Autonomic Control of Blood Pressure

Question 1

what is the single most important mechanism providing short-term regulation of arterial pressure?

Answer 1

the arterial baroreceptor, most importantly high-pressure carotid sinus, then aortic arch

Question 2

how is MAP monitored?

Answer 2

1. high-pressure arterial baroreceptors (on arterial side, most importantly carotid sinus then aortic arch)
2. renal juxtaglomerular apparatus
3. low-pressure baroreceptors (AKA volume or cardiopulmonary receptors, on venous side and atrium)
4. chemoreceptors (although mostly for respiratory control)

Question 3

how are adjustments to MAP made?

Answer 3

via the ANS and release of specific hormones

Question 4

where do all baroreceptors feed back to?

Answer 4

the nucleus tractus solitarius (NTS) in medulla

Question 5

where do carotid baroreceptors feed back to?

Answer 5

from carotid sinus; transmitted thru Hering’s nerve to CN IX in high neck, then to NTS

Question 6

where do aortic baroreceptors feed back to?

Answer 6

from aortic arch; transmitted thru CN X to NTS

Question 7

what happens if there is stretching of distensible vessel walls at the high-pressure baroreceptors?

Answer 7

stretching at carotid sinus and oartic arch causes reflex vasodilation and bradycardia

Question 8

peripheral chemoreceptors

Answer 8

located in carotid and aortic bodies, and in close contact with arterial blood

• when arterial pressure falls below a critical level, the receptors are stimulated b/c less blood flow causes less O2
• signals transmitted from chemoreceptors and baroreceptors pass thru Hering’s and IX nerves (if carotid) and vagus (if aortic) to NTS to elevate MAP back to normal

Question 9

is the chemoreceptor reflex a powerful MAP controller?

Answer 9

not until MAP falls below 80 mmHg (like during hemorrhage)

-thus it’s at lower pressures that this reflex becomes important to prevent further decreases in MAP

Question 10

central chemoreceptors

Answer 10

in the medulla

• sensitive to decreases in brain pH reflecting increase in arterial PCO2
• causes increase in SNS output

Question 11

what do low PO2 and high PCO2 act on, what what is their effect?

Answer 11

low PO2 acts on peripheral chemoreceptors, and high PCO2 acts on central chemoreceptor
-they act in concert to enhance vasoconstriction

Question 12

low-pressure baroreceptors

Answer 12

in cardiovascular system

-detect changes in venous pressure/volu

Question 13

what happens to baroreceptor APs and receptor potentials in response to higher pressure?

Answer 13

higher pressure (steps) –> higher receptor potential (depolarization) –> more frequent APs

Question 14

structure of baroreceptors in carotid sinus and aortic arch, and what are they sensitive to?

Answer 14

branched terminals of myelinated and unmyelinated sensory nerve fibers, intermeshed within elastic layers
-they are sensitive to stretch

Question 15

what does an increase in transmural pressure difference do?

Answer 15

enlarges the vessel, deforming the receptors, and increasing firing rate of the baroreceptor’s sensory nerve
-the signal is frequency modulated

Question 16

to what do baroreceptors respond to, and when are they most sensitive?

Answer 16

they respond rapidly to changes in MAP and are most sensitive in normal operating range ~100 mmHg for carotid sinus, and 130 mmHg for aortic arch (less sensitive)
-the slope of dI/dP is maximum at this time

Question 17

when do carotid sinus baroreceptors not work?

Answer 17

they are not stimulated by pressures between 0 to 50 or 60 mmHg
-above these levels, they respond progressively more rapidly and reach a maximum at 180 mmHg, and are optimal at 100 mmHg

Question 18

what happens to the rate of impulse firing during systole and diastole?

Answer 18

the rate increases in the fraction of a second during each systole, and decreases again during diastole

Question 19

do baroreceptors respond more to rapidly changing pressures or stationary pressures?

