The endothelium is described as physiologically critical to regulating capillary exchange and altering:

1. The lining was once thought to be merely passive. 2. Recent studies have changed how the endothelium is regarded. 3. They show it is physiologically critical to certain activities. 4. Those include helping to regulate capillary exchange. 5. They also include altering blood flow. _Source: OpenStax Anatomy and Physiology 2e (CC BY 4.0), Ch 20 'The Cardiovascular System: Blood Vessels and Circulation', sections 20.1-20.5_