction
o Rest or digest system: decrease MAP
o PSYN also stimulates contraction of the veins increase venous return increase
EDV increase SV increase MAP
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- SYN neural - nerves innervating blood vessels or the heart directly
o Neurotransmitter: norepinephrine
o Increases force of contraction/SV by increasing the slope of the pacemaker potential
o Increase amount of calcium entering cardiac contractile cells = increase SV
o On blood vessels: general vasoconstriction but there are also cholinergic vasodilator
fibers that go to blood vessels in skeletal muscle and release Ach = vasodilation
- SYN hormones
o SYN innervates the adrenal gland and releases Ach onto it
o Adrenal gland releases primarily epinephrine with a little bit of nor. which circulate
in the blood and bind to their receptors (ac bd)
Alpha1: general vasoconstriction throughtout the body
Beta2: only epi binds, norepinephrine doesn’t
- VDMs from exercising tissue can block the effects of epinephrine and norepinephrine on
blood vessel receptors = vasoconstriction can be shut off in exercising muscle which is what
you want to do because you want to increase blood flow to those areas
Baroreceptor reflex
- Homeostatic mechanism (negative feedback)
- Function: maintains normal mean arterial pressure for proper perfusion of tissues
throughout the body
o If BP is too low = pass-out because blood flow to brain is too little
o If BP is too high = cardiovascular disease
- Involves rapid adjustments to both cardiac output and total peripheral resistance (neural
effects are very rapid)
- This mechanism is in effect when you have been sitting down for too long and then you
stand up blood pools to legs, bp drops baroreceptor reflex helps bring bp up again
- Keeps blood pressure at constant levels
- Heart regulates cardiac output and blood vessels regulate TPR
Relies on specialized structures:
- 1. Mechanoreceptors: found in areas of the cardiovascular system
o Higher pressure baroreceptors in carotid arch and aortic sinuses (involved in reflex)
o Low pressure blood volume receptors (not involved in baroreceptor reflex)
- 2. Cardiovascular control centre in medulla and brainstem
o Receives information from receptors and ensures input from receptors matches a
set point if input from the receptors changes, it causes a change in CO or TPR to
ensure MAP is maintained
o Regulates the output of information
o Negative feedback system
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High pressure baroreceptors
- Receptors are located in the walls (tunica externa) of:
o Aortic arch
In the walls of the aorta
o Carotid sinus
Swelling of the internal and external carotid artery
- Stretch sensitive receptors constantly monitor changes in arterial pressure and relay info to
CV centre
o There is information going to the CV centre even when MAP is at its proper level
o Blood pressure increase = firing rate increases
o Blood pressure decreases = firing rate decreases
o They monitor the stretch of the blood vessels
- Increased BP stretches walls of blood vessels increase AP firing rate by the
baroreceptors signals sent to CV centre in the brainstem
- Decreased BP less stretch of the blood vessels decrease AP firing rate signals to the
CV centre that blood pressure has dropped
Low pressure volume receptors
- (Not involved in baroreceptor reflex but involved in CV regulatory mechanisms)
- Located in left and right atrium
o Predominantly in the right atrium because that is where the venous side of the
systemic circulation is (contains majority of our blood)
o Since they are volume receptors they must be close to where most of the blood is
contained
- Detect fullness of circulation
- Help regulate blood volume
- Blood pressure is related to blood volume:
o The higher the blood volume the higher the MAP (and visa versa)
o Dehydrated = low blood volume, low blood pressure
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