Common carotid artery

 ( Arteries Of The Head And Neck ) 

The average diameters of the common carotids in adult males and females are 6.5 mm and 6.1 mm respectively.

During the thoracic part of its course, the left common carotid artery is related to the following structures: In front, it is separated from the manubrium of the Human sternum by the sternohyoid and sternothyroid muscles, the anterior portions of the left pleura and lung, the left brachiocephalic vein, and the remains of the thymus; behind, it lies on the trachea, esophagus, left recurrent laryngeal nerve, and thoracic duct.

To its right side, below is the brachiocephalic trunk, and above, the trachea, the inferior thyroid veins, and the remains of the thymus; to its left side are the left vagus and , left pleura, and lung. The left subclavian artery is posterior and slightly lateral to it.

Each vessel passes obliquely upward, from behind the sternoclavicular joint to the level of the upper border of the thyroid cartilage, where it divides.

At the lower neck, the two common carotid arteries are separated from each other by a very narrow interval which contains the trachea; but at the upper part, the thyroid gland, the larynx and human pharynx separate the two arteries.

The common carotid artery is contained in a sheath known as the carotid sheath, which is derived from the deep cervical fascia and encloses also the internal jugular vein and vagus nerve, the vein lying lateral to the artery, and the nerve between the artery and vein, on a plane posterior to both. On opening the sheath, each of these three structures is seen to have a separate fibrous cover.

At approximately the level of the fourth vertebra, the common carotid artery splits ("" in literature) into an internal carotid artery (ICA) and an external carotid artery (ECA). While both branches travel upward, the internal carotid takes a deeper (more internal) path, eventually travelling up into the skull to supply the brain. The external carotid artery travels more closely to the surface, and sends off numerous branches that supply the neck and face.

At the lower part of the neck, the common carotid artery is very deeply seated, being covered by the integument, superficial fascia, the platysma muscle, deep cervical fascia, the sternocleidomastoid muscle, the sternohyoid, sternothyroid, and the omohyoid; in the upper part of its course it is more superficial, being covered merely by the integument, the superficial fascia, the platysma, deep cervical fascia, and medial margin of the sternocleidomastoid.

When the sternocleidomastoid muscle is drawn backward, the artery is seen to be contained in a triangular space known as the carotid triangle. This space is bounded behind by the sternocleidomastoid, above by the stylohyoid and the posterior belly of the digastric muscle, and below by the superior belly of the omohyoid.

This part of the artery is crossed obliquely, from its medial to its lateral side, by the sternocleidomastoid branch of the superior thyroid artery; it is also crossed by the superior and middle thyroid veins (which end in the internal jugular vein); descending in front of its sheath is the descending branch of the hypoglossal nerve, this filament being joined by one or two branches from the cervical nerves, which cross the vessel obliquely.

Sometimes the descending branch of the hypoglossal nerve is contained within the sheath.

The superior thyroid vein crosses the artery near its termination, and the middle thyroid vein a little below the level of the cricoid cartilage; the anterior jugular vein crosses the artery just above the clavicle, but is separated from it by the sternohyoid and sternothyroid.

Behind, the artery is separated from the transverse processes of the cervical vertebrae by the longus colli and longus capitis muscles, the sympathetic trunk being interposed between it and the muscles. The inferior thyroid artery crosses behind the lower part of the vessel.

Medially, it is in relation with the esophagus, trachea, and thyroid gland (which overlaps it), the inferior thyroid artery and recurrent laryngeal nerve being interposed; higher up, with the larynx and pharynx. Lateral to the artery, inside the carotid sheath with the common carotid, are the internal jugular vein and vagus nerve.

At the lower part of the neck, on the right side of the body, the right recurrent laryngeal nerve crosses obliquely behind the artery; the right internal jugular vein diverges from the artery. On the left side, however, the left internal jugular vein approaches and often overlaps the lower part of the artery.

Behind the angle of bifurcation of the common carotid artery is a reddish-brown oval body known as the carotid body. It is similar in structure to the coccygeal body which is situated on the median sacral artery.

The relations of the cervical region of the common carotid artery may be discussed in two points:

• Internal relations of organs present inside the carotid sheath
• two external relations of carotid sheath

In other cases, the artery on the right side may arise as a separate branch from the arch of the aorta, or in conjunction with the left carotid.

The left common carotid varies in its origin more than the right.

In the majority of abnormal cases it arises with the brachiocephalic trunk; if that artery is absent, the two carotids arise usually by a single trunk.

It is rarely joined with the left subclavian artery, except in cases of transposition of the aortic arch.

Very rarely, the common carotid artery ascends in the neck without any subdivision, either the external or the internal carotid being absent; and in a few cases the common carotid has itself been found to be absent, the external and internal carotids arising directly from the arch of the aorta.

This peculiarity existed on both sides in some instances, on one side in others.

Typically, blood flow velocities in the common carotid artery are measured as peak systolic velocity (PSV) and end diastolic velocity (EDV).

In a study of normative men aged 20–29 years, the average PSV was 115 cm/sec and EDV was 32 cm/sec. In men 80 years and older, the average PSV was 88 cm/sec and EDV was 17 cm/sec.