Baroreceptor Reflexes

Human Anatomy and Physiology Premium Course

Human Anatomy and Physiology Study Course

Get Instant Access

Arterial Baroreceptors

It is only logical that the reflexes that homeostatically regulate arterial pressure originate primarily with arterial receptors that respond to changes in pressure. High in the neck, each of the two major vessels

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

Carotid Artery Function

Internal carotid arteries

To brain

Common carotid arteries

Aortic arch

Internal carotid arteries

Baroreceptors Firing

Carotid sinus baroreceptors

baroreceptor

Carotid sinus baroreceptors

FIGURE 14-56

Locations of arterial baroreceptors.

To brain i

FIGURE 14-55

Sequence of events by which a decrease in blood volume leads to a decrease in mean arterial pressure.

supplying the head, the common carotid arteries, divides into two smaller arteries (Figure 14-56). At this division, the wall of the artery is thinner than usual and contains a large number of branching, vinelike nerve endings. This portion of the artery is called the carotid sinus (the term "sinus" denotes a recess, space, or dilated channel). Its nerve endings are highly sensitive to stretch or distortion. Since the degree of wall stretching is directly related to the pressure within the artery, the carotid sinuses serve as pressure receptors, or baroreceptors. An area func tionally similar to the carotid sinuses is found in the arch of the aorta and is termed the aortic arch baroreceptor. The two carotid sinuses and the aortic arch baroreceptor constitute the arterial baroreceptors. Afferent neurons from them travel to the brainstem and provide input to the neurons of cardiovascular control centers there.

Action potentials recorded in single afferent fibers from the carotid sinus demonstrate the pattern of baroreceptor response (Figure 14-57). In this experiment the pressure in the carotid sinus is artificially controlled so that the pressure is either steady or pul-satile—that is, varying as usual between systolic and diastolic pressure. At a particular steady pressure, for example, 100 mmHg, there is a certain rate of discharge by the neuron. This rate can be increased by raising the arterial pressure, or it can be decreased by lowering the pressure. Thus, the rate of discharge of the carotid sinus is directly proportional to the mean arterial pressure.

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

Baroreceptor Reflexes

40 80 120

Mean arterial pressure (mmHg)

FIGURE 14-57

Effect of changing mean arterial pressure (MAP) on the firing of action potentials by afferent neurons from the carotid sinus. This experiment is done by pumping blood in a nonpulsatile manner through an isolated carotid sinus so as to be able to set the pressure inside it at any value desired.

40 80 120

Mean arterial pressure (mmHg)

FIGURE 14-57

Effect of changing mean arterial pressure (MAP) on the firing of action potentials by afferent neurons from the carotid sinus. This experiment is done by pumping blood in a nonpulsatile manner through an isolated carotid sinus so as to be able to set the pressure inside it at any value desired.

Baroreceptor Discharge

FIGURE 14-58

Neural components of the arterial baroreceptor reflex. If the initial change were a decrease in arterial pressure, all the arrows in the boxes would be reversed.

If the experiment is repeated using the same mean pressures as before but allowing pressure pulsations, it is found that at any given mean pressure, the larger the pulse pressure, the faster the rate of firing by the carotid sinus. This responsiveness to pulse pressure adds a further element of information to blood pressure regulation, since small changes in factors such as blood volume may cause changes in arterial pulse pressure with little or no change in mean arterial pressure.

The Medullary Cardiovascular Center

The primary integrating center for the baroreceptor reflexes is a diffuse network of highly interconnected neurons called the medullary cardiovascular center, located in the brainstem medulla oblongata. The neurons in this center receive input from the various baroreceptors. This input determines the outflow from the center along neural pathways that terminate upon the cell bodies and dendrites of the vagus (parasympathetic) neurons to the heart and the sympathetic neurons to the heart, arterioles, and veins. When the arterial baroreceptors increase their rate of discharge, the result is a decrease in sympathetic outflow to the heart, arterioles, and veins, and an increase in parasympa-thetic outflow to the heart (Figure 14-58). A decrease in baroreceptor firing rate results in just the opposite pattern.

As parts of the baroreceptor reflexes, angiotensin II generation and vasopressin secretion are also altered so as to help restore blood pressure. Thus, decreased arterial pressure elicits increased plasma concentrations of both these hormones, which raise arterial pres-

FIGURE 14-58

Neural components of the arterial baroreceptor reflex. If the initial change were a decrease in arterial pressure, all the arrows in the boxes would be reversed.

sure by constricting arterioles. For simplicity, however, we focus in the rest of this chapter mainly on the sympathetic nervous system when discussing reflex control of arterioles. The roles of angiotensin II and vaso-pressin will be described further in Chapter 16 in the context of their effects on salt and water balance.

Operation of the Arterial Baroreceptor Reflex

Our description of the arterial baroreceptor reflex is now complete. If arterial pressure decreases as during a hemorrhage (Figure 14-59), this causes the discharge rate of the arterial baroreceptors to decrease. Fewer impulses travel up the afferent nerves to the medullary cardiovascular center, and this induces (1) increased heart rate because of increased sympathetic activity to the heart and decreased parasympathetic activity, (2) increased ventricular contractility because of increased sympathetic activity to the ventricular myocardium,

(3) arteriolar constriction because of increased sympathetic activity to the arterioles and increased plasma concentrations of angiotensin II and vasopressin, and

(4) increased venous constriction because of increased sympathetic activity to the veins. The net result is an increased cardiac output (increased heart rate and stroke volume), increased total peripheral resistance (arteriolar constriction), and return of blood pressure toward normal.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

III. Coordinated Body Functions

14. Circulation

© The McGraw-Hill Companies, 2001

I Parasympathetic discharge to heart

SA node t Heart rate

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • manuel
    Are baroreceptors lipid or protein?
    8 years ago
  • amethyst
    Where is the primary integration center for baroreceptor reflexes located?
    8 years ago
  • giordano
    What is a baroreceptor function?
    8 years ago
  • abeba tesfalem
    When carotid and aortic baroreceptors slow their discharge rate (rate of firing)?
    8 years ago
  • Adiam
    What part of the body is the baroreceptor?
    8 years ago
  • Luka
    What is the function of baroreceptors in the walls of the aorta and carotid arteries?
    8 years ago
  • harold
    Why is the aorta and internal carotid the site for baroreceptor?
    8 years ago
  • PRIAMUS ROPER
    Are baroreceptors afferent nerves?
    8 years ago
  • dean turner
    What are baroreceptors reflexes?
    8 years ago
  • ren
    What is the pressoreceptors reflex controlling mean arterial pressure?
    8 years ago
  • mary
    Where are the baroreceptors found in our bodies?
    8 years ago
  • nasih
    What body functions depend on barrorecptor reflex?
    7 years ago
  • Pekka Immonen
    Does the heart rat e increase when the cartid and aortic barorecptors slow their discharge?
    7 years ago
  • christine
    When aortic and carotid baroceptors slow their doscharge?
    7 years ago
  • mildred
    What is the function of the baroreceptors in aorta and carotid artery?
    6 years ago

Post a comment