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The Cardiovascular System

Objectives

  • To study the relationship between structure and function in the cardiovascular system.

Heart

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.

Examine slide #23 (heart and associated structures, elastic stain). If you lack this slide, start with the aorta slide (slide #29) as described on the next page until the heart slide becomes available. First locate the aorta, a straight strip of dark tissue, and follow it toward the heart mass until you come to the aortic valve, a thin loose flap of tissue protruding on one side. Just below the valve is some deep pink-staining material where the aorta joins the ventricular muscle. This is the fibrous heart skeleton. The free edge of the ventricular muscle to which the aorta and valve are attached, is the endocardial surface. The epicardium is not present.

   

At 10x look for groups of larger, paler-staining Purkinje fibers located between the main mass of myocardium and the endocardium. These are conducting fibers in the subendocardial layer. The Purkinje fibers are modified cardiac muscle fibers and have an accumulation of glycogen in the central portions of the cell. The myofibrils are pushed to peripheral locations, leaving a pale "empty" center around the nucleus since the glycogen is lost during sample preparation.

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.

Blood Vessels

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.

Elastic Arteries

Study the aorta of an older individual slide #29, (elastic tissue stain, not counterstained with hematoxylin).The slides contain only one wall of the vessel. The innermost layer is the tunica intima which can be recognized as having a smooth more sharply defined free-edge than the opposite surface and no visible blood vessels. The intima is seen to be covered by the vessel lining, a layer of thin endothelial cells which may have been torn off in many places.

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.

Muscular Arteries

Good examples of muscular arteries with their accompanying veins can be seen within the connective tissue of the gall bladder (slide #64). Examine the connective tissue forming the looser edge of the tissue. Even though many of the vessels in this slide contain blood cells, the flattened endothelial cells are visible. In these arteries, the intima is composed of the endothelium and a small amount of connective tissue which are bounded by the red staining internal elastic membrane. The wavy appearance of this elastic membrane is probably due to vessel contraction during fixation since in living vessels it appears as a smooth line. The media consists of smooth muscle arranged in a tight helix and some collagen and elastic fibers.

Veins

In slide #64, there will be blood veins which accompany the muscular arteries. At low magnification, compare the relative thickness of the vessel walls. It should be obvious that for vessels with comparable sized lumens, the veins have a thinner wall and lack an internal elastic membrane. Practice distinguishing arteries and veins.

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 slide #64, study the smallest branches of the vascular system can be observed scattered throughout the adipose and connective tissue. Arterioles have 1 to 3 layers of muscle in the media. The endothelial cells border a vessel lumen that has a very regular, round appearance. The smallest arterioles have a single muscle layer and the lumen diameter (10 µm) is slightly larger than that of an erythrocyte (6 µm). The adventitia is composed of collagenous connective tissue.

   

At high magnification, capillaries can be located in the connective tissue (slide #64). Look for tiny circular structures containing a single red blood cell. In cross-section, capillaries consist of a circular lumen made up of a very thin wall and, sometimes, a single endothelial cell nucleus can be seen. Look for longitudinal sections in which the lumen is only about 1 red cell diameter in thickness.

In venules, the endothelial cells rest on connective tissue and their irregular lumens are the diameter of 2–3 erythrocytes. There is usually no smooth muscle present in the smallest venules. Compare venules with arterioles in slide #64.

Lymphatics (Slide #27, the even slide is better), Several large lymphatic vessels are present with characteristic thin irregularly shaped walls and valves that extend into the lumen. The shape of the lumen is irregular and it may contain lymph, a few white blood cells and, occasionally, erythrocytes.

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.