Chapter 134: Carotid and Aortic Bodies

INTRODUCTION

The control of ventilation is achieved by regulating and processing complex inputs from central and peripheral chemoreceptors to the central nervous system. The main peripheral chemoreceptors in the body are the carotid bodies and the aortic bodies.

ANATOMY AND PHYSIOLOGY

The carotid body is a collection of sensory chemoreceptors located near the common carotid artery bifurcation. Its primary role is to detect changes in the composition of arterial blood such as oxygen tension, CO₂ tension, pH, and temperature, and relay the information to the central respiratory center. The carotid body is composed of glomus cells, which exist in two types: type I and type II. After sensing changes in the arterial blood, type I glomus cells release neurotransmitters, acetylcholine, adenosine triphosphate (ATP), and dopamine, which generate an action potential that travels via glossopharyngeal nerve (CN IX) to the central respiratory center. Type II glomus cells are supporting cells that do not participate directly in respiratory regulation.

Whereas central chemoreceptors largely respond to changes in H⁺ concentration in direct correlation with Paco₂, carotid body chemoreceptors respond mainly to changes in arterial oxygen tension, Pao₂. The action potential output of type I glomus cells is minimal when Pao₂ remains greater than 100 mm Hg. When Pao₂ is less than 100 mm Hg, the glomus cells respond by releasing stored neurotransmitters, resulting in immediate information relay to the central respiratory center. The degree of response is exponential, as Pao₂ continues to fall below 100 mm Hg. Changes in Paco₂, pH, and temperature in the arterial blood are also able to elicit the glomus cell’s response, albeit not to the level of Pao₂.

Although the carotid body is not believed to directly initiate a modulatory response, a fall in Pao₂ will increase the ventilatory drive. When the carotid body is activated, a reflex increase in minute volume ventilation promotes CO₂ removal from alveoli and decreased alveolar PAco₂ ensues. This reduction in alveolar PAco₂, along with increased alveolar and arterial Po₂, minimizes hypoxia. Consequently, adequate tissue oxygen supply is maintained. The response of carotid bodies to the combination of hypoxemia and hypercapnia is greater than the sum of the individual responses to each component. Notably, separate carotid baroreceptors modulate cardiovascular response to changes in blood pressure.

Aortic bodies are sensory chemoreceptors and baroreceptors scattered throughout the aortic arch and its branches. Similar to the carotid body, aortic body chemoreceptors sense changes in Pao₂, Paco₂, and pH in the arterial blood. Signals from aortic body chemoreceptors travel via the vagus nerve (CN X) to the medulla where respiratory centers are stimulated, increasing ventilatory drive.

CELLULAR MECHANISMS