Prelab Exercise 3 –CIRCULATORY SYSTEM

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CIRCULATORY SYSTEM Arteries and veins are classified according to their size and the histology of their walls. In each case, one should analyze thoroughly the tissue composition of each of the three tunicas: intima, media, and adventitia. The histology of the media and adventitia is particularly important in determining the type of vessel. Although vessels are subdivided into categories on the basis of size and composition of the wall, there are no abrupt boundaries between categories. Rather, the histology changes gradually from the larger to the smaller vessels (see summary table, below).

ARTERIES – there are several types of arteries: elastic arteries; muscular ateries and arterioles.

Large conducting (“elastic”) arteries have a relatively thin intima (endothelium and thin

layer of CT). Also, smooth muscle cells interdigitate with prominent lamellae of elastic fibers in the thicker media. The adventitia also contains elastic fibers along with dense collagen fibers and is thinner (relatively speaking) than in muscular arteries. There are also vasa vasorum and nervi vascularis in the adventitia.

Medium-size, distributing (“muscular”) arteries. In general, the muscular arteries correspond to the smaller named arteries which you dissect in gross anatomy lab. The intima and internal elastic lamina are well-developed. The media has a predominance of smooth muscle, with sparse elastic fibers. The adventitia is relatively thicker than in the elastic arteries (see the figure below).

Prelab Exercise 3 –CIRCULATORY SYSTEM

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Arterioles. Arterioles contain only a few layers of smooth muscle in the media and are quite small (40-200 µm diameter). These vessels provide much of the peripheral resistance to blood flow and therefore are involved in the regulation of blood flow and pressure.

CAPILLARIES Capillaries consist of an endothelium with a basement membrane. They are specialized to permit (and in some cases restrict) the passage of things between the blood stream and the interstitial spaces. Review the fine structure of the three types of capillaries (continuous, fenestrated, and discontinuous). Especially note how capillary structure is adapted to the physiologic requirements of each organ. For example, compare the capillaries of the brain to those of the liver.

Prelab Exercise 3 –CIRCULATORY SYSTEM

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Types of microcirculation formed by small blood vessels. (1) The usual sequence of arteriole –> metarteriole –> capillary –> venule and vein. (2) An arteriovenous anastomosis. (3) An arterial portal system, as is present in the kidney glomerulus. (4) A venous portal system, with a second capillary bed, is present in the liver.

Prelab Exercise 3 –CIRCULATORY SYSTEM

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VEINS

Veins have larger lumens, much less smooth muscle, a thinner intima (without internal elastic lamina) and a thick connective tissue adventitia. Valves appear at intervals through the venous system.

Diagram comparing the structure of a muscular artery (left) and accompanying vein (right). Note that the tunica intima and the tunica media are highly developed in the artery but not in the vein.

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CHARACTERISTICS OF BLOOD VESSELS

ARTERIES VESSEL DIAMETER INNER LAYER MIDDLE LAYER OUTER LAYER (TUNICA INTIMA) (TUNICA MEDIA) (TUNICA ADVENTITIA) Elastic artery > 1 cm Endothelium Smooth muscle Connective tissue Connective tissue Elastic lamellae Elastic fibers Thinner than tunica media Muscular artery 2 - 10 mm Endothelium Smooth muscle Connective tissue Large Connective tissue Collagen fibers Some elastic fibers Prominent internal Relatively little Thinner than tunica media elastic lamina elastic tissue Ext. elastic lamina Small 0.1 - 2 mm Endothelium Smooth muscle Connective tissue Connective tissue (8-10 cell layers) Some elastic fibers Internal elastic Collagen fibers Thinner than tunica media lamina Arteriole 40-200 µm Endothelium Smooth muscle Thin, ill-defined sheath of Connective tissue (1-2 cell layers) connective tissue Capillary 4 - 10 µm Endothelium None None

VEINS VESSEL DIAMETER INNER LAYER MIDDLE LAYER OUTER LAYER (TUNICA INTIMA) (TUNICA MEDIA) (TUNICA ADVENTITIA) Venule 50 - 100 µm Endothelium Smooth muscle Connective tissue Pericytes (1 or 2 cell layers) Some elastic fibers Thicker than tunica media Small vein 0.1 - 1 mm Endothelium Smooth muscle Connective tissue (2 or 3 layers Some elastic fibers continuous with Thicker than tunica media tunica intima) Medium vein 1 - 10 mm Endothelium Smooth muscle Connective tissue Connective tissue Collagen fibers Some elastic fibers

Prelab Exercise 3 –CIRCULATORY SYSTEM

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Internal elastic membrane Thicker than tunica media in some cases Large vein > 1 cm Endothelium Smooth muscle Connective tissue Connective tissue (2-15 layers) Some elastic fibers Cardiac muscle Much thicker than tunica near heart media. sm. mus. fascicles Collagen fibers

LYMPHATIC VESSELS

The smallest lymphatics are often difficult to distinguish from capillaries and small venules except for their greater variability in form and cross-sectional area. Larger lymphatics resemble small venous channels, but have somewhat thinner walls, less well-defined tunicas, and an abundance of valves. Lymphatic channels should contain lymph fluid (and some lymphocytes) in contrast to venous and arterial channels, which will contain all blood elements.

CHECK LIST

VESSELS: Understand the architecture of both arteries and veins. Be aware of the approximate diameter, composition, organization and relative thickness of:

-tunica intima -tunica media -tunica adventitia

Define:

-endothelium -vasa vasorum -nervi vascularis -internal elastic lamina -external elastic lamina -metarteriole -arteriovenous anastomosis -continuous and fenestrated capillaries -pericytes -lymphatic vessels with valves.

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THE HEART Cardiac muscle is found in only one site in the body, namely the wall of the heart. The structure of this type of muscle tissue was studied specifically during the Muscle lab. This lab will consider the muscle in context of the entire organ. The walls of all chambers possess similar layers: endocardium, myocardium, and epicardium, but the relative thickness of these layers and the detailed organization of the tissues composing them vary radically in different chambers. (Note the respective continuity of these layers with tunica intima, tunica media, tunica adventitia of blood vessels.) Also, there are highly specialized structures such as the valves and their reinforcing mechanisms, which require careful analysis. Because the heart is fairly complex, it becomes important in the study of any section to determine precisely where it was cut with reference to the total cardiac anatomy. The “cardiac skeleton” is dense connective tissue between the atrium and ventricles that provide an anchoring point for cardiac muscle and that limit the direct spread of electrical activity in atrial muscle to the ventricles. The thin leaflets of the atrioventricular valve intervening between the two heart chambers attach to the cardiac skeleton. The epicardium (visceral pericardium) and subepicardial fat are outside of the myocardium. This is the location of the coronary blood vessels.

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The cardiac conduction system is comprised of specially modified cardiac muscle cells (Purkinji fibers). These are designed to transmit the electrical signal from the A-V node to the ventricles through various branches. Their location, and the direction of transmission of the cardiac impulse is shown in the figure below.

CHECK LIST

Know the histology of the heart, including the morphology of atrium and ventricle. Recognize the epicardium, myocardium and endocardium.

Understand the structure and importance of the cardiac skeleton.

Explain the role of Purkinje fibers in the heart and know their structures.