baroreceptor

Baroreceptors (or baroceptors) are sensors located in the blood vessels of the human body.^[1] They detect the pressure of blood flowing through them, and can send messages to the central nervous system to increase or decrease total peripheral resistance and cardiac output. Baroreceptors act immediately as part of a negative feedback system called the baroreflex,^[2] as soon as there is a change from the usual blood pressure mean arterial blood pressure, returning the pressure to a normal level. They are an example of a short term blood pressure regulation mechanism. Baroreceptors detect the amount of stretch of the blood vessel walls, and send the signal to the nervous system in response to this stretch.^[1]

Types of baroreceptors

Baroreceptors can be divided into two categories, high pressure arterial baroreceptors and low pressure baroreceptors (also known as cardiopulmonary^[3] or volume receptors^[4]).

Arterial (high pressure) baroreceptors

Arterial baroreceptors are present in the aortic arch and the carotid sinuses of the left and right internal carotid arteries. The baroreceptors found within the aortic arch enable the examination of the blood being delivered to all the blood vessels via the systemic circuit, and the baroreceptors within the carotid arteries monitor the blood pressure of the blood being delivered to the brain.^[1]

Arterial baroreceptors are stimulated by pressure changes in the arteries. The baroreceptors can identify the changes in the blood pressure which can increase or decrease the heart rate. They are sprayed nerve endings that lie in the tunica adventitia of the artery, not drug-binding molecules as the term receptor may suggest. A change in the mean arterial pressure induces depolarization of these sensory endings which results in action potentials. These action potentials are conducted to the central nervous system by axons and have a direct effect on the cardiovascular system through autonomic neurons^[5]. Hormone secretions which target the heart and blood vessels are affected by the stimulaton of baroreceptors.

If blood pressure falls, such as in hypovolaemic shock, baroreceptor firing rate decreases. 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)^[6]. If the arterial pressure is severely lowered, the baroreflex is activated^[7].

Baroreceptors respond very quickly to maintain a stable blood pressure, but they only respond to short term changes. Over a period of 1–2 days they will reset to a new value^[8]. Thus, in people with essential hypertension the baroreceptors behave as if the elevated blood pressure is normal and aim to maintain this high blood pressure. The receptors then become less sensitive to change^[9].

Low pressure baroreceptors

These are found in large systemic veins and in the walls of the right atrium of the heart^[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 which have profound effects on the retention of salt and water and 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 which 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, and if they fail to operate correctly, it can have some effects on the body. Pressure changes in the blood vessels would not be detected as quickly as in the presence 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].

References

1. ^ ^{a b c} Stanfield, CL; Germann, WJ. (2008) Principles of Human Physiology, Pearson Benjamin Cummings. 3rd edition, pp.424.
2. ^ Stanfield, CL; Germann, WJ. (2008) Principles of Human Physiology, Pearson Benjamin Cummings. 3rd edition, pp.427.
3. ^ Levy, MN; Pappano, AJ. (2007) Cardiovascular Physiology, Mosby Elsevier. 9th edition, pp.172.
4. ^ ^{a b} Stanfield, CL; Germann, WJ. (2008) Principles of Human Physiology, Pearson Benjamin Cummings. 3rd edition, pp.430-431.
5. ^ Stanfield, CL; Germann, WJ. (2008) Principles of Human Physiology, Pearson Benjamin Cummings. 3rd edition, pp.424-425.
6. ^ Bray, JJ; Cragg, PA; Macknight, ADC; Mills, RG. (1999) Lecture Notes on Human Physiology, Blackwell Publishing. 4th edition, pp.379.
7. ^ Guyton, AC; Hall, JE. (2006) Medical Physiology, Elsevier Saunders. 11th edition, pp.258.
8. ^ Guyton, AC; Hall, JE. (2006) Medical Physiology, Elsevier Saunders. 11th edition, pp.211.
9. ^ Levy, MN; Pappano, AJ. (2007) Cardiovascular Physiology, Mosby Elsevier. 9th edition, pp.171.
10. ^ Guyton, AC. (1991) Blood pressure control – special role of the kidneys and body fluids, Science, Vol. 252, pp.1813.

External links

• MeSH Baroreceptors