External carotid artery

The external carotid artery is one of the two terminal branches of the common carotid artery, the other being the internal carotid artery. It primarily supplies blood to the face, scalp, neck, and external structures of the head.

Location

The external carotid artery originates from the common carotid artery at the level of the superior border of the thyroid cartilage (around the fourth cervical vertebra). From its origin, it ascends through the neck, passing deep to the sternocleidomastoid muscle and in close proximity to the mandibular angle. It travels superficially, lying outside the carotid sheath, and runs upward toward the parotid gland, where it divides into its terminal branches—the superficial temporal artery and the maxillary artery. Along its course, the external carotid artery gives off several important branches that supply the structures of the face, neck, scalp, and jaws.

Structure and Anatomy

Origin

The external carotid artery arises from the common carotid artery at the level of the superior border of the thyroid cartilage, which corresponds to the level of the fourth cervical vertebra (C4). It is one of the two terminal branches of the common carotid artery, the other being the internal carotid artery. The bifurcation occurs in the carotid triangle of the neck, a region bordered by the sternocleidomastoid muscle, the superior belly of the omohyoid muscle, and the posterior belly of the digastric muscle.

Course

After its origin, the external carotid artery ascends through the neck, running more superficially than the internal carotid artery. It courses upward, initially positioned deep to the sternocleidomastoid muscle and the posterior belly of the digastric muscle. As it travels, it passes behind the ramus of the mandible and into the parotid gland, where it continues toward the level of the neck of the mandible. At this point, the artery terminates by bifurcating into its two terminal branches: the superficial temporal artery and the maxillary artery.

Branches

The external carotid artery is known for giving off multiple important branches along its course, which supply the face, neck, scalp, and various structures of the head. These branches can be categorized into anterior, posterior, medial, and terminal branches:

• Anterior Branches:
  • Superior Thyroid Artery: This is the first branch of the external carotid artery, which descends toward the thyroid gland, supplying it and nearby muscles.
  • Lingual Artery: This artery runs forward and upward, supplying the tongue, floor of the mouth, and parts of the oral cavity.
  • Facial Artery: This artery ascends to supply the facial muscles, skin, and structures such as the lips and nose. It runs along the lower border of the mandible and curves over it to reach the face.
• Posterior Branches:
  • Occipital Artery: This branch ascends to supply the posterior scalp and parts of the neck and muscles at the back of the head.
  • Posterior Auricular Artery: This smaller artery ascends behind the ear and supplies the auricle and the scalp behind the ear.
• Medial Branch:Ascending Pharyngeal Artery: This slender branch ascends vertically, supplying parts of the pharynx, prevertebral muscles, and structures in the neck.
• Terminal Branches:
• • Superficial Temporal Artery: This terminal branch continues its ascent above the zygomatic arch, supplying the scalp and temporal region.
  • Maxillary Artery: This large terminal branch runs anteriorly and medially into the infratemporal fossa, supplying deep structures of the face, including the maxilla, mandible, nasal cavity, and palate.

Relationships with Surrounding Structures

Throughout its course, the external carotid artery is closely related to several important anatomical structures:

• Carotid Sheath: The external carotid artery lies outside the carotid sheath, unlike the internal carotid artery, which is enclosed within the sheath along with the internal jugular vein and vagus nerve.
• Sternocleidomastoid Muscle: Initially, the artery runs deep to the sternocleidomastoid muscle in the carotid triangle before becoming more superficial.
• Mandible and Parotid Gland: The external carotid artery runs behind the ramus of the mandible and through the parotid gland, where it bifurcates into its terminal branches.

Branching Patterns and Variations

While the external carotid artery follows a fairly predictable course, variations in its branching patterns can occur. The origin or course of its branches, such as the facial or lingual arteries, may differ between individuals, which can have implications during surgical procedures in the head and neck region.

Termination

At the level of the neck of the mandible, within the parotid gland, the external carotid artery divides into two terminal branches:

• Superficial Temporal Artery: This artery ascends above the zygomatic arch to supply the scalp and portions of the forehead.
• Maxillary Artery: This artery dives deeper into the infratemporal fossa, supplying structures in the deep face, including the maxilla, mandible, and teeth.

