Heart valves must be shaped so as to exactly fit together and prevent backflow of blood. If congenitally abnormal or misshapen due to infection or other problem, regurgitation of blood back through the valve will occur and this is detectable as a murmur.

Study of the circulatory system outside the heart includes the arteries and veins, their smaller branches, and the microvasculature between the arterial supply and the venous system. The relative thicknesses and the general shapes of these vessels in transverse section is shown in the diagram.

Arteries and Arterioles

Examine a section of the aorta and identify

• The three basic layers or tunics (intima, media, and adventitia) of this large vessel
• Note their distinctive features.

Which is thicker in the media: one of the layers of smooth muscle or a layer of elastic material?

Clinical note: Degeneration of components such as elastic fibers in the tunica media of arteries weakens the arterial wall and can lead to blood leakage into or through the wall (a “dissected” artery) or to formation of a circumscribed bulge called an aneurysm. Either type of defect can have disastrous results in a major artery such as the aorta.

Now for muscular and elastic arteries.