12/1/2014 Medical Physiology Autonomic Control of Blood Pressure Mary Lou Vallano Flashcards
What do local intrinsic mechanisms for vascular regulation aim to do?
Regulate regional blood flow (metabolic, myogenic)
What do neuronal and hormonal mechanisms for vascular regulation aim to do?
Regulate mean arterial pressure (MAP) to maintain adequate tissue perfusion
What is Mean Arterial Pressure equal to?
MAP = CO (HR x SV) x TPR
How is MAP monitored?
- high- pressure arterial baroreceptors
- renal juxtaglomerular apparatus
- low-pressure baroreceptors
How are adjustments made during vascular regulation?
Via de ANS and release of specific hormones
Is the arterial baroreceptor reflex the single most important mechanism providing short-term regulation of arterial pressure?
Yes
Acute regulation of arterial pressure is primarily mediated by the ___, with contributions by ___ (aka low-pressure baroreceptors or volume receptors) and ___ (primarily concerned with respiratory control)
high-pressure baroreceptor reflex
cardiopulmonary receptors
chemoreceptors
Chronic regulation of arterial pressure is linked to volume control by the ___
kidneys
Where are baroreceptors located in the body?
In the walls of several large systemic arteries (in particular the carotid sinus and aortic arch)
What is the mechanism of baroreceptor vascular control?
- a rise in arterial pressure stretches the baroreceptors
- baroreceptors transmit more action potentials to the CNS medullary control centers
- “feedback” signals are then sent back through the ANS to the circulation to reduce arterial pressure downward toward the normal level
* a fall in arterial pressure has the converse effects on ANS efferent output
Signals from the “carotid baroreceptors” in the carotid sinus are transmitted through small Hering’s nerves to ___ nerves in the high neck, and then to the ___ in the medulla
glossopharyngeal (IX)
nucleus tractus solitarius (NTS)
Signals from the “aortic baroreceptors” in the aortic arch are transmitted through the ___ also to ___ in the medulla
vagus nerves (X)
nucleus tractus solitarius (NTS)
The two most important high-pressure loci are the ___ and the ___
carotid sinus;
aortic arch
What does stretching of the distensible vessel walls at either the carotid sinus or the aortic arch lead to?
reflex vasodilation and bradycardia
Peripheral chemoreceptors located in the carotid and aortic bodies, are in close contact with ___
arterial blood
The chemoreceptor reflex is not a powerful arterial pressure controller until the arterial pressure falls below ___
80 mm Hg (e.g., hemorrhage)
A low PO2 acting on the peripheral chemoreceptor and a high PCO2 acting on the central chemoreceptor act in concert to enhance ___
vasoconstriction
The cardiovascular system also has ___ that detect changes in venous pressure/volume
low-pressure baroreceptors
What are baroreceptors in the carotid sinus and aortic arch sensitive to?
Stretch
Do baroreceptors increase their rate of firing in response to stretch (increase MAP) in a frequency-dependent manner?
Yes
Carotid sinus baroreceptors (which transmit impulses in Hering’s nerves) are not stimulated by pressures between ___
0 and 50 to 60 mm Hg
How do carotid sinus baroreceptors respond above 60 mm Hg?
Progressively more rapidly and reach a maximum at about 180 mm Hg
How are the responses of the aortic baroreceptors different from those of the carotid receptors?
They operate, in general, at arterial pressure levels about 30 mm Hg higher (higher threshold means less sensitive)
Does the baroreceptor feedback mechanism function most effectively in the pressure range where it is most needed?
Yes
The baroreceptors respond ___ to changes in arterial pressure
rapidly
With baroreceptors, the rate of impulse firing ___ in the fraction of a second during each systole and ___ again during diastole
increases; decreases
The baroreceptors respond much more to a ___ changing pressure than to a stationary pressure
rapidly
The baroreceptor reflex adapts to ___ changes in mean arterial pressure
long-term
In hypertension, the set point is ___ so the curve is parallel and shifted to the ___
increased; right
Why is the baroreceptor system called a pressure buffer system?
Because it opposes either increases or decreases in arterial pressure
*the nerves from the baroreceptors are sometimes referred to as “buffer nerves”
What is a primary purpose of the arterial baroreceptor system?
To reduce the minute-by-minute variation in arterial pressure to about one-third that which would occur if the baroreceptor system was not present
In most tissues, sympathetic nerve fibers innervate all vessels except the ___
capillaries
Are precapillary sphincters and metarterioles innervated in some tissues?
Yes –> e.g., mesenteric blood vessels, but it’s usually not as dense as in the small arteries, arterioles, and veins
The innervation of the small arteries and arterioles allows sympathetic stimulation to ___ resistance to blood flow and thereby to ___ rate of blood flow through the tissues
increase; decrease
What are the consequences of innervation of the large vessels, particularly of the veins?
It makes it possible for sympathetic stimulation to decrease the volume of these vessels–>this can push blood into the heart and thereby play a major role in regulation of cardiac output
What does the vasoconstrictor area of the vasomotor center do under normal conditions? What does this cause?
