Physiology 3120 Lecture Notes - Lecture 36: Baroreflex, External Carotid Artery, Carotid Sinus
15 May 2018
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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
find more resources at oneclass.com
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