This chapter should help you to:

· Name the types, subtypes, and major functions of each circulatory system component.
· Name the 3 tissue layers (tunics) that make up the walls of all circulatory system components, and know the type of tissue fi,und in each tunic.
· Compare the circulatory system components in terms of size and wall structure.
· Relate the wall structure of each circulatory system component to its major functions. · Describe the heart's impulse-generating and conducting system in terms of structure, function, location, and how the impulse is conveyed to the cardiac muscle fibers.
· Recognize the types of vessels present in a slide or photomicrograph of an organ and identify the tunics, valves, cell and tissue types, and other named structural components.
· Distinguish between cardiac muscle and Purkinje fibers and identify the endocardium, myocardium, epicardium, and valves in a slide or photomicrograph of the heart.
· Predict the functional consequences of a structural defect in one of the tunics of any component of the circulatory system.



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SYNOPSIS

I. GENERAL FEATURES OF THE CIRCULATORY SYSTEM

A. General Function: The circulatory system is responsible for the transport and homeostatic distribution of oxygen, nutrients, wastes, body fluids and solutes, body heat, and immune system components.

B. The Subsystems:

1. The cardiovascular system is comparable to a closed system of plumbing, through which the blood circulates with the aid of an in-line pump. It has 4 types of components: the heart, a muscular pump; the arteries, which carry blood away from the heart toward the tissues; the veins, which return blood from the tissues to the heart; and the capillaries, which intervene between the arteries and veins, allowing exchange of nutrients, oxygen, and waste products between the blood and other tissues.
2. The lymphatic vascular system comprises an additional set of vessels, in which lymph moves in only one direction (toward the junction of the lymph vessels with the large veins in the neck). This system lacks a separate pump. It includes 3 types of vessels: the lymphatic capillaries, which are blind-ending, endothelial tubes that collect lymph (excess tissue fluid, cellular debris and lymphocytes) from the intercellular spaces; the lymphatic vessels, which collect lymph from lymphatic capillaries; and the lymphatic ducts, which collect lymph from the smaller lymphatic vessels and empty it into the large jugular and subclavian veins.

C. Walls of the Blood and Lymphatic Vessels: Components of the circulatory system are described in terms of their wall structure (II). Vessel walls are constructed on a general plan of 3 concentric layers or tunics. The borders between the tunics of lymphatic vessels are less, distinct than in blood vessels. Local weakening of vessel walls as a result of embryonic defects, disease, or lesions may lead to the development of a thin-walled outpockcting. or aneurysm, that may rupture and cause a hernorrhage.

1.The tunica intima, the innermost layer, borders the lumen. The intima of arteries and veins and that of the heart (the endocardium are virtually identical. It consists of the endothelium (a simple squamous epithclium that borders the lumen and is underlain by a thin basal larnina) and the subendothelial layer of connective tissue. Capillaries are often composed solely of the endothclium. In arteries, the intima is separated frorn the tunica media by a fenestrated layer of elastin called the internal elastic lamina,
2. The tunica media, is the middle layer. In blood vessels, it consists mainly of circumferen tially arranged vascular smooth muscle fibers. Arteries generally have a thicker media, containing more muscle and elastic fibers, than that of veins or lymphatic vessels. Large arteries often exhibit an external elastic lamina between the media and the tunics adventitia. The media of the heart (myocardium) is several times thicker than that of the largest artery (aorta) and is composed of cardiac muscle.
3. The tunica adventitia, the outermost layer, consists chiefly of type 1 collagen and elastic fibers that anchor the vessel in the surrounding tissues. In veins, the adventitia is the thickest layer; in large veins, it may contain longitudinal bundles of smooth muscle. In large vessels of each type, the adventitia contains small blood vessels (vasa vasorum) that supply oxygen and nutrients to cells in the vessel wall too far from the lumen to be nourished by diffusion. The outer layer of the heart (cpicardiurn) is not an adventitia, but rather a serosa, composed of connective tissue covered on its outer surface by a mesothelium. The smooth surface helps reduce friction between the beating heart and surrounding structures.

