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The Cardiovascular System
Embryologically the heart, like the rest of the cardiovascular system, develops from a tubular channel. Its basic organization of tissues is divided into three layers called endocardium, myocardium and epicardium, corresponding roughly to the tunicas intima, media and adventitia, respectively, of the blood vessels. As in blood vessels, the lining is endothelium and the remaining tissues are muscle, connective tissue nerves and blood vessels.
The endocardial layer is thin and consists of endothelium on the surface with underlying collagenous and elastic tissue. The myocardium is the thickest layer and consists of cardiac muscle with intervening connective tissue, blood vessels and nerves. Between the endocardium and myocardium is the subendocardial layer, where nerves and the impulse-conducting system (Purkinje fibers) are located in a bed of connective tissue. The epicardium or outer layer consists of connective tissue with a large amount of adipose tissue covered on its outer edge by a mesothelium which lines the pericardial cavity. Coronary vessels and nerves are present in the epicardium.
Under high magnification, examine the endocardium. The endothelium is not readily visible over the entire endocardial surface because of damage, but a few flattened nuclei may be located. The endocardial layer also contains collagenous (pink) and elastic (black) fibers. At 4x, examine the extent of the heart skeleton at the base of the aortic valve. It is thickest at the base of the valve and extends as a collar (seen as a dark red line) along the initial part of the aorta. The alternating light and dark staining stripes are a sectioning artifact reflecting the fact that this is a relatively hard structure. Endothelium extends over the surface of the valve, covering both surfaces and continues on to form the lining of the aorta. The elastic fibers are more numerous on the ventricular side of the valve, the side which expands most when the valve is closed and blood in the aorta exerts backward pressure. Their elastic recoil helps to open the valve. The collagenous fibers, with more tensile strength, are on the aortic or "holding" side of the valve.
The structure of the various blood vessels is closely related to their function. The vessels which receive blood from the heart, the elastic arteries, have thick, strong walls to cope with the sudden high pressure produced during diastole; they contain abundant elastic material to allow stretch so that the vessel lumen may accommodate the change of volume. They also have a thick, outer coat of collagenous connective tissue whose tensile strength prevents over-distension of the elastic tissue. The elastic recoil of these elastic arteries is responsible for maintaining a continuous, though decreased, flow of blood to smaller vessels during systole.
Further along the arterial system, elastic components gradually diminish. Most of the muscle is arranged circularly, in the middle layer of the vessel wall (the tunica media). These muscular arteries contribute to the regulation of the amount of blood flowing into a region.
Maintenance of blood pressure and the control of blood flow into capillary beds is affected through the action of nervous and humoral agents on the smallest vessels in the arterial system, the arterioles. The amount of muscle present decreases gradually from about 3 layers of muscle cells, to only 1 around the smallest precapillary arterioles.
Capillary beds are the major site of the exchanges between blood and tissues. The walls of the capillary vessels consist of a layer of flattened endothelial cells, pericytes, a basement membrane and a few associated connective tissue fibers. The lumen of the smallest capillaries is just large enough to allow the passage of erythrocytes in single file. Exchange of materials across the capillary wall depends on the nature of the vessel; in discontinuous capillaries the endothelial cell poses no barrier; in continuous capillaries transport involves facilitated transfer across the endothelial cell by vesicles. Fenestrated capillaries have pores which are usually covered by a diaphragm and are intermediate in permeability between continuous and discontinuous capillaries.
The capillary networks drain into thin-walled venules made up of an endothelium surrounded by connective tissue. Muscle cells appear as the venules unite, forming larger vessels and they eventually develop a continuous muscle coat. Some elastic fibers may be present in the larger veins; however, at no point are the muscular and elastic components as abundant or as clearly organized as in arteries of comparable size.
The tunica media contains bundles of elastic fibers with gaps between them lying within a background of very fine collagen; the elastic membranes stain a deeper red-purple than the collagen. The cellular components of this layer consist of smooth muscle cells which will be difficult to see.
The outermost layer, the tunica adventitia, consists of connective tissue with thick collagenous fibers. The adventitia contains numerous blood vessels (vasa vasorum; or blood vessels of the blood vessels) and nerves. In some slides a mesothelium covers limits the adventitia.
In slide #30 (aorta) the elastic elements are not differentially stained since this is a routine H&E stain. Identify the features studied in slide #29. The slides of aorta will be either cross or longitudinal section. Compare #30 even and odd; in longitudinal sections, smooth muscle cells of the media will be in cross-section and appear round, rather than elongated, with round nuclei.
Arterioles, Venules & Capillaries
The definition of what constitutes an arteriole is extraordinarily variable. For this course we will define them as containing 1 to 3 layers of smooth muscle.
In venules, the endothelial cells rest on connective tissue and their irregular lumens are the diameter of 23 erythrocytes. There is usually no smooth muscle present in the smallest venules. Compare venules with arterioles in slide #64.
Slide #22 is a section of atrium covered by endocardium which includes some pectinate muscles. The section was probably taken from an area near the A.V. node. Under very low power, locate the epicardial and endocardial surfaces. The endocardial surface consists of compact, pink-staining tissue made up of fine closely woven collagen and elastic fibers while the epicardial surface appears ragged by comparison since it contains loosely arranged bundles of coarser collagenous fibers. Both the mesothelial covering of the epicardium and the endothelium of the endocardium have been damaged during tissue preparation and will probably not be seen. The myocardium consists of widely spaced groups of cardiac muscle fibers interspersed with loose connective and adipose tissue. It is much thinner than the ventricular myocardium and relatively large blood vessels and nerves may be seen within abundant connective tissue. The loosely packed muscle fibers are thinner than those of the ventricle.