The provided text is an excerpt from a medical textbook that explains the structure and function of veins, describes the pathophysiology of chronic venous disorders, and details the common venous disorders of chronic venous insufficiency, deep vein thromboembolism (DVT), varicose veins, and venous ulcers. The text explains the risk factors, clinical presentation, diagnosis, and treatment of each disorder. It also explains the use of diagnostic tests, prophylactic and treatment medications, and surgical interventions used in the management of venous disorders.