II. BLOOD VESSELS

Blood vessels are classified according to type and siae. Comparisons are based on structure (Fig 1 1-1) and function and often focus on the thickness and composition of the tunics (Tables 1 I - I and 1 1-2).

A. Blood Capillaries: These are the smallest vascular channels in the body, with an average diameter of 7-8 um. Their walls consist of a single layer of simple squamous epithelial (endothelial) cells rolled into a tube covered on the outer surface by a thin basal lamina. The cells attach to one another at their borders by junctional complexes, including tight (occluding) junctions and gap junctions. Some blood capillaries have fenestrations (pores) in their endo thelial linings.

1, Capillary beds. The basic plan of the arterial tree is that a small number of large-diameter vessels branch to feed a successively larger number of smaller-diameter vessels. Capillaries are the smallest-diameter vessels in this chain and hence are the most numerous. They commonly occur as components of a profusion of anastomosing (interconnecting) channels referred to as a capillary bed (Fig 11 I).
2. Cells of capillaries a. Endothelial cells, the chief structural component of capillaries, are simple squamous epithelial cells of mesenchymal origin joined by interccllular junctions (including zonulae occludentcs) to form an epithelial tube. The nucleus causes the center of each cell to bulge into the capillary lumen, but the cell thins toward its periphery to as little as 0.2 um. There are abundant pinocytotic vesicles throughout the cytoplasm and small amounts of major organelles and filaments near the nucleus. Key functions carried out by endothelial cells of capillaries and larger vessels include the following: (I) converting angiotensin L to angiotensin 11 (angiotensin regulates blood pressure by causing arterial smooth muscle contraction); (2) inactivating a variety of bioactive compounds leg, bradykinin, serotonin, prostaglandins, norepinephrinc, and thrombin), thus regulating their effects; (3) breaking down lipoproteins (lipolysis) to yield triglycerides and cholesterol (substrates for energy metabolism, hormone synthesis, and cell membrane assembly); (4) preventing thrombus (clot) formation (endothelial cells release pros tacyclin, an inhibitor of platelet aggregation; damage to these cells may induce local clotting by decreasing prostacyclin release and uncovering the basal lamina, whose collagen stimulates thrombogenesis); and (5) participating in capillary transport (II.A.4). b, Pericytes, or adventitial cells, are small mesenchymal cells scattered along capillaries. Each is surrounded by its own basal lamina and clings by long cytoplasmic processes to the outside of capillaries. They may or may not be contractile. These mesenchymal stem cells may differentiate into a variety of cell types
3, Types of capillaries. As for all vessels, capillaries are classified by wall structure. a. Continuous capillaries have a smooth, nonporous, endothclial lining in which the cells attach tightly to each other by junctional complexes. Structures containing continuous capillaries include muscles, the brain, and peripheral nerves. b, Fenestrated capillaries have endothelial cells perforated by pores (fenestrae), There are 2 types, one with unobstructed pores and another with pores covered by thin diaphragms that limit the size of macromolecules that can pass. Fenestrated capillaries occur in tissues where a rapid exchange of materials between tissues and the blood is required. Organs containing fcnestrated capillaries include the kidneys, intestines, and certain endocrine glands. c, Sinusoidal capillaries have 6 distinctive features. They (l) have unusually wide lumens (30-40 CLm); (2) follow a tortuous path; (3) have gaps between their endothelial cells, often large enough to allow cells to pass; (4) have abundant fenestrations: (5) often have phagocytic cells interspersed among their endothelial cells; and (6) are surrounded by a discontinuous basal lamina.
4. Transport across capillary walls. Capillaries are termed exchange vessels, because capil lary beds serve as major sites for the exchange of oxygen, nutrients, and many other substances between blood and other tissues. Most mechanisms of transcapillary transport are not well understood, but morphologic bases exist for at least 4 types. Fenestrae penetrate completely through the cndothelial lining, allowing passive diftusion. Some have diaphragms, whose composition is poorly understood. Intercellular clefts are spaces between neighboring endothelial cells through which particles and even some cells may pass. These are especially numerous in sinusoidal capillaries. Pinocytosis is the process by which small amounts of plasma or tissue fluid are cndocytosed by endothelial cells. This mechanism is followed by transport of membrane-bound pinocytotic vesicles across the cndothelial cytoplasm in either direction. Diapedesis is the process by which some Icukocytes pass from the blood into the tissues. It may involve opening of the junctions between endothclial cells by means of locally released substances, cg, histamine, which is involved in inflammation and increases vascular permcability.

