about three layers of cells. They line the oral, nasal, and external auditory meatus, (ear), providing lubrication and protection against pathogens. The lips are also protected by specialized sensory cells called Meissner's corpuscles. Blood, lymph and nerve supply Blood supply Blood circulates from the upper systemic loop originating at the aortic arch, and includes: the brachiocephalic artery, left common carotid artery and left subclavian artery. The head and neck are emptied of blood by the subclavian vein and jugular vein. The brachiocephalic artery or trunk is the first and largest artery that branches to form the right common carotid artery and the right subclavian artery. This artery provides blood to the right upper chest, right arm, neck, and head, through a branch called right vertebral artery. The right and left vertebral artery feed into the basilar artery and upward to the Posterior cerebral artery, which provides most of the brain with oxygenated blood. The posterior cerebral artery and the posterior communicating artery are within the circle of Willis. The left common carotid artery divides to form the: internal carotid artery (ICA) and an external carotid artery (ECA). The ICA supplies the brain. The ECA supplies the neck and face. The left subclavian artery and the right subclavian artery, one on each side of the body form the internal thoracic artery, the vertebral artery, the thyrocervical trunk, and the costocervical trunk. The subclavian becomes the axillary artery at the lateral border of the first rib. The left subclavian artery also provides blood to the left upper chest and left arm. Blood–brain barrier The Blood–brain barrier (BBB) is semi-permeable membrane that controls the capillary leak potential of the circulatory system. In most parts of the body, the smallest blood vessels, called capillaries, are lined with endothelial cells, which have small spaces between each individual cell so substances can move readily between the inside and the outside of the capillary. This is not in the case of brain. In the brain, the endothelial cells fit tightly together to create a tight junction and substances cannot pass out of the bloodstream. Specialized glial cells called astrocytes form a tight junction or protective barrier around brain blood vessels and may be important in the development of the BBB. Astrocytes may also be responsible for transporting ions (electrolytes) from the brain to the blood. Venous drainage Blood from the brain and neck flows from: (1) within