The baroreflex or baroreceptor reflex is one of the body's

homeostatic In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism an ...

mechanisms that helps to maintain

blood pressure Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure ...

at nearly constant levels. The baroreflex provides a rapid

negative feedback loop Negative feedback (or balancing feedback) occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by other ...

in which an elevated blood pressure causes the

heart rate Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...

to decrease. Decreased blood pressure decreases baroreflex activation and causes heart rate to increase and to restore blood pressure levels. Their function is to sense pressure changes by responding to change in the tension of the arterial wall The baroreflex can begin to act in less than the duration of a cardiac cycle (fractions of a second) and thus baroreflex adjustments are key factors in dealing with

postural hypotension Orthostatic hypotension, also known as postural hypotension, is a medical condition wherein a person's blood pressure drops when standing up or sitting down. Primary orthostatic hypertension is also often referred to as neurogenic orthostatic hyp ...

, the tendency for blood pressure to decrease on standing due to gravity. The system relies on specialized

neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa ...

s, known as

baroreceptor Baroreceptors (or archaically, pressoreceptors) are sensors located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch. They sense the blood pressure and relay the information to the brain, so that ...

s, chiefly in the

aortic arch The aortic arch, arch of the aorta, or transverse aortic arch () is the part of the aorta between the ascending and descending aorta. The arch travels backward, so that it ultimately runs to the left of the trachea. Structure The aorta begins ...

and

carotid sinus In human anatomy, the carotid sinus is a dilated area at the base of the internal carotid artery just superior to the bifurcation of the internal carotid and external carotid at the level of the superior border of thyroid cartilage. The carot ...

es, to monitor changes in blood pressure and relay them to the

medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (invol ...

. Baroreceptors are stretch receptors and respond to the pressure induced stretching of the blood vessel in which they are found. Baroreflex-induced changes in blood pressure are mediated by both branches of the

autonomic nervous system The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies internal organs, smooth muscle and glands. The autonomic nervous system is a control system t ...

: the

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of ...

and sympathetic nerves. Baroreceptors are active even at normal blood pressures so their activity informs the brain about both increases and decreases in blood pressure. The body contains two other, slower-acting systems to regulate blood pressure: the heart releases atrial natriuretic peptide when blood pressure is too high, and the kidneys sense and correct low blood pressure with the

renin–angiotensin system The renin–angiotensin system (RAS), or renin–angiotensin–aldosterone system (RAAS), is a hormone system that regulates blood pressure, fluid and electrolyte balance, and systemic vascular resistance. When renal blood flow is reduced, ...

Anatomy

Baroreceptors are present in the atria of the

heart The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...

and

vena cavae In anatomy, the venae cavae (; singular: vena cava ; ) are two large veins (great vessels) that return deoxygenated blood from the body into the heart. In humans they are the superior vena cava and the inferior vena cava, and both empty into the ...

, but the most sensitive baroreceptors are in the

es and

. While the carotid sinus

axons travel within the

glossopharyngeal nerve The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior (closer to the nose) to the vagus nerve. ...

(CN IX), the aortic arch baroreceptor axons travel within the

vagus nerve The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and righ ...

(CN X). Baroreceptor activity travels along these nerves directly into the central nervous system to excite

glutamatergic Glutamatergic means "related to glutamate". A glutamatergic agent (or drug) is a chemical that directly modulates the excitatory amino acid (glutamate/aspartate) system in the body or brain. Examples include excitatory amino acid receptor agonis ...

neurons within the

solitary nucleus In the human brainstem, the solitary nucleus, also called nucleus of the solitary tract, nucleus solitarius, and nucleus tractus solitarii, (SN or NTS) is a series of purely sensory nuclei (clusters of nerve cell bodies) forming a vertical column ...

(SN) in the brainstem. Baroreceptor information flows from these NSS neurons to both parasympathetic and sympathetic neurons within the brainstem. The SN neurons send excitatory fibers (

) to the

caudal ventrolateral medulla The ventrolateral medulla, part of the medulla oblongata of the brainstem, plays a major role in regulating arterial blood pressure and breathing. It regulates blood pressure by regulating the activity of the sympathetic nerves The sympathetic n ...

(CVLM), activating the CVLM. The activated CVLM then sends inhibitory fibers ( GABAergic) to the

rostral ventrolateral medulla The rostral ventrolateral medulla (RVLM), also known as the pressor area of the medulla, is a brain region that is responsible for basal and reflex control of sympathetic activity associated with cardiovascular function. Abnormally elevated sympa ...

(RVLM), thus inhibiting the RVLM. The RVLM is the primary regulator of the

sympathetic nervous system The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of t ...