B, Arteries: Arteries have a thicker tunica media than veins do. The media is best cxemplitied in medium-sized (muscular) arteries. Large (elastic) arteries contain more elastin in their media and adventitia than any other vessels. Arteries are also distinguished by refractile eosinophilic internal and external elastic laminac. In most tissues and organs, arteries are accompanied by veins. In cross sections through paired vessels, the arteries appear rounder than veins, with thicker walls and smaller lumens. For more detail, see Table 11-1.

C, Veins: In cross sections, veins often appear collapsed. They have thinner walls than arteries and are more likely to contain erythrocytes in their lumen in sectioncd tissue. They are charac tcrized by a thicker adventitia, which in larger veins may contain longitudinal smooth muscle. Veins contain valves that help maintain unidirectional blood Row. These extensions of the intima into the lumen of the vein are composed of a tibroelastic connective tissue core covered on both sides by a layer of endothelium. Since blood pressure is greatly diminished in veins, valves are needed to ensure blood flow back to the heart and to help prevent blood from pooling. Pooling can lead to clot formation and obstruct blood flow. For more detail, sec Table 112.

D. Portal Vessels: Portal vessels carry blood directly from one capillary (or sinusoidal) bed to another without first returning to the heart. Examples include the hepatic portal vein between the intestines and the liver, the hypophyseal portal veins in the pituitary gland, and the efferent arterioles of the renal cortex.

E. Carotid and Aortic Bodies: These unencapsulated chemoreceptors comprise clumps and cords of epithelioid cells permeated by fenestrated and sinusoidal capillaries. Carotid bodies lie at the bifurcation of the common carotid artery. The left aortic body is in the wall of the aorta, near the origin of the subclavian artery. The right aortic body is in the angle between the common carotid and subclavian. Changes in blood oxygen, CO2 or pH levels generate nerve impulses in their rich supply of unmyelinated nerve endings.'I'hese signals are carried to the brain by the glossopharyngeal nerve and elicit the physiologic response appropriate to maintain ing homeostasis.

F, Carotid Sinus: This unencapsulated mcchanoreceptor at the hifurcation of the common carotid consists of a dilation of the arterial lumen (sinus) and a thinned media, whose outer portion contains many large nerve endings. The sinus acts as a baroreceptor, responding to increased blood pressure by generating impulses that are carried by the glossopharyngeal nerve to the brain, where they elicit peripheral vasodilation and reflexive slowing of the heart.

G, Arteriovenous Anastomoses: These are direct connections between arteries and veins that regulate blood flow by smobth muscle contraction. When they are open, more blood passes directly from the artcrial circulation to the venous circulation, bypassing the capillary bed. Complex anastomoses between arterioles and venules, called glomera, occur mainly in the finger pads, nail bcd, and ears. The arterioles of glomera lack an internal elastic larnina and have more smooth muscle in their media, which, on contraction, can completely or partially close the vessels. Arteriovenous anastomoses allow efficient management of blood distribution during stress, heavy exertion, and temperature changes. They are also important in regulating blood pressure and other physiologic processes such as erection and menstruation.