Relations to Cranial Nerves

The external carotid artery and its branches have important relationships with cranial nerves:

• Facial Nerve (CN VII): As the external carotid artery passes through the parotid gland, it runs near the facial nerve, which divides into branches within the gland to supply the muscles of facial expression.
• Hypoglossal Nerve (CN XII): The external carotid artery lies in close relation to the hypoglossal nerve, particularly near the origin of the lingual artery, which supplies the tongue.
• Glossopharyngeal Nerve (CN IX): This nerve passes near the external carotid artery and its branches, particularly the ascending pharyngeal artery.

Clinical Landmarks

Due to its relatively superficial course, the external carotid artery is accessible for procedures and assessments. It can be palpated in the carotid triangle, and its pulse can be felt in regions such as the mandibular angle, making it clinically important in assessing vascular health in the head and neck region.

Surrounding Vascular Structures

The external carotid artery runs in proximity to other major vascular structures, including:

• Internal Jugular Vein: This large vein runs parallel and lateral to the external carotid artery in the neck, draining deoxygenated blood from the brain and face.
• Internal Carotid Artery: While both arteries originate from the common carotid artery, the internal carotid artery does not give off branches in the neck and instead supplies the brain and orbit. The external carotid artery, on the other hand, supplies structures outside the cranium.

Function

Blood Supply to the Face

One of the primary functions of the external carotid artery is to supply blood to the face. Through its branches, including the facial artery, superior thyroid artery, and lingual artery, the external carotid artery provides oxygenated blood to key facial structures:

• Facial Artery: This artery nourishes the skin and muscles of the face, including the lips, cheeks, and nose, ensuring these tissues receive the necessary oxygen and nutrients for normal functioning, healing, and maintaining facial aesthetics.
• Lingual Artery: Supplies the tongue and floor of the mouth, contributing to functions like speech and swallowing.
• Superior Thyroid Artery: Provides blood to the thyroid gland and larynx, ensuring these structures can regulate metabolism and maintain healthy vocal function.

Blood Supply to the Scalp and Forehead

The external carotid artery plays a crucial role in vascularizing the scalp and forehead through its superficial temporal artery and occipital artery branches. These arteries provide oxygenated blood to the skin, hair follicles, and muscles of the scalp:

• Superficial Temporal Artery: This terminal branch runs along the side of the head and supplies the upper face, scalp, and temporal region.
• Occipital Artery: This branch supplies the posterior scalp, muscles, and skin at the back of the head. It anastomoses with other arteries, ensuring the scalp receives a continuous blood supply.

This extensive vascular network helps maintain the health and functionality of the scalp tissues and supports hair growth.

Blood Supply to the Neck and Pharynx

The external carotid artery is responsible for delivering blood to the muscles and tissues of the neck and pharynx. Through branches such as the ascending pharyngeal artery and occipital artery, the external carotid artery ensures the pharynx, a vital structure involved in swallowing and breathing, receives sufficient blood supply. The sternocleidomastoid muscle, a key muscle responsible for head and neck movement, also receives blood from the occipital artery and superior thyroid artery.

The pharynx is involved in important bodily functions such as speech, swallowing, and the passage of air to the respiratory system. The arterial supply provided by the external carotid artery ensures these activities can be carried out effectively.

Supply to the Parotid Gland

The external carotid artery travels through the parotid gland (the largest salivary gland), and its terminal branches, particularly the superficial temporal artery, supply the gland itself. The parotid gland produces saliva, which plays an essential role in digestion and oral hygiene. By delivering oxygenated blood to the gland, the external carotid artery ensures that the parotid gland can maintain its salivary function.

Blood Supply to the Oral Cavity and Tongue

Through its lingual artery branch, the external carotid artery provides blood to the oral cavity, including the tongue, floor of the mouth, and adjacent structures. This is critical for functions such as:

• Speech: Blood supply to the tongue allows the muscles to function properly, facilitating articulation and speech.
• Swallowing: The lingual artery also supplies muscles involved in swallowing, ensuring smooth and coordinated movements of the tongue and other structures.
• Taste Sensation: By nourishing the tongue and taste buds, the external carotid artery supports taste perception.

