What are 2 locations where baroreceptors are seen?

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Arterial baroreceptors
Low-pressure baroreceptors
Baroreceptor dysfunction
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Arterial blood pressure is normally regulated within a narrow range, with a mean arterial pressure typically ranging from 85 to 100 mmHg in adults. It is important to tightly control this pressure to ensure adequate blood flow to organs throughout the body. This is accomplished by negative feedback systems incorporating pressure sensors (i.e., baroreceptors) that sense the arterial pressure. The most important arterial baroreceptors are located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch (Figure 1). These receptors respond to stretching of the arterial wall so that if arterial pressure suddenly rises, the walls of these vessels passively expand, which increases the firing frequency of action potentials generated by the receptors. If arterial blood pressure suddenly falls, decreased stretch of the arterial walls leads to a decrease in receptor firing.

The carotid sinus baroreceptors are innervated by the sinus nerve of Hering, which is a branch of the glossopharyngeal nerve (IX cranial nerve). The glossopharyngeal nerve synapses in the nucleus tractus solitarius (NTS) located in the medulla of the brainstem. The aortic arch baroreceptors are innervated by the aortic nerve, which then combines with the vagus nerve (cranial nerve X) traveling to the NTS. The NTS modulates the activity of sympathetic and parasympathetic (vagal) neurons in the medulla, which in turn regulate the autonomic control of the heart and blood vessels.

Of these two sites for arterial baroreceptors, the carotid sinus is quantitatively the most important for regulating arterial pressure. The carotid sinus receptors respond to pressures ranging from 60-180 mmHg (Figure 2). Receptors within the aortic arch have a higher threshold pressure and are less sensitive than the carotid sinus receptors. Maximal carotid sinus sensitivity occurs near the normal mean arterial pressure; therefore, very small changes in arterial pressure around this "set point" dramatically alters receptor firing so that autonomic control can be altered in such a way that the arterial pressure remains very near to the set point. This set point changes during exercise, hypertension, and heart failure. In chronic hypertension, for example, the response curve shifts to right thereby increasing the set point. This explains, in part, how arterial pressure can remain elevated during chronic hypertension.

Baroreceptors are sensitive to the rate of pressure change as well as to the steady or mean pressure. Therefore, at a given mean arterial pressure, decreasing the pulse pressure (systolic minus diastolic pressure) decreases the baroreceptor firing rate. This is important during conditions such as hemorrhagic shock in which pulse pressure as well as mean pressure decreases. The combination of reduced mean pressure and reduced pulse pressure amplifies the baroreceptor response.

Although the baroreceptors can respond to either an increase or decrease in systemic arterial pressure, their most important role is responding to sudden reductions in arterial pressure (Figure 3). This can occur, for example, when a person suddenly stands up or following blood loss (hemorrhage). A decrease in arterial pressure (mean, pulse or both) results in decreased baroreceptor firing. Autonomic neurons within the medulla respond by increasing sympathetic outflow and decreasing parasympathetic (vagal) outflow. Under normal physiological conditions, baroreceptor firing exerts a tonic inhibitory influence on sympathetic outflow from the medulla. Therefore, acute hypotension results in a disinhibition of sympathetic activity within the medulla, so that sympathetic activity originating within the rostral ventrolateral medulla increases. These autonomic changes cause vasoconstriction (increased systemic vascular resistance, SVR), tachycardia and positive inotropy. The latter two changes increase cardiac output. Increases in cardiac output and SVR lead to a partial restoration of arterial pressure.

It is important to note that baroreceptors adapt to sustained changes in arterial pressure. For example, if arterial pressure suddenly falls when a person stands, the baroreceptor firing rate will decrease; however, after a period of time, the firing returns to near normal levels as the receptors adapt to the lower pressure. Therefore, the long-term regulation of arterial pressure requires activation of other mechanisms (primarily hormonal and renal) to maintain normal blood pressure.

Revised 12/8/16

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.

Sensors detecting blood pressure

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 a proper blood pressure can be maintained.

Baroreceptors are a type of mechanoreceptor sensory neuron that are excited by a 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 vascular resistance.[1] Baroreceptors act immediately as part of a negative feedback system called the baroreflex,[2] 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 solitary nucleus 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 (also known as cardiopulmonary[3] or volume receptors[4]).

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 (synapses) transmit this information to neurons within the solitary nucleus[5] which lies in the medulla. Reflex responses from such baroreceptor activity can trigger increases or decreases in the heart rate. Arterial baroreceptor sensory endings are simple, splayed 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.[6] 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 (cranial nerve IX). Signals from the aortic baroreceptors travel through the vagus nerve (cranial nerve X).[7] Carotid sinus baroreceptors are responsive to both increases or decreases in arterial pressure, while aortic arch baroreceptors are only responsive to increases in arterial pressure.[5] 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.[8]

Electrical stimulation of baroreceptors has been found to activate the baroreflex, reducing sympathetic tone throughout the body and thereby reducing blood pressure in patients with resistant hypertension.[9]

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 atrial volume receptors).[4] 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.[citation needed]

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.[10]

Baroreceptors can also become oversensitive in some people (usually the carotid baroreceptors in older males). This can lead to bradycardia, dizziness and fainting (syncope) from touching the neck (often whilst shaving). This is an important cause to exclude in men having pre-syncope or syncope symptoms.

References

^ Heesch, C. M. (December 1999). "Reflexes that control cardiovascular function". The American Journal of Physiology. 277 (6 Pt 2): S234–243. doi:10.1152/advances.1999.277.6.S234. ISSN 0002-9513. PMID 10644250.
^ Stanfield, CL; Germann, WJ. (2008) Principles of Human Physiology, Pearson Benjamin Cummings. 3rd edition, pp.427.
^ Levy, MN; Pappano, AJ. (2007) Cardiovascular Physiology, Mosby Elsevier. 9th edition, pp.172.
^ a b Stanfield, CL; Germann, WJ. (2008) Principles of Human Physiology, Pearson Benjamin Cummings. 3rd edition, pp.430-431.
^ a b Costanzo, Linda S. (2017-03-15). Physiology (Sixth ed.). Philadelphia, PA. ISBN 9780323511896. OCLC 965761862.
^ Stanfield, CL; Germann, WJ. (2008) Principles of Human Physiology, Pearson Benjamin Cummings. 3rd edition, pp.424-425.
^ Bray, JJ; Cragg, PA; Macknight, ADC; Mills, RG. (1999) Lecture Notes on Human Physiology, Blackwell Publishing. 4th edition, pp.379.
^ Levy, MN; Pappano, AJ. (2007) Cardiovascular Physiology, Mosby Elsevier. 9th edition, pp.171.
^ Wallbach, M; Koziolek, MJ (9 November 2017). "Baroreceptors in the carotid and hypertension-systematic review and meta-analysis of the effects of baroreflex activation therapy on blood pressure". Nephrology, Dialysis, Transplantation. 33 (9): 1485–1493. doi:10.1093/ndt/gfx279. PMID 29136223.
^ Guyton, Arthur C. (1991). "Blood Pressure Control-Special Role of the Kidneys and Body Fluids". Science. 252 (5014): 1813–1816. Bibcode:1991Sci...252.1813G. doi:10.1126/science.2063193. JSTOR 2875873. PMID 2063193.