The correct option is Endothelium.
Explanation
The walls of blood vessels (arteries and veins) are organized into three distinct concentric layers, or tunics, surrounding the central lumen. These layers differ in structure and function to support blood flow and pressure regulation.
Detailed Analysis
- Tunica Intima (Innermost Layer): This layer is in direct contact with the blood flowing through the lumen. It is composed of a single layer of simple squamous epithelial cells known as the endothelium. This smooth lining minimizes friction as blood moves through the vessel.
- Tunica Media (Middle Layer): This layer consists primarily of smooth muscle cells and sheets of elastin. It is responsible for regulating the diameter of the blood vessel through vasoconstriction and vasodilation.
- Tunica Externa (Outermost Layer): Also known as the tunica adventitia, this layer is composed largely of loosely woven collagen fibers that protect and reinforce the vessel, anchoring it to surrounding structures.
- Cartilage: Cartilage is a flexible connective tissue found in areas such as the joints, ear, nose, and trachea. It is not a structural component of blood vessel walls.
Key Takeaway: The tunica intima is the endothelial lining; the tunica media is the muscular layer; and the tunica externa is the collagenous connective tissue layer.