Answer 19

they respond more to rapidly changing pressures
-ex: if MAP is 150 mmHg, but is rising rapidly, the rate of impulse transmission may be twice that when the pressure is stationary at 150 mmHg

Question 20

what happens to the baroreceptor reflex in HTN?

Answer 20

since it adapts to long-term changes in MAP, the curve is parallel and shifts to right (meaning it’s not as sensitive)

Question 21

what would happen to MAP if baroreceptors were absent?

Answer 21

MAP would fluctuate wildly if the baroreceptors were denervated, and no longer be constantly monitored
-the MAP would have a very wide range, instead of the usual ~100mmHg

Question 22

what is the primary purpose of arterial baroreceptor system?

Answer 22

reduce the minute-by-minute variation in arterial pressure to about one-third that which would occur if the baroreceptor system was not present

Question 23

in general, how many vessels does the SNS innervate? what does this mean?

Answer 23

in most tissues they innervate all vessels except capillaries

• precapillary spinchters and metarterials are innervated in some, but not as densely
• innervation of small arteries/arterioles allows SNS stimulation to increase R and decrease Q
• innervation of large vessels, especially veins, allows SNS to vasoconstrict to push blood into heart to regulate CO (decrease compliance of vessels)

Question 24

what does the vasoconstrictor area of the vasomotor center do?

Answer 24

it transmits signals continuously to the sympathetic vasoconstrictor nerve fibers over the entire body

• causes slow firing of these fibers at a rate of about 1/2 to 2 impulses per second
• these impulses normally maintain a partial state of contraction in blood vessels, called vasomotor tone

Question 25

what does total spinal anesthesia do? and how can it be reversed?

Answer 25

it blocks all transmission of sympathetic nerve impulses from the spinal cord to the periphery

• thus, the MAP falls from 100 to 50 mmHg, demonstrating the effect of losing vasoconstrictor tone throughout the body
• it can be reversed by injecting NE so vessels constrict and MAP rises to an even greater level (~140 mmHg, for several minutes) until the NE is metabolized, and MAP decreases again

Question 26

what does an increase in MAP cause?

Answer 26

activates negative feedback via high-pressure baroreceptors (detector)

• go via their afferent pathways to coordinating center (NTS in medulla)
• efferent pathways cause vasodilation of veins/arterioles thru peripheral circulatory system, and decreased HR and strength of contraction
• bradycardia and vasodilation will decrease MAP

Question 27

what happens when one changes posture from standing to lying down?

Answer 27

there is an increase in (central) venous return and SV, which leads to an increase in MAP

• increases firing rate of baroreceptor afferent fibers, which feed back to NTS in medulla
• effectors increase PNS and decrease SNS, which feed back to SA node
• work together to decreases HR and CO (thus decrease SV)
• SNS decreases peripheral resistance in arterioles, decrease venous tone in veins, and decrease contractility in ventricles to decrease SV and CO
• altogether, will decrease MAP

Question 28

what happens when one stands up from lying down?

Answer 28

pooling of blood in veins will decrease arteriolar pressure

• the baroreceptor fires less, thus causing and increase in sympathetic outflow (increased HR, contractility, CO, arterolar and vein constriction to increase TPR
• minimizes the decrease in pressure in head and upper body

Question 29

what does the carotid sinus massage, or release from a Valsalva maneuver do?

Answer 29

stimulates the baroreceptors and reflexly slows heart in people with atrial tachycardia (atria flutter or fibrillation) to decrease HR

Question 30

carotid sinus syndrome

Answer 30

patients have hyper-sensitive baroreceptors such that even mild external pressure to the neck causes a strong reflex, and may even stop the heart for 5-10 seconds (syncope)

Question 31

what is the Valsalva maneuver used for?

Answer 31

test the integrity of the baroreceptor reflex

• patient expires against a closed glottis, which increases intrathoracic pressure, thus decreasing venous return to heart, CO, and MAP
• intact baroreceptors sense decrease in MAP, and direct increase in SNS and decrease in PNS outflow to heart and vessels
• HR is remeasured (should have gone up)
• rebound decrease in HR is noted after release