Carotid Arteries
Arising from the brachiocephalic trunk of the aortic arch, and the aortic arch itself, the common carotid arteries ultimately branch into the external and internal carotid arteries that provide the oxygenated blood supply to the neck and head. The right and left common carotid arteries, the right and left external carotid arteries, and the right and left internal carotid arteries form what anatomists term the carotid arterial system.
The common carotid arteries normally vary in origin. Although the right common carotid artery arises from the brachiocephalic artery, the left common carotid artery normally splits directly off the aortic arch. Normally the left common carotid branches from the highest point of the aortic arch near the junction of the neck and thorax. The common carotids then branch into the external and internal carotid arteries.
The respective external carotid arteries arising from the common carotid arteries supply blood to branches that provide blood to the pharyngeal and facial regions, superior thyroid, neck, and the skull. The internal carotids that arise from the common carotids supply blood to the brain, the orbit of the eye, the middle ear, and the hypophysis.
In the early embryo (4 weeks), primitive carotid arteries branching from the developing pharyngeal arterial system and a pair of dorsal aortae provide early circulation to the developing brain. The generation, fusion, and degeneration of embryonic arteries that accompany the development of the pharyngeal arch system into the ascending aorta and aortic arch region of the ascending aorta--and of carotid arteries derived from them--are typical of a head-to-tail (craniocaudal) scheme of development. This dynamic development usually explains the varied and often tortuous (twisting and turning) origin and course of vessels derived from the pharyngeal branch system around adult organs (e.g., the esophagus and trachea).
At about the level of the thyroid cartilage in the adult, the right and left common carotid arteries divide into the right and left sets of internal and external carotid arteries. Where this division takes place, the common carotids expand in diameter (dilate). Anatomists commonly refer to the enlargement of a vessel or chamber as a sinus. Accordingly, the carotid sinus refers to the location where the dilation occurs (dilatation of the vessel). Because the carotid arteries are dilated, the walls of the vessels are stretched thinner than in adjacent areas.
At the carotid sinus, the carotid system is most vulnerable to external pressure and compression.
Sufficient pressure on the carotid arteries can cause fainting or a more severe and prolonged lack of consciousness (carotid sinus syncope). The carotids are most sensitive to pressure at the point at which they split (bifurcate). The physiological mechanism for syncopal episodes involves the activation (excitation) of pressure receptors (baroreceptors) that may lead to a rapid and dramatic decrease in arterial pressure of 20-25 mm Hg (mercury) or more. It is this sudden loss or arterial pressure--and concurrent loss of oxygenated blood supply to the brain--that causes the fainting (syncopal) episode. If sufficient or prolonged pressure is applied, it is possible to cause a temporary cardiac arrest a few seconds in duration. In individuals with healthy hearts, the heart usually begins to beat again due the internal (intrinsic) contractility of cardiac muscle. Prolonged compression of the carotids may be fatal.
Within what is termed the carotid triangle (an area of the anterior neck defined by muscles--specifically by the sternocleidomastoid, and portions of the omohyoid, stylohyloid, and digastric muscles) gentle pressure is often applied to the external carotid artery to determine the presence of a pulse. The pulsations of the heart are easily felt because within the carotid triangle the external carotid is essentially only covered by skin and superficial fascia. According the external carotid is easily felt (palpated) and is highly vulnerable to external pressure and injury.
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