Figure 1216

Schematic diagram of a large vein. The cellular and extracellular components are labeled. The tunica intima consists of an endothelial lining and a small amount of connective tissue. The tunica media contains circumferentially arranged smooth muscle cells. The tunica adventitia, in addition to an extensive collagenous and elastic fiber component, contains bundles of longitudinally arranged smooth muscle cells near the tunica media. f„ tunica. (Based on Rhodin JAG. Handbook of Physiology. New York: Oxford University Press, 1980.)

t. adventitia unmyelinated nerve collagen fibers bundles of smooth muscle cells t. media^

t. intima lamina elastic fibers fibroblasts blood vessel bundles of smooth muscle cells sel wall. Along with the usual collagen and elastic fibers and fibroblasts, the tunica adventitia also contains longitudinally disposed smooth muscle cells.

Atypical Veins

In several locations, veins with a highly atypical structure are present. For example, venous channels in the cranial cavity, called venous or dural sinuses, are essentially broad spaces in the dura mater that are lined with endothelial cells. Veins in certain other locations (e.g., retina, placenta, trabeculae of the spleen) also have atypical walls and are discussed in the chapters that describe these organs.

heart

The heart is a muscular pump that maintains unidirectional flow of blood

The heart contains four chambers, the right and left atria and right and left ventricles, through which blood is pumped (see Figs. 12.2 and 12.17). Valves guard the exits of the chambers, preventing backflow of blood. An intera trial septum and an interventricular septum separate the right and left sides of the heart. The right atrium receives blood returning from the body via the inferior and superior venae cavae, the two largest veins of the body. The right ventricle receives blood from the right atrium and pumps it to the lungs for oxygenation via the pulmonary arteries. The left atrium receives the oxygenated blood returning from the lungs via the four pulmonary veins. The left ventnele receives blood from the left atrium and pumps it into the aorta for distribution into the systemic circulation.

The walls of the heart contain

• A musculature of cardiac muscle for contraction to propel the blood.

• A fibrous skeleton, which consists of four fibrous rings surrounding the valve orifices, two fibrous trigones connecting the rings, and the membranous part of the interventricular and interatrial septa. The fibrous rings are composed of dense irregular connective tissue. They encircle the base of the two arteries leaving the heart (aorta and pulmonary trunk) and the openings between the atria and the ventricles (right and left A-V orifices) (Fig. 12.18). These rings provide the attachment site for the leaflets of all four valves of the heart that allow blood flow in only one direction through the openings. The membranous part of the interventricular septum is devoid of cardiac muscle; it consists of dense connective tissue that contains a short length of the un-

aorta superior vena cava

Purkinje fibers sinuatrial (S-A) node pulmonary trunk left bundle right ventricle right bundle inferior vena cava

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