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Core
The brain could be considered to be the most important structure within the head and neck region. It requires 0.75-1L of blood every minute, which is about 15 to 20% of the heart's output. It also uses about 20% of the oxygen consumed by the whole body, even though it makes up only about 2% of body weight. This means that the arterial supply to the head and neck, and the subsequent venous drainage, needs to be able to handle a large volume of blood effectively.
It is important to remember that the vascular system in the human body is, for the most part, symmetrical: there is a left and a right vessel unless otherwise stated. There are also some normal anatomical variants (differences between healthy people). For example, the common carotid artery usually splits in two (bifurcates) at the level of C4 (the fourth vertebra of the neck), but in some people this happens at C3 or C5.
There are some abbreviations of vessel names used in this article:
- CCA = Common Carotid Artery
- ECA = External Carotid Artery
- ICA = Internal Carotid Artery
- IJV = Internal Jugular Vein
- EJV = External Jugular Vein
Blood travelling to the brain begins in the heart. This oxygenated blood travels towards the head via the common carotid and vertebral arteries.
The arch of the aorta has three branches: the left subclavian artery, the left common carotid artery, and the brachiocephalic trunk (a short vessel that supplies the right side of the head, neck and arm). The brachiocephalic trunk then divides into the right common carotid artery and the right subclavian artery. Both common carotid arteries ascend in a carotid sheath, a fibrous tissue sleeve that contains the common carotid artery (medially), the internal jugular vein (laterally), and the vagus nerve (posteriorly).
At the level of C4 (the superior border of the thyroid cartilage) the common carotid bifurcates into the internal and external carotid arteries. At this level, the internal and external carotid arteries look similar, so it can be difficult to tell them apart, but there are clues. The internal carotid artery appears more bulbous at its origin because of the carotid sinus (a widening of the artery wall that senses blood pressure). The carotid body (a small structure that senses the oxygen level in the blood) lies near the bifurcation. The external carotid artery will exit the carotid sheath. The other main difference between the internal and external carotid arteries is the lack of branches from the ICA in the neck. The ICA ascends up the neck and enters the base of the skull via the carotid canal (a channel through the base of the skull) and continues up through the cavernous sinus (a blood-filled space on either side of the pituitary gland). It then helps to form the Circle of Willis, a ring of arteries at the base of the brain that supplies the brain's structures. The ring links the internal carotid arteries to the vertebral arteries (through the basilar artery, described below). A join between two vessels like this is called an anastomosis. The Circle of Willis will be covered in the neuroanatomy unit found here.
The ECA is the major blood supply to the extra-cranial structures of the head and neck (i.e. things on the outside of the skull). The eight branches it gives off are the superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, maxillary and superficial temporal arteries. They can be remembered by the following mnemonic and are shown on the image below.
Table - Mnemonic for the branches of the external carotid artery
SimpleMed original by Dr. Ethan Kavanagh
Diagram - Branches of the external carotid artery
SimpleMed original by Dr. Maddie Swannack
The maxillary and superficial temporal arteries are the terminal branches of the ECA (the two branches where the ECA ends). They arise high up in the parotid gland (the large salivary gland in front of the ear).
The important branches of the maxillary artery are:
- Middle Meningeal Artery: this artery can tear after an injury to the side of the head (the temporal region), causing an extradural haemorrhage (bleeding between the skull and the dura, the tough membrane around the brain).
- Sphenopalatine Artery: contributes to the anastomosis that supplies the nasal cavity and, in a small number of nosebleeds (particularly in the elderly) is the culprit vessel. It is also located quite far back within the nasal cavity. A nosebleed from the back of the nose (posterior epistaxis) can be more difficult to control with simple pressure alone. A suspected posterior nosebleed needs admission to hospital, where treatment can include packing the nose, cautery (sealing the bleeding point with heat or a chemical) or surgery.
The superficial temporal artery runs just in front of the ear, where its pulse can be felt.
The forehead has overlapping arterial supplies. The frontal branch of the superficial temporal artery (the superficial temporal artery is a terminal branch of the ECA) contributes alongside the supraorbital and supratrochlear branches of the ophthalmic artery (these run above the eye socket). The ophthalmic artery arises from the ICA and enters the eye socket (the orbit). Its forehead branches join (anastomose) with branches of the ECA, so the forehead is supplied by both the internal and external carotid arteries.
The posterior circulation of the brain is supplied by the vertebral arteries. The vertebral arteries ascend through the transverse foramina (small holes in the side of each vertebra) of the cervical vertebrae (with the exception of C7). They then enter the subarachnoid space (the fluid-filled space around the brain) between C1 (also called the atlas) and the occipital bone. Both vertebral arteries then pass through the foramen magnum (the large hole in the base of the skull), curve around the medulla (the lowest part of the brainstem), and join to form the basilar artery (a single artery formed by the two vertebral arteries). The basilar artery then contributes to the Circle of Willis via the posterior cerebral arteries, which connect with the ICA through the posterior communicating arteries, to supply the brain.
Inside the skull, the dura (the tough outer membrane around the brain) forms dural folds. Two of these folds are the falx cerebri, which runs from the front to the back of the skull on the sagittal plane and separates the two hemispheres of the brain, and the tentorium cerebelli, which lies roughly horizontally and separates the occipital lobes (at the back of the brain) from the cerebellum below. Within these dural folds, between the periosteal and meningeal layers of dura, exist channels called dural venous sinuses, which carry venous blood.
The image below shows the different sinuses in the skull and how they relate to the dural folds. The sinuses converge at the confluence of sinuses (where the main sinuses meet at the back of the skull). Blood then drains through the transverse sinuses to the sigmoid sinuses (the S-shaped sinuses on each side). Each sigmoid sinus leaves the skull through the jugular foramen (an opening in the base of the skull) and becomes an internal jugular vein. Each internal jugular vein then travels through the neck in the carotid sheath.
Diagram - The dural venous sinuses
SimpleMed original by Dr. Maddie Swannack
Both the left and the right IJVs descend the neck and join with the subclavian veins to form the brachiocephalic veins (large veins that carry blood from the head, neck and arms towards the heart), which then go on to form the superior vena cava.
As the IJV descends, it receives blood from a number of different vessels. These vessels include:
- Facial Vein
- Lingual Vein
- Occipital Vein
- Superior and Middle Thyroid Veins
Learning the path of the internal jugular vein is important, as it is used on the right side to assess the jugular venous pressure (JVP). The JVP gives an estimate of the pressure in the right atrium (the heart chamber that receives blood returning from the veins of the body). For more information on the JVP, see this article.
Diagram - The position for measuring the jugular venous pressure
SimpleMed original by Dr. Bethany Turner
The external jugular veins drain the scalp and face, as well as some of the deeper structures within the face. Each external jugular vein is formed by the connection of two veins. The first is the posterior auricular vein (auricular means of the ear), which drains the area of the scalp superior and posterior to the ear. The second is the posterior branch of the retromandibular vein (the vein behind the jaw), which is formed by the maxillary and superficial temporal veins and drains the face. The EJV descends down the neck and feeds into the subclavian vein.
Edited by: Dr. Maddie Swannack
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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