Endothelium is a thin layer of cells that lines the interior surface of blood vessels, lymphatic vessels, and the heart. It is a type of epithelial tissue that is composed of a single layer of flattened cells called endothelial cells.

Endothelial cells are specialized cells that have a number of important functions. They form a barrier between the blood and the surrounding tissues, regulating the exchange of nutrients, gases, and other substances between the blood and the tissues. They also play a key role in the regulation of blood pressure and blood clotting, and they produce a number of important signaling molecules that help to control the growth and function of other cells in the body.

In addition to these functions, endothelial cells also play a critical role in the immune response. They help to recruit immune cells to sites of infection or injury, and they can also act as antigen-presenting cells, presenting foreign antigens to T cells and initiating an immune response.

Overall, the endothelium is a vital component of the circulatory system, playing a key role in maintaining the health and function of the body’s tissues and organs.