Baroreceptors are sensors located in the blood vessels of all vertebrate animals.
They sense the blood pressure and relay the information to the brain, so that a
proper blood pressure can be maintained. Baroreceptors are a type of
mechanoreceptor sensory neuron that is excited by stretch of the blood vessel.
Thus, increases in the pressure of blood vessel triggers increased action
potential generation rates and provides information to the central nervous
system. This sensory information is used primarily in autonomic reflexes that in
turn influence the heart cardiac output and vascular smooth muscle to influence
total peripheral resistance. Baroreceptors act immediately as part of
a negative feedback system called the baroreflex, as soon as there is a change
from the usual mean arterial blood pressure, returning the pressure toward
a normal level. These reflexes help regulate short-term blood pressure. The
nucleus tractus solitarii in the medulla oblongata of the brain recognizes
changes in the firing rate of action potentials from the baroreceptors, and
influences cardiac output and systemic vascular resistance.
Baroreceptors can be divided into two categories based on the type of blood
vessel in which they are located: high-pressure arterial baroreceptors and
low-pressure baroreceptors. Arterial baroreceptors
Arterial baroreceptors are stretch receptors that are stimulated by
distortion of the arterial wall when pressure changes. The baroreceptors can
identify the changes in both the average blood pressure or the rate of change in
pressure with each arterial pulse. Action potentials triggered in the
baroreceptor ending are then directly conducted to the brainstem where central
terminations transmit this information to neurons within the solitary nucleus.
Reflex responses from such baroreceptor activity can trigger increases or
decreases in the heart rate. Arterial baroreceptor sensory endings are simple,
sprayed nerve endings that lie in the tunica adventitia of the artery. An
increase in the mean arterial pressure increases depolarization of these
sensory endings, which results in action potentials. These action potentials are
conducted to the solitary nucleus in the central nervous system by axons and have
a reflex effect on the cardiovascular system through autonomic neurons.
Hormone secretions that target the heart and blood vessels are affected by the
stimulation of baroreceptors. At normal resting blood pressures,
baroreceptors discharge with each heart beat. If blood pressure falls, such as
on orthostatic hypotension or in hypovolaemic shock, baroreceptor firing
rate decreases and baroreceptor reflexes act to help restore blood pressure by
increasing heart rate. Signals from the carotid baroreceptors are sent via the
glossopharyngeal nerve. Signals from the aortic baroreceptors travel through the
vagus nerve. Arterial baroreceptors inform reflexes about arterial blood
pressure but other stretch receptors in the large veins and right atrium convey
information about the low pressure parts of the circulatory system.
Baroreceptors respond very quickly to maintain a stable blood pressure, but
their responses diminish with time and thus are most effective for conveying
short term changes in blood pressure. In people with essential hypertension the
baroreceptors and their reflexes change and function to maintain the elevated
blood pressure as if normal. The receptors then become less sensitive to
change. Low-pressure baroreceptors
The low-pressure baroreceptors, are found in large systemic veins, in
pulmonary vessels, and in the walls of the right atrium and ventricles of the
heart. The low-pressure baroreceptors are involved with the regulation of
blood volume. The blood volume determines the mean pressure throughout
the system, in particular in the venous side where most of the blood is held.
The low-pressure baroreceptors have both circulatory and renal effects; they
produce changes in hormone secretion, resulting in profound effects on the
retention of salt and water; they also influence intake of salt and water. The
renal effects allow the receptors to change the mean pressure in the system
in the long term. Denervating these receptors 'fools' the
body into thinking that it has too low blood volume and initiates mechanisms
that retain fluid and so push up the blood pressure to a higher level than it
would otherwise have. Baroreceptor dysfunction
Baroreceptors are integral to the body’s function: Pressure changes in the blood
vessels would not be detected as quickly in the absence of baroreceptors. When
baroreceptors are not working, blood pressure continues to increase, but,
within an hour, the blood pressure returns to normal as other blood
pressure regulatory systems take over. Baroreceptors can also become
oversensitive in some people. This can lead to bradycardia, dizziness and
syncope from touching the neck. This is an important cause to exclude in men
having pre-syncope or syncope symptoms. References
^ C. M. Heesch. Reflexes that control cardiovascular function. Am.J.Physiol
277:S234-S243, 1999. ^ Stanfield, CL; Germann, WJ. Principles
of Human Physiology, Pearson Benjamin Cummings. 3rd edition, pp.427.
^ Levy, MN; Pappano, AJ. Cardiovascular Physiology, Mosby Elsevier. 9th edition,
pp.172. ^ a b Stanfield, CL; Germann, WJ.
Principles of Human Physiology, Pearson Benjamin Cummings. 3rd edition,
pp.430-431. ^ Stanfield, CL; Germann, WJ. Principles
of Human Physiology, Pearson Benjamin Cummings. 3rd edition, pp.424-425.
^ Bray, JJ; Cragg, PA; Macknight, ADC; Mills, RG. Lecture Notes on Human
Physiology, Blackwell Publishing. 4th edition, pp.379.
^ Levy, MN; Pappano, AJ. Cardiovascular Physiology, Mosby Elsevier. 9th edition,
pp.171. ^ Guyton, AC. Blood pressure control –
special role of the kidneys and body fluids, Science, Vol. 252, pp.1813.
External links Baroreceptors at the US National Library
of Medicine Medical Subject Headings