Arteries are tubes that deliver blood from the heart to the tissuess and organss of the body. The arterial system is a high-pressure system that must accommodate the systolic and diastolic pressures generated by the strong contractions of the heart's left ventricle.
To withstand and adapt to these pressures the arteries are surrounded by a varying degree of smooth muscle which contracts or relaxes in response to adrenergic and other locally produced peptides. (See epinephrine, norepinephrine, alpha and beta receptors.)
Arteries deliver blood that has passed through the pulmonary circulation and has become enriched or saturated with oxygen. Hemoglobin molecules within red blood cells bind to up to four molecules of elemental oxygen and gradually release them one by one as the blood cells enter oxygen-poor environments of the distal tissues. This phenomenon is possible due to the oxyhemoglobin dissociation curve (a sigmoid curve illustrating hemoglobin's affinity for oxygen).
The aorta is the root artery and it receives blood directly from the left ventricle of the heart. As the aorta branches and these arteries branch in turn, they become successively smaller in diameter and are called arterioles. Finally, in the most distal parts, they are called capillaries. Capillaries have no smooth muscle surrounding them and have the diameter of only a few red blood cells. At the level of the capillaries oxygen is released to diffuse into tissue cells. The capillaries are continuous with venules. Venules pool together to form larger vessels, each helping to transport wastes and oxygen-poor red blood cells up through the low-pressure system of the veins.
Over time abnormally high pressures (high blood pressure), cholesterol, smoking and possibly many other inflammatory agents (see diabetes, vasculitis, C-reactive protein) are all involved in damaging the endothelium of the vessels, resulting in poor or otherwise abnormal circulation. Simply put, hypertension leads to aneurysms in major vessels whereas hypercholesterolemia leads to coronary artery occlusions (which may result in myocardial infarctions) and diabetes/smoking and vasculitides lead to peripheral (small) artery damage.
The arterial system is extremely important in sustaining life. Its proper functioning is responsible for the delivery of oxygen and nutrients to all cells, as well as the removal of waste products, maintenance of optimum pH and mobilization of the elements of the immune system. Barring trauma, infection and malignancy, it is most often the vascular system which determines whether we continue living or not. In First World countries the two leading causes of death, myocardial infarction and stroke, are each directly the result of an arterial system that has been slowly compromised by years of deterioration.