Autonomic Control of Blood Pressure Flashcards
Vascular Regulation Overview
-local intrinsic mechanisms for vascular regulation are primarily aimed at regulating regional blood flow (metabolic, myogenic)
-neural and hormonal mechanisms are often aimed at regulating mean arterial blood pressure (MAP) to maintain adequate tissue perfusion
-monitored at several points:
A) high-pressure arterial baroreceptors
B) renal juxtaglomerular apparatus
C) low-pressure baroreceptors
-adjustments are made via the ANS, and release of specific hormones
Arterial baroreceptor reflex
- acute regulation of arterial pressure is primarily mediated by the high pressure baroreceptor reflex, with contributions by cardiopulmonary receptors (aka low-pressure baroreceptors or volume receptors) and chemoreceptors (respiratory control)
- stretch receptors, baroreceptors in walls of several large systemic arteries
- rise in arterial pressure stretches the baroreceptors and causes them to transmit more APs to the CNS medullary control centers. Feedback signals are then sent back to ANS to the circulation to reduce arterial pressure downward toward the normal level
- signals from carotid baroreceptors in the carotid sinus arte transmitted through small Hering’s nerves to glossopharyngeal nerves (IV) in the high next, and then to the nucleus tractus solitarius (NTS) in the medulla
- signals from aortic baroreceptors in aortic arch are transmitted through the vagus nerves (X) also to NTS in the medulla
High pressure baroreceptors
- most important are carotid sinus and aortic arch
- stretching of the distensible vessel walls at either site leads to reflex vasodilation and bradycardia
Peripheral chemoreceptors
- in the carotid and aortic bodies
- close contract with arterial blood
- when arterial pressure falls below critical level, the receptors become stimulated because diminished blood flow causes decreased oxygen, and excess buildup of CO2 and H+ ions
- signals from chemoreceptors, and baroreceptors fibers through Hering’s nerves and the vagus nerves into the vasomotor center to elevate the arterial pressure back toward normal
- not powerful arterial pressure controller unless under 80 mm Hg
Chemical chemoreceptors
-in the medulla are sensitive to decrease in brain pH (reflecting an increase in arterial PCO2) and cause an increase in SNS output
Low pressure baroreceptors
-detect changes in venous pressure/volume
Baroreceptors and response to stretch
- they increase their rate of firing in response to stretch (increased MAP) in a frequency dependent manner
- they are branched terminals of myelinated and unmyelinated sensory nerve fibers
- following a large initial depolarization (dynamic component) is a more modest but steady depolarization (static componenet)
- graded response whose amplitude is proportional to the degree of stretch
Baroreceptors respond rapidly to changes in arterial pressure in specific ranges
- carotid sinus baroreceptos are not stimulated between 0-50 to 60 mm Hg, above they respond more and reach maximum at 180
- aortic baroreceptors similar just at levels 30 mm Hg higher
- in normal operating range ~100 mm Hg, slight change in pressure causes strong change in baroreflex signal to readjust arterial pressure back toward normal
- change faster to rapidly changing pressure than static one (twice as fast for rapidly rising 150 mm Hg than stationary one)
- adapt to long term changes in MAP- in hypertension curve shifted to right
Primary purpose of arterial baroreceptor system
-to reduce the minute by minute variation in arterial pressure to about 1/3 which would occur if the baroreceptor system was not present
Sympathetic nervous system innervation of systemic blood vessels
- sympathetic nerve fibers innervate all vessels except the capillaries
- precapillary sphincters and metarterioles are innervated in some tissues e.g. mesenteric blood vessels, but it’s usually not as dense as the small arteries, arterioles, and veins
- the innervation of the small arteries and arterioles allows sympathetic stimulation to increase resistance to blood flow and thereby to decrease rate of blood flow through the tissues
- the innervation of large vessels, particularly of the veins, 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
Vasomotor tone
- vasoconstrictor area of the vasomotor center transmits signals continuously to the sympathetic vasoconstrictor nerve fibers over the entire body, slowing firing to about one half to 2 impulses per second
- impulses maintain a partial state of contraction in the blood vessels called vasomotor tone
- when no vasoconstrictor tone arterial pressure falls from 100 to 50 mm Hg
Increase in MAP activates homeostatic negative feedback
- activates high pressure baroreceptors 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)
- thus reflex bradycardia and vasodilation lead to a decrease in MAP counteracting initial stimulus (baroreceptor reflex)
- after entering tractus solitarius of medulla, secondary signals inhibit the vasoconstrictor center and excite vagal parasympathetic
- effects: vasodilation, decreased HR and strength of heart contraction
Baroreceptor reflex from standing to lying
- standing to lying down increases venous return and thus stroke volume which increases MAP
- increased MAP enhances the firing rate of high pressure baroreceptors
- the CNS medullary center responds by increasing efferent activity of the PNS in conjuction with decreasing efferent activity of the SNS
- this reflex leads to bradycarida and vasodilation returning MAP back towards the normal operating range
Carotid sinus massage
- or release from a Valsalva maneuver
- stimulates the baroreceptors and reflexly slows the heart in people with atrial tachycardia
- 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
Valsalva maneuver
- can be used to test the integrity of the baroreceptor reflex. The subject is asked to expire against a closed glottis (as during heavy lifting, coughing, defacating)
- this causes an increase in intrathoracic pressure and a decrease in venous return to the heart, which decreases CO and MAP
- 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
