Tunica intima of vessel

The tunica intima of vessel (also called the tunica interna) is the innermost layer of a blood vessel, lying in direct contact with the circulating blood. It consists of a simple squamous epithelium known as the endothelium, which rests on a basement membrane and a thin layer of loose connective tissue. This layer is continuous with the endocardium that lines the chambers of the heart.

The endothelium functions as a selectively permeable barrier, regulating the exchange of substances between the bloodstream and surrounding tissues. It also secretes chemical mediators that influence vascular tone by inducing contraction or relaxation of the smooth muscle within the vessel wall. Under physiological conditions, the endothelium inhibits platelet adhesion and aggregation, promoting smooth and unobstructed blood flow. When endothelial integrity is compromised, however, platelets adhere to the exposed surface and initiate thrombus formation. During inflammatory responses, endothelial cells express adhesion molecules that facilitate leukocyte attachment and migration into the affected tissues to support immune defense.