Vessels

Blood Vessels

Blood vessels resemble the branching of a tree in that the arteries start as large vessels and divide into smaller and smaller branches. The smallest arteries are arterioles, which are con­tinuous with the smallest blood vessels, capil­laries.

Arteries and arterioles are tubular structures that carry blood away from the heart. Like all blood vessels, they are lined with endothelium. The walls of arteries tend to be thick and elastic, properties that are important in maintaining blood pressure. Smooth muscle in the walls of smaller arteries controls the diameter of these vessels.

Capillaries are tiny tubes composed almost entirely of endothelium, a continuation of the simple squamous epithelium that lines the heart and blood vessels. These thin-walled vessels are only large enough in diameter to accommodate a single file of erythrocytes. The wall acts as a selectively permeable membrane that permits water, oxygen, and nutrients to leave the blood for tissue cells and permits waste products from tissue cells to enter the blood. Much of the fluid that passes out of the capillaries into tissue spaces again returns to the blood by passing back through the capillary walls. Some fluid remains in the tissues, and excess fluid normally is removed by lymph vessels.

Capillaries unite to form venules, which merge into larger and larger veins. veins are larger in diameter than the arteries they parallel and have much thinner walls. venous blood pressure is typically quite low. Contraction of skeletal muscles in the limbs and trunk squeezes the thin-walled veins, assisting the flow of venous blood back toward the heart. valves, usually consisting of two cusps each, are scat­tered at irregular intervals throughout the venous and lymphatic systems.

Lymphatic Vessels

The walls of capillaries are thin enough to permit fluid as well as nutrients and gases to escape into spaces between tissue cells. Some of this extracellular fluid (ECF) does not reenter the vascular space directly but is instead recov­ered by thin-walled lymphatic vessels. Lym­phatic vessels resemble veins in that they contain numerous valves permitting flow only toward the heart. The smallest lymphatic vessels are blind capillary-sized structures that begin in intercellular spaces, where they accumulate extracellular fluid. Fluid within the lymphatic vessels, called lymph, is transported to larger and larger lymph vessels and finally emptied into the cranial vena cava or one of its tributar­ies. The tracheal trunks, two large lymph vessels draining the head and neck, usually terminate in the jugular veins. Lymph from the caudal half of the body is delivered to the large thoracic duct (of which there may be one or two), which traverses the thoracic cavity adja­cent to the aorta to empty its lymph into the cranial vena cava.

Movement of lymph is driven largely by gravity or changing pressures of adjacent struc­tures. For example, contraction of a muscle applies pressure to the adjacent lymphatic vessels and forces the lymph farther toward the heart, since the valves prevent backflow. The lymph is filtered by nodular structures called lymph nodes (see Chapter 16) scattered along the course of most lymphatic vessels.