H. Blood and Nerve Supply to Blood Vessels: Oxygen, nutrients, and wastes cannot reach all cells in the walls of large arteries and veins by simple diffusion from the lumen. The vasa vasorum ("vessels of` the vessels") form a capillary network to distribute blood to cells in the walls of these vessels. The walls of all blood vessels except capillaries and some venulcs contain a rich nerve supply. Unmyelinated vasomotor fibers (sympathetic fibers) arise in the sympa thetic ganglia, ramify in the adventitia, and terminate in small knohlike endings in the media. They stimulate smooth muscle contraction. Arteries usually contain more of these. Small intra adventitial ganglia are fhund in the aorta and some other large arteries. Myelinated fihers occur in bundles in the advcntitia. Their unmyelinated (free) nerve endings appear to be sensory Many terminate in the adventitia; some extend to the intima.

III. HEART

A. Chambers: The heart has 4 chambers: 2 atria, thinner-walled chambers located at the base (top) of the heart, which collect returning blood; and 2 ventricles, thicker-walled chambers located in the body and apex of the heart. See section IV Lor a description of the route of the blood through these chambers.

B. Tunics: The walls of the heart have 3 layers or tunics.

1. The endocardium (inner layer) has the same basic structure as the intima of the vessels and has 3 major components. The innermost layer is the endothelium, underlain by a thin, continuous basal lamina. Surrounding this is a layer of subendothelial connective tissue with elastic fibers and some smooth muscle cells. The subendocardium is a layer of areolar tissue with small blood vessels, nerves, and, in the ventricles, branches of the impulse conducting system (bundle branches and Purkinje fibers) (III.E).
2. The myocardium is the middle layer. This layer consists mainly of cardiac muscle tibcrs and carries out the forceful contractions that allow the heart to serve as a pump. It is homologous to the much thinner media of vessels. It contains the impulse-conducting system and parts of the cardiac skeleton (II1.C). Each cardiac muscle fiber is surrounded by an cndornysium. and each fascicle of fibers is surrounded by perimysium. The muscles in the atria and ventricles differ in some important respects. a. Atrial cardiac muscle. Muscle in the atrial myocardium is arranged in overlapping networks, giving the inner surface of the atria the appearance of woven bundles of muscle (musculi pectinati). Muscle cells in the outer myocardium form a complex helical pattern around the chamber-, resembling the arrangement in the ventricles. Collagen and elastic fibers are interspersed among the muscle cells. Compared with ventricular cardiac muscle, atrial cells (I)are somewhat smaller, (2) have many granules containing atrial natriuretic factor, (3) have a less extensive T-tubule system, (4) have more gap junc tions, (5) conduct impulses at 3 higher rate, and (6) contract more rhythmically. b, Ventricular cardiac muscle, Muscle in the ventricular myocardium forms complex layers of cells wound helically around the ventricular cavity. This aids in "wringing out" the heart during contraction, which increases the percentage of blood in the cavity that is expelled during each contraction. The superficial muscle layers surround both ventricles, whereas the deeper muscle layers surround each ventricle and contribute to the inter ventricular septum. There may also be differences in metabolic activity between the cells of these inner and outer layers. Elastic connective tissue is less abundant in ventricular than atrial myocardium. 3. The epicardium, or visceral pericurdium. is the outermost tunic. While occupying the same relative position as the tunica advcntitia, it is a serosa rather than an adventitia. It consists of a single layer of squamous mcsothelial cells, a thin basal lamina, and a layer of subepicardiai connective (areolar) tissue that binds the epicardium to the myocardium. The smooth meso thelial surface reduces the friction, generated during contraction, between the heart and the surrounding structures.

