13-1 Arteries, arterioles, capillaries, venules, and veins differ in size, structure, and function Kelsee, Aimee

How it works Blood leaves heart through pulmonary trunk and aorta Vessels branch forming major arteries that carry blood to body organs More branching within organs forms arterioles Arterioles provide blood to over 10 billion capillaries Vital functions of the cardiovascular system occur within the capillaries Blood flowing out of capillary network enters venules (slender vessels that form veins)

Structure of vessel walls The walls of arteries and veins contain three distinct layers: Tunica intima- the innermost layer of a blood vessel, includes the endothelial lining of the vessel and an underlying layer of connective tissue dominated by elastic fibers Tunica media- the middle layer, contains smooth muscle tissue in the framework of collagen and elastic fibers. When these smooth muscles contract, vessel diameter decreases; when they relax vessel diameter increases Tunica externa- outer layer that forms a sheath of connective tissue around the vessel. Its collagen fibers may intertwine with those of adjacent tissues, stabilizing and anchoring the blood vessel.

Arteries Blood passes through elastic arteries, muscular arteries, and arterioles Elastic arteries: large resilient vessels Ex. Pulmonary trunk and aorta and major arterial branches Muscular arteries: medium sized/distribution arteries, distribute blood to skeletal muscles and internal organs Ex. External carotid arteries of the neck Arterioles: much smaller than muscular arteries

Elastic Arteries Walls are dominated by elastic fibers Absorb pressure changes that occur during cardiac cycle During ventricular systole (blood pressure rises quickly) elastic arteries are stretched, diameter increases During ventricular diastole, (blood pressure declines) the elastic fibers go back to normal

Muscular Arteries and Arterioles Muscular arteries contain more smooth muscle cells than elastic fibers Tunica media of an arteriole contains two layers of smooth muscle cells These muscle layers change the diameter, which alters blood pressure

Capillaries Single layer of endothelial in basement membrane No tunica externa or tunica media Function in a network called capillary bed The only blood vessels whose walls permit exchange between blood and surrounding interstitial fluid Walls are thin and diffusion distances are short Diameter is small, causing the blood flow to be slow which gives adequate time for diffusion Each capillary is protected by a precapillary sphincter

The process A single arteriole feeds dozens of capillaries which connect to several venules Blood usually flows from arterioles to venules at a constant rate however it is variable Vasomotion Smooth muscle fibers respond to local changes in interstitial fluid Autoregulation

The process cont. Alternate routes for blood flow are formed by anastomosis outlet The joining of blood vessels Circumstances can cause blood to completely bypass a capillary bed through an arteriovenous anastomosis vessel that connects an arteriole to venule A single capillary bed can be supplied by an arterial anastomosis More than one artery fuses and gives rise to arterioles If one or more artery is blocked the others can still deliver blood and tissue will not be damaged

Veins Collect blood from tissues and organs and deliver it back to heart Venules- the smallest Medium sized veins- similar to size of muscular arteries Contain valves to prevent backflow of blood Large veins- include two vena cavae and tributaries in abdominopelvic and thoracic cavities Thin walls due to lower pressure Pressure is so low it cannot overcome the force of gravity