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The Circulatory System SUMMARY OF KEY POINTS Heart

  • The heart has three major layers: (1) the inner endocardium of endothelium and subendothelial connective tissue; (2) the myocardium of cardiac muscle; and (3) the epicardium, connective tissue with many adipocytes and covered by mesothelium.

  • The cardiac conducting system stimulates rhythmic contractions and consists of modified cardiac muscle fibers forming the sinoatrial (SA) and atrioventricular (AV) nodes, the atrioventricular bundle (of His), left and right bundle branches, and Purkinje fibers.

  • Purkinje fibers, located in the subendocardial layer of both ventricles, are distinguished from contractile fibers by their greater diameter, abundant glycogen, and more sparse bundles of myofibrils.

  • Masses of dense irregular connective tissue make up the cardiac skeleton, which surrounds the bases of all heart valves, separates the atria from the ventricles, and provides insertions for cardiac muscle.

  • Macroscopically visible blood vessels have three major layers or tunics: (1) The intima includes the endothelium, connective tissue, and an internal elastic lamina in larger vessels; (2) the media contains alternating layers of smooth muscle and collagen or elastic lamellae; and (3) the adventitia (or externa) contains connective tissue, small vessels (vasa vasorum), and nerves.

  • Through the vasculature, endothelial cells are not simply heart and vessel liners; they actively produce factors that prevent blood clotting, factors that cause adjacent smooth muscle cells to contract or relax, and factors that initiate inflammation at sites of damage or infection.

  • Arteries are grouped by size and wall composition: (1) large elastic arteries, with fenestrated elastic laminae in the thick tunica media; (2) muscular, medium-sized arteries; and (3) small arteries, with fewer than 10 layers of smooth muscle in the media.

  • A microvasculature too small for surgical manipulation permeates most organs and consists of (1) arterioles, with one to three smooth muscle layers; (2) capillaries, consisting only of an intima endothelial layer; and (3) venules, with large lumens and thin walls, which drain capillaries.

  • Terminal arterioles branch into metarterioles, in which smooth muscle sphincters contract to resist blood flow and relax cyclically to allow pulsatile flow of blood into an anastomosing capillary bed, where metabolic exchange with surrounding cells occurs.

  • Capillaries are classified as three structural and functional types, with features that allow different degrees of molecular or even cellular exchange: (1) continuous capillaries with many tight junctions so that all exchange must occur through the cells; (2) fenestrated capillaries with small pores or fenestrations through the cells; and (3) discontinuous capillaries, or sinusoids, with larger lumens, large spaces between the endothelial cells, and a discontinuous basal lamina.

  • Capillary beds generally drain into venules, the last segment of the microvasculature; postcapillary venules are the sites at which white blood cells enter damaged or infected tissues.

  • The endothelium of continuous capillaries and postcapillary venules is frequently surrounded by thin cells called pericytes, whose contractions ...

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