C. Cardiac Skeleton: The dense fibrous connective tissue scaffolding into which the cardiac muscle fibers insert and from which the cores of the cardiac valves extend is the cardiac skeleton, or fibrous skeleton of the heart. It has 3 major groups of components. The annuli fihrosae are rings of dense connective tissue that surround and rcinfhrce the valve openings in the atriovcntricular canals and at the origins of the aorta and pulmonary artery. The trigona fibrosae are 2 triangular masses of dense connective tissue, occasionally containing some cartilage, that lie between the 2 groups of annuli tibrosae. The septum membranaceum is a dense tibrous plate that forms the superior portion of the otherwise muscular interventricular septum. Together with the arrangement of the muscle fibers, the fibrous skeleton directs the force of myocardial contraction so that the heart "wrings out" the blood in its chambers. Portions of the skeleton may become calcified during disease and aging.

D. Cardiac Valves: These control the direction of blood flow through the heart. Each is a fold of. endocardium enclosing a platelike core of dense connective tissue that is anchored in, and continuous with, the annuli fibrosae. The tricuspid valve, located between the right atriurn and ventricle, has 3 cusps (flaps). The free edge of each cusp is anchored to papillary muscles in the floor of each ventricle by fibrous cords called chordae tendinae, The bicuspid or mitral valve, located between the left atrium and ventricle, has 2 cusps, each anchored by chordae tendinac to papillary muscles in the ventricle floor. ?'he semilunar valves, each composed of 3 semilunar cusps, are not attached by chordae tendinae. Each has a characteristic thickening (nodule) at the center of its free edge. The 2 scmilunar valves are the aortic valve, between the left ventricle and the aorta, and the pulmonary valve, between the right ventricle and the pulmonary artery.

E. Impulse-Generating and Conducting System: This system comprises unusual cardiac muscle cells specialized for the initiation and conduction of electrochemical impulses. The distribution of these cells allows the impulses they carry to coordinate the contraction of the myocardium surrounding the chambers of the heart.

1. The sinoatrial (SA) node, or pacemaker node, is a small cell mass in the median wall of the right atrium, near the opening of the superior vena cava. All cardiac muscle cells contract spontaneously; the cells with the fastest intrinsic rhythm generate impulses that lead the surrounding cells to contract faster. Since the cells of the SA node have the fastest intrinsic rhythm, they serve as the pacemaker for the rest of the heart. Autonomic nerve fibers and ganglia located near the SA node do not directly dictate heart rhythm, but can modulate the heart rate. Impulses generated in the SA node travel rather slowly through ordinary atrial cardiac muscle cells to the atrioventricular node. This slow conduction allows the atria to complete their contraction before the ventricles begin theirs.
2. The atrioventricular (AV) node is a cluster of cells located on the right side of the interatrial septum. As the impulse leaves the AV node, it passes directly and rapidly along the atrio- ventricular bundle.
3. The atrioventricular (AV) bundle (of His), is a bundle of specialized cardiac muscle fibers, approximately 15 mm long and 2-3 mm wide, that passes from the interatrial septum into the interventricular scptum. It terminates by giving off a smaller bundle (bundle branch) to each ventricle.
4. The right and left bundle branches travel a short distance before branching further to form the Purkinje fibers
5. Purkinje fibers are cardiac muscle cells specialized to conduct electrochemical impulses. They are wider than typical cardiac muscle cells and contain sparse myofilaments that are concentrated at the cell periphery. They are generally wider than the cells in bundle branches and, like typical cardiac muscle cells, may have one or 2 central nuclei and are connected by intercalated disks. The impulses are transmitted through gap junctions between the Purkinje fibers and the cardiac muscle cells they contact.
6. Ventricular cardiac muscle cells are the last link in the impulse conduction chain. They not only contract in response to the impulse, but also can propagate (albeit more slowly) the impulses they receive from Purkinje fibers and pass them on to their neighbors. Thus, the cardiac musculature functions effectively as a syncytium, its cells contracting as one in a synchronous, coordinated manner.