Blood Supply to the Thyroid Gland and Larynx

The superior thyroid artery, a branch of the external carotid artery, is the primary blood supplier to the thyroid gland and larynx:

• Thyroid Gland: The superior thyroid artery ensures the thyroid gland receives the oxygen and nutrients needed to regulate metabolism, growth, and energy production. Proper blood flow is critical for maintaining hormone production, which influences the body’s metabolic rate.
• Larynx: The artery supplies the vocal cords and surrounding muscles, which are essential for phonation (the production of sound). This arterial supply supports the health and functionality of the larynx, ensuring that it can modulate voice and protect the airway during swallowing.

Supply to the Posterior Neck and Scalp

The occipital artery, a branch of the external carotid artery, is responsible for delivering blood to the posterior neck and scalp. This artery supplies oxygenated blood to the muscles and skin of the back of the head, including the trapezius muscle and other cervical muscles that contribute to head and neck movements. Additionally, the occipital artery forms anastomoses with other arteries in the scalp, ensuring a stable blood supply even if another vessel is compromised.

Collateral Circulation and Anastomoses

The external carotid artery plays a key role in maintaining collateral circulation in the head and neck. Its branches form numerous anastomoses (connections) with other arteries, including the internal carotid artery, vertebral artery, and subclavian artery. These anastomotic networks ensure that if one artery becomes blocked or narrowed, alternative pathways can supply blood to vital structures. For example:

• Facial Artery anastomoses with branches from the ophthalmic artery (a branch of the internal carotid artery), ensuring that blood can still reach parts of the face and forehead if the internal carotid artery is compromised.
• Occipital and Superficial Temporal Arteries form anastomoses with each other and with branches of the vertebral and subclavian arteries, ensuring consistent blood flow to the scalp and neck.

Thermoregulation of the Head and Face

Through its extensive network of arteries supplying the scalp, face, and neck, the external carotid artery also plays a role in thermoregulation. Increased blood flow through the facial and scalp arteries allows the body to dissipate heat through the skin in warm environments, while reduced blood flow conserves heat in colder conditions. This regulation is vital for maintaining body temperature, especially in the exposed regions of the head and face.

Nutrient Delivery and Waste Removal

Like all arteries, the external carotid artery is responsible for delivering oxygen, glucose, and other nutrients to the tissues it supplies. In addition to nutrient delivery, the external carotid artery plays a role in removing waste products from these tissues by ensuring continuous blood flow. This function is essential for maintaining the health of facial skin, muscles, glands, and bones, as well as for promoting healing and tissue repair after injury or surgery.

Clinical Significance

The external carotid artery is clinically significant due to its role in supplying blood to the face, neck, scalp, and associated structures. Its accessibility and extensive branching make it a key focus in various medical and surgical procedures.

Facial and Head Surgeries

The external carotid artery and its branches are often encountered during facial surgeries, such as facelifts, neck dissections, and tumor resections. Surgeons must be cautious to avoid damaging these arteries to prevent excessive bleeding and ensure proper healing of the facial tissues.

Arterial Pulsation and Diagnostic Assessment

Pulses of the external carotid artery, particularly in the facial artery and superficial temporal artery, can be palpated and used in diagnostic assessments. These palpable pulses help clinicians assess vascular health and detect abnormalities such as arterial narrowing (stenosis) or vascular occlusion.

Vascular Conditions and Aneurysms

The external carotid artery is susceptible to atherosclerosis, which can lead to reduced blood flow to facial and scalp structures. Additionally, aneurysms or arteriovenous malformations (AVMs) involving branches of the external carotid artery can result in complications, including ischemia, tissue damage, or excessive bleeding.

Embolization in Head and Neck Tumors

The branches of the external carotid artery are often involved in procedures such as embolization to reduce blood flow to tumors or malformations in the head and neck before surgery, minimizing the risk of excessive bleeding.