Pericardium (per-ih-KAR-dee-um): The sac that surrounds the heart and the roots of the great vessels; it consists of an outer fibrous pericardium and an inner double-layered serous membrane (serous pericardium).
Fibrous pericardium (FY-brus per-ih-KAR-dee-um): The outermost, tough, inextensible layer of the pericardium, composed of dense irregular connective tissue; it anchors the heart and limits its distention.
Parietal serous pericardium (puh-RYE-uh-tul SIR-us per-ih-KAR-dee-um): The inner surface of the fibrous pericardium; it is composed of a mesothelium and an underlying layer of loose connective tissue, forming the outer wall of the pericardial cavity.
Epicardium (ep-ih-KAR-dee-um): The outer layer of the heart wall, equivalent to the visceral serous pericardium; it consists of a mesothelium and underlying connective tissue containing blood vessels and adipose tissue.
Visceral serous pericardium (VIS-uh-rul SIR-us per-ih-KAR-dee-um): The layer of the serous membrane that adheres directly to the surface of the heart; it is identical to the epicardium.
Myocardium (my-oh-KAR-dee-um): The thick, middle layer of the heart wall; it is composed primarily of cardiac muscle cells, responsible for the heart's pumping action.
Endocardium (en-doh-KAR-dee-um): The thin, innermost layer of the heart wall, lining the heart chambers and covering the valve leaflets; it is composed of an endothelium and a thin layer of underlying connective tissue.
Endothelium (en-doh-THEE-lee-um): A simple squamous epithelium that lines the entire inner surface of the cardiovascular system (heart chambers, blood vessels, and lymphatics); it acts as a selective barrier and regulates coagulation and vascular tone.
Internal elastic lamina (in-TUR-nul ih-LAS-tik LAM-uh-nuh): A prominent layer of fenestrated elastic material (visible as a wavy line) located between the tunica intima and the tunica media in arteries and arterioles; it is typically well-defined in muscular arteries.
Tunica intima (TOO-nih-kuh IN-tih-muh): The innermost layer of a blood vessel wall; it consists of the endothelium, its basement membrane, and a subendothelial layer of connective tissue, and in arteries, the internal elastic lamina.
Tunica media (TOO-nih-kuh MEE-dee-uh): The middle layer of a blood vessel wall; it is primarily composed of concentric layers of smooth muscle cells and varying amounts of elastic fibers; this layer is thickest in muscular arteries.
External elastic lamina (eks-TUR-nul ih-LAS-tik LAM-uh-nuh): A layer of elastic material located between the tunica media and the tunica adventitia in some large arteries; it is generally thinner and less prominent than the internal elastic lamina.
Tunica adventitia (TOO-nih-kuh ad-ven-TISH-uh): Alternate term for the tunica externa. Primarily composed of dense connective tissue. In larger vessels (e.g., elastic arteries and large veins), there may be small blood vessels (vaso vasorum) present in this layer.
Elastic lamina (ih-LAS-tik LAM-uh-nuh): A general term for the fenestrated sheets of elastin found in the walls of arteries (specifically the internal and external elastic laminae); they provide recoil capability to the vessel wall.
Elastic artery (ih-LAS-tik AR-tuh-ree): The largest arteries (e.g., aorta, common carotids) located near the heart; their tunica media is rich in elastic sheets to dampen the pulsatile force of the heart and maintain blood pressure during diastole.
Muscular artery (MUS-kyoo-ler AR-tuh-ree): Medium-sized arteries that distribute blood to organs (e.g., femoral, radial arteries); their tunica media is dominated by smooth muscle, allowing them to regulate blood flow via vasoconstriction and vasodilation.
Tunica externa (TOO-nih-kuh eks-TUR-nuh): The outermost layer of a blood vessel wall (also called tunica adventitia); it is composed of loose connective tissue, blood vessels (vasa vasorum), and nerves, and functions to anchor the vessel to surrounding tissues.
Venule (VEN-yool): The smallest veins, which collect blood from capillaries; their wall is thin, consisting of an endothelium and often only a few scattered smooth muscle cells, but they are a primary site for leukocyte emigration during inflammation.
Capillary (KAP-ih-lair-ee): The smallest blood vessels, composed of only a single layer of endothelium and its basement membrane; these are the primary sites for gas, nutrient, and waste exchange between blood and tissues.
Arteriole (ar-TIR-ee-ohl): A small artery that leads into a capillary bed; its wall contains 1-3 layers of smooth muscle, making it the key regulator of blood flow into the capillaries and a major determinant of systemic blood pressure.
Lumen (LOO-men): The inner, open space or cavity within a tubular organ, such as the heart chambers, blood vessels, or the intestines, through which fluid (blood or chyme) passes.
Valve leaflet (valv LEEF-let): A fold of endocardium and underlying dense core of connective tissue that forms the cusps of the heart valves (e.g., mitral, tricuspid, aortic, pulmonary); they ensure unidirectional blood flow by closing tightly.