The cardiovascular system consists of a closed system or arteries, arterioles, capillaries, venules, and veins, in addition to an open system of lymphatic vessels, which connect to the blood circulation by way of the veins. Each blood vessel type, in turn, is comprised of cellular and noncellular components, the nature of which differs not only between blood vessel types (e.g., arteries and veins) but also between different organs (e.g., capillaries in the heart and brain). Blood vessels are highly dynamic entities; they participate in many different functions, including the control of vasomotor tone and maintenance of blood pressure, the selective permeability of cells and solutes between blood and underlying tissue, innate and acquired immunity, regeneration and repair, and the regulation of hemostasis. Thus, vascular pathophysiology may involve not only distinct vascular beds but also selected functions. The vascular wall is integrally linked to hemostasis and thrombosis, and the theme of vascular disease is weaved throughout all sections of this volume. In the current section, we have chosen to cover a few additional examples of such disorders.

Vascular disorders may be classified in a number of different ways. For example, they may be categorized according to whether they are congenital or acquired. Among the more prominent congenital disorders are connective tissue disorders, arteriovenous malformations, hereditary hemorrhagic telangiectasia, and cerebral cavernous malformations. As discussed in Chapters 69, 70 and 71, recent studies have shed fascinating light on the molecular mechanisms underlying these defects. Vascular disease may be classified according to whether it involves primarily veins, arteries, small blood vessels, or lymphatics. Atherosclerosis is so common in the Western world that is often viewed synonymously with arterial disease. However, there are many other forms of arterial pathology, including arteriosclerosis, aneurysm formation (Chapter 72), and vasculitis (which is covered under the section on complex diseases). The most recognizable disorders associated with veins are superficial and deep thrombosis and varicose veins. Venous thromboembolism is discussed in detail in other chapters in the current volume. Another approach to vascular disease is to consider it from the perspective of the organ or vascular bed affected. For example, pulmonary vascular disorders, including pulmonary hypertension and diseases associated with the bronchial circulation, are considered quite separately from disorders of the kidney vasculature (e.g., renal artery stenosis and glomerulonephritis). Chapter 73 showcases how a specialist from a single discipline (neurology) approaches distinct vascular disease phenotypes in their organ of interest (subarachnoid hemorrhage and intracranial bleeding). Chapter 74 provides another example of how one approaches vascular disease from the perspective of a single organ, namely, the eye. In reading this chapter, we gain an appreciation for the complexity of the eye vasculature and the breadth of disorders that affect it. Indeed, in perusing the chapters in this section, it becomes clear that subspecialization in medicine, though a necessary development over the past 50 years, has limited our capacity to fully grasp the vasculature—warts and all—as an organizing principle in health and disease.