, sending excitatory fibers (

) to the sympathetic preganglionic neurons located in the intermediolateral nucleus of the spinal cord. Hence, when the baroreceptors are activated (by an increased blood pressure), the NTS activates the CVLM, which in turn inhibits the RVLM, thus decreasing the activity of the sympathetic branch of the autonomic nervous system, leading to a relative decrease in blood pressure. Likewise, low blood pressure activates baroreceptors less and causes an increase in sympathetic tone via "disinhibition" (less inhibition, hence activation) of the RVLM. Cardiovascular targets of the sympathetic nervous system includes both blood vessels and the heart. Even at resting levels of blood pressure, arterial baroreceptor discharge activates SN neurons. Some of these SN neurons are tonically activated by this resting blood pressure and thus activate excitatory fibers to the

nucleus ambiguus The nucleus ambiguus ("ambiguous nucleus" in English) is a group of large motor neurons, situated deep in the medullary reticular formation named by Jacob Clarke. The nucleus ambiguus contains the cell bodies of neurons that innervate the muscle ...

and dorsal nucleus of vagus nerve to regulate the

parasympathetic nervous system The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part o ...

. These parasympathetic neurons send axons to the heart and parasympathetic activity slows cardiac pacemaking and thus

. This parasympathetic activity is further increased during conditions of elevated blood pressure. The parasympathetic nervous system is primarily directed toward the heart.

Activation

The

baroreceptors Baroreceptors (or archaically, pressoreceptors) are sensors located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch. They sense the blood pressure and relay the information to the brain, so that ...

are stretch-sensitive

mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, a ...

s. At low pressures, baroreceptors become inactive. When blood pressure rises, the carotid and aortic sinuses are distended further, resulting in increased stretch and, therefore, a greater degree of activation of the baroreceptors. At normal resting blood pressures, many baroreceptors are actively reporting blood pressure information and the baroreflex is actively modulating autonomic activity. Active baroreceptors fire

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...

s ("spikes") more frequently. The greater the stretch the more rapidly baroreceptors fire action potentials. Many individual baroreceptors are inactive at normal resting pressures and only become activated when their stretch or pressure threshold is exceeded. Baroreceptor mechanosensitivty is hypothesised to be linked to the expression of

PIEZO1 Piezo1 is a mechanosensitive ion channel protein that in humans is encoded by the gene ''PIEZO1''. Piezo1 and its close homolog Piezo2 were cloned in 2010, using an siRNA-based screen for mechanosensitive ion channels. Structure and function ...

and

PIEZO2 Piezo-type mechanosensitive ion channel component 2 is a protein that in humans is encoded by the PIEZO2 gene. It has a homotrimeric structure, with three blades curving into a nano-dome, with a diameter of 28 nanometers. Function Piezos are ...

on neurons in the pretrosal and nodose ganglia. Baroreceptor action potentials are relayed to the

, which uses frequency as a measure of blood pressure. Increased activation of the solitary nucleus inhibits the vasomotor center and stimulates the vagal nuclei. The end-result of baroreceptor activation is inhibition of the

and activation of the

. The sympathetic and

branches of the

have opposing effects on blood pressure. Sympathetic activation leads to an elevation of total peripheral resistance and

cardiac output In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: th ...

via increased contractility of the heart,

, and arterial

vasoconstriction Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vess ...

, which tends to increase blood pressure. Conversely,

activation leads to decreased

via decrease in

, resulting in a tendency to lower blood pressure. By coupling sympathetic inhibition and

activation, the baroreflex maximizes blood pressure reduction. Sympathetic inhibition leads to a drop in peripheral resistance, while parasympathetic activation leads to a depressed

(

reflex bradycardia Reflex bradycardia is a bradycardia (decrease in heart rate) in response to the baroreceptor reflex, one of the body's homeostatic mechanisms for preventing abnormal increases in blood pressure. In the presence of high mean arterial pressure, the ...

) and contractility. The combined effects will dramatically decrease blood pressure.In a similar manner, sympathetic activation with

inhibition allows the baroreflex to elevate blood pressure.

Set point and tonic activation

Baroreceptor firing has an inhibitory effect on sympathetic outflow. The sympathetic neurons fire at different rates which determines the release of norepinephrine onto cardiovascular targets. Norepinephrine constricts blood vessels to increase blood pressure. When baroreceptors are stretched (due to an increased blood pressure) their firing rate increases which in turn decreases the sympathetic outflow resulting in reduced norepinephrine and thus blood pressure. When the blood pressure is low, baroreceptor firing is reduced and this in turn results in augmented sympathetic outflow and increased norepinephrine release on the heart and blood vessels, increasing blood pressure.

Effect on heart rate variability

The baroreflex may be responsible for a part of the low-frequency component of heart rate variability, the so-called Mayer waves, at 0.1 Hz.

Baroreflex activation therapy

High blood pressure

The baroreflex can be used to treat resistant hypertension. This stimulation is provided by a pacemaker-like device. While the devices appears to lower blood pressure, evidence remains very limited as of 2018.

Heart failure

The ability of baroreflex activation therapy to reduce sympathetic nerve activity suggests a potential in the treatment of chronic heart failure, because in this condition there is often intense sympathetic activation and patients with such sympathetic activation show a markedly increased risk of fatal arrhythmias and death. One trial has already shown that baroreflex activation therapy improves functional status, quality of life, exercise capacity and N-terminal pro-brain natriuretic peptide.

References

* * * {{Reflex Autonomic nervous system Reflexes Cardiovascular physiology Vagus nerve Blood pressure Human homeostasis