F. Blood Supply to the Heart: The coronary arteries arise near the origin of the aorta and supply oxygen-rich blood to the myocardium. Blockage of a coronary vessel or its branches by a thrombus or atherosclerotic plaques (fatty deposits in the media and intima) may rob the tissue supplied by the vessel of oxygen and nutrients. This ischemia can lead to localized tissue necrosis, called an infarction, Tissues with high energy and oxygen demands, such as the brain and myocardium, are particularly susceptible to infarction. The density of capillaries in cardiac muscle is even greater than in skeletal muscle and is a diagnostic feature of this tissue in histologic section. Most of the venous blood returns through the coronary sinus to the superior vena cava as it enters the heart.

G. Lymphatics of the Heart: The myocardium contains abundant lymphatic capillaries. These begin as blind-ending tubes in the myocardium (near the endocardium) and drain into larger lymphatic vessels in the epicardial connective tissue.

H. Innervation of the Heart: Many myelinated and unmyelinated autonomic motor fibers (both sympathetic and parasympathetic) enter the base (top) of the heart and ramify, forming plexuses and innervating several ganglia. Although there are no myoneural junctions in the heart, the autonomic nervous system can adjust the heart rate to meet changing demands by various organs and tissues. Generally, sympathetic stimulation increases and parasympathetic stimulation decreases the heart rate.

IV. ROUTE OF THE BLOOD

The route taken by the blood through the cardiovascular system may be summarized as follows. Venous blood returns to the heart via the superior and inferior venae cava. It enters the right atrium, which contracts and forces blood through the tricuspid valve into the right ventricle. Contraction of the right ventricle forces blood through the pulmonary (semilunar) valve into the pulmonary artery, through which it reaches the capillaries surrounding the alveoli in the lungs. Here, the blood picks up oxygen and releases carbon dioxide and other volatile wastes. Newly oxygenated blood is collected in the pulmonary veins and carried to the left atrium, which contracts to force it through the bicuspid (mitral) valve and into the left ventricle. The left ventricle then contracts, forcing blood through the aortic (semilunar) valve and into the aorta (a large elastic artery) for distribution to the body. The aorta gives oft, numerous branches (distributing arteries) through which blood passes to arteries of successively smaller diameters (muscular arteries, arterioles) until it reaches the capillary beds, where it releases its oxygen and nutrients to the tissues and picks up carbon dioxide and other metabolic by-products. Some fluid also escapes from the capillaries into intercellular tissue spaces; part of this excess tissue fluid returns to the capillary lumen before the blood leaves the tissue. The blood in the capillary bed enters the venules and then veins of increasing diameters (medium-sized veins, large veins), finally returning to the heart through the largest veins, the superior and inferior venae cava.

V. LYMPHATIC VESSELS

A. Lymphatic Vessels and Ducts: These have walls that resemble those of veins. The beaded appearance of lymphatic ducts and vessels reflects the presence of valves that control the direction of lymph flow. The adventitiais thin and lacks smooth muscle. The media contains both longitudinal and circular smooth muscle, but longitudinal fibers predominate.

B. Lymphatic Capillaries: These resemble blood capillaries in that they are simple squamous endothelial tubes. They differ from blood capillaries in that they have a larger diameter (up to 100 IJ-m) and a thinner basal lamina. They lack fenestrations and have fewer tight junctions (zonulae occludentes) than blood capillaries.

C. Route of the Lymph: The route taken by the lymph is unidirectional. Excess tissue fluid not returned to the blood capillaries (IV) is collected by blind-ending lymphatic capillaries in the region of the blood capillary beds and carried through lymphatic vessels to lymphatic ducts. There is one major lymphatic duct for each side of the body, the thoracic duct on the left and the right lymphatic duct on the right. The lymphatic ducts return lymph to the blood by emptying into the venous system at the junction of the jugular and subclavian veins in the neck. The lymphatic system is discussed further in Chapter 14.

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