It transmits signals continuously to the sympathetic vasoconstrictor nerve fibers over the entire body, causing slow firing of these fibers at a rate of about one half to two impulses per second
What is vasomotor tone?
Impulses that normally maintain a partial state of contraction in the blood vessels
What happens during total spinal anesthesia?
Blocking of all transmission of sympathetic nerve impulses from the spinal cord to the periphery –> this results in the arterial pressure falling demonstrating the effect of losing vasoconstrictor tone throughout the body.
If norepi is injected a few minutes after total spinal anesthesia, the vessels once again become ___ and the arterial pressure ___ to a level even greater than normal for 1 to 3 minutes, until the norepinephrine is metabolized
constricted; rises
Is there a loss of vasomotor tone during total spinal anesthesia?
Yes
Describe the baroreceptor reflex
- an increase in MAP (stimulus) activates high-pressure baroreceptors (detectors on the high pressure/arterial portion of the CV system) whose signal is carried via afferent nerve pathways to a central coordinating center in the medulla oblongata (integrator/set point)
- the coordinating center uses efferent ANS pathways to decrease heart rate and dilate vessels (effectors)
- reflex bradycardia and vasodilation lead to a decrease in MAP, counteracting the initial stimulus (negative feedback)
What are the net effects of the baroreceptor signals entering the tractus solitarius of the medulla, leading to secondary signals inhibiting the vasoconstrictor center and exciting the vagal parasympathetic center?
(1) vasodilation of the veins and arterioles throughout the peripheral circulatory system
(2) decreased heart rate and strength of heart contraction
Excitation of the baroreceptors by high pressure in the arteries reflexly causes the arterial pressure to ___ because of both a decrease in peripheral resistance and a decrease in cardiac output
decrease
A decrease in MAP reflexly causes the pressure to ___ back toward normal (tachycardia & vasoconstriction)
rise
The baroreceptor reflex ___ to long-term changes in mean arterial pressure .
adapts
* in hypertension, the set point is raised
A change in posture from standing to lying down ___ venous return and thus stroke volume which ___ MAP
increases; increases
Increased MAP ___ the firing rate of the high-pressure baroreceptors
enhances
During the baroreceptor reflex when changing posture, the CNS medullary center responds by ___ efferent activity of the PNS in conjunction with
___ efferent activity of the SNS
increasing; decreasing
The baroreceptor reflex when changing posture leads to ___ and ___, thus returning MAP back towards the normal operating range
bradycardia; vasodilation
What does the carotid sinus massage (or release from a Valsalva maneuver) do?
Stimulates the baroreceptors and reflexly slows the heart in people with atrial tachycardia (e.g., atria flutter or fibrillation)
What happens to patients with carotid sinus syndrome?
Have hyper-sensitive baroreceptors such that even mild external pressure to the neck elicits a strong reflex, even stopping the heart for 5-10 seconds
Do both sympathetic and parasympathetic nerves innervate the SA and AV nodes?
Yes
What innervates the myocardium?
Sympathetic nerves
Arterial and venous vessels throughout most of the body are innervated by ___ nerves, whereas the ___ nervous system innervates vessels in the genital organs and lower GI tract
sympathetic; parasympathetic
Autonomic efferent activity is regulated by the ___ reflex, in response to changes in MAP detected at the carotid sinus and aortic arch baroreceptors
baroreceptor
Is the ability of the baroreceptors to maintain relatively constant MAP in the upper body important when a person stands up after having been lying down?
Yes
What happens after immediately standing?
The arterial pressure in the head and upper part of the body tends to fall, and marked reduction of this pressure could cause dizziness or loss of consciousness (e.g., orthostatic hypotension, postural syncope)
How is loss of consciousness after immediately standing avoided?
The falling pressure at the baroreceptors elicits an immediate reflex, resulting in strong sympathetic discharge throughout the body
This minimizes the decrease in pressure in the head and upper body
What is utility of the Valsalva maneuver?
It can be used to test the integrity of the baroreceptor reflex
Describe the Valsava Maneuver
- expire against a closed glottis (as during heavy lifting, coughing, defecating)
- increase in intrathoracic pressure and a decrease in venous return to the heart, which decreases CO and MAP occur
- intact baroreceptors will sense the decrease in MAP and direct an increase in SNS and decrease in PNS outflow to the heart and vessels
- the increase in HR is measured
- a rebound decrease in HR is noted after release from the maneuver
A decrease in MAP (e.g., hemorrhage, changing posture from lying to standing) ___ the stretch sensed by baroreceptors, decreasing baroreceptor impulses sent to the control centers in the medulla
decreases
To counteract the decrease in MAP, the medullary control center enhances ___ discharge, leading to vasoconstriction + increased rate and strength of cardiac contractility in conjunction with decreased ___ discharge, also resulting in increased heart rate
SNS; PNS
Where are cardiovascular control centers for both SNS & PNS?
In the medulla oblongata
