Chapter 22: Cardiovascular System: Vessels & Circulation

A comparison of vessel structure reveals that arteries and veins share three distinct wall layers: the inner intima, the middle media, and the outer adventitia. Arteries, which handle high pressure, have thick, elastic walls and smaller lumens, while veins are thinner-walled, more distensible, and contain one-way valves in the limbs to ensure blood flows toward the heart, aided by the skeletal muscle pump. The arterial hierarchy moves from large, resilient elastic arteries (conducting arteries) to medium-sized muscular arteries (distribution arteries) and finally to small arterioles, which control flow into the tiny exchange vessels called capillaries. Capillary structure varies, including continuous capillaries, porous fenestrated capillaries, and irregular sinusoids, all organized into interconnected networks called capillary beds, where blood flow is regulated locally by precapillary sphincters. The venous system is crucial because it functions as a large blood reservoir, normally holding between 65 and 70 percent of the total blood volume; mobilization of this reserve through venoconstriction helps maintain adequate circulation during significant blood loss. Major systemic arteries branch from the aorta, including the brachiocephalic trunk and carotid arteries supplying the head and upper limbs, where the brain's circulation is ensured by the anastomotic cerebral arterial circle. Visceral branches of the abdominal aorta include the unpaired celiac trunk and mesenteric arteries, while the paired iliac arteries supply the pelvis and lower limbs. Venous blood drains into either the superior vena cava (SVC) or the inferior vena cava (IVC), with the unique hepatic portal system routing nutrient-laden blood from the digestive organs directly to the liver for necessary processing before it reaches the IVC. The chapter also addresses vessel pathology, such as arteriosclerosis and the formation of aortic aneurysms, and describes the essential shunts in fetal circulation—the foramen ovale and the ductus arteriosus—that bypass the developing lungs and typically close at birth.