CVS

Cardiovascular System Histology

The circulatory system pumps and directs blood cells and substances carried in blood to all tissues of the body. It includes both the blood and lymphatic vascular systems, and in an adult the total length of its vessels is estimated at between 100,000 and 150,000 kilometers. The blood vascular system , or cardiovascular system, consists of the following structures:
■ heart.
■ Arteries.
■ Capillaries.
■ Veins.
HEART
Cardiac muscle in the four chambers of the heart wall contracts rhythmically, pumping the blood through the circulatory system. The right and left ventricles propel blood to the pulmonary and systemic circulation, respectively; right and left atria receive blood from the body and the pulmonary veins, respectively. The walls of all four heart chambers consist of three major layers: the internal endocardium; the middle myocardium; and the external epicardium.
■ Endocardium, consists of a very thin inner layer of endothelium and supporting connective tissue, a middle myoelastic layer of smooth muscle fibers and connective tissue, and a deep layer of connective tissue called the subendocardial layer that merges with the myocardium, Branches of the heart’s impulse-conducting system, consisting of modified cardiac muscle fibers, are located in the subendocardial layer.
■ Myocardium, the thickest layer consists mainly of cardiac muscle with its fibers arranged spirally around each heart chamber. Because strong force is required to pump blood through the systemic and pulmonary circulations, the myocardium is much thicker in the walls of the ventricles, particularly the left, than in the atrial walls.
■ Epicardium, is a simple squamous mesothelium supported by a layer of loose connective tissue containing blood vessels and nerves. The epicardium corresponds to the visceral layer of the pericardium, the membrane surrounding the heart.
Where the large vessels enter and leave the heart, the epicardium is reflected back as the parietal layer lining the pericardium. During heart movements, underlying structures are cushioned by deposits of adipose tissue in the epicardium and friction within the pericardium is prevented by lubricant fluid produced by both layers of serous mesothelial cells.

Within these major layers the heart contains other structures important for its overall function of moving blood. Dense fibrous connective tissue of the cardiac skeleton forms part of the interventricular and interatrial septa, surrounds all valves of the heart, and extends into the valve cusps and the chordae tendineae to which they are attached. These regions of dense irregular connective tissue perform the following functions:
■Anchoring and supporting the heart valves.
■Providing firm points of insertion for cardiac muscle.
■Helping coordinate the heart beat by acting as electrical insulation between atria and ventricles.
Within the subendocardial layer and adjacent myocardium, modified cardiac muscle cells make up the impulse conducting system of the heart, which generates and propagates waves of depolarization that spread through the myocardium to stimulate rhythmic contractions, This system consists of two nodes of specialized myocardial tissue in the right atrium:


1- Sinoatrial (SA) node (or pacemaker), Located in the right atrial wall near the superior vena cava, the SA node is a 6- to 7-mm3 mass of cardiac muscle cells with smaller size, fewer myofibrils, and fewer typical intercalated disks than the neighboring muscle fibers.
2- Atrioventricular (AV) node.
3- AV bundle (of His).
4- Subendocardial conducting network.
At the apex of the heart, the bundles branch further into a subendocardial conducting network of myofibers, usually called Purkinje fibers. These are pale-staining fibers, larger than the adjacent contractile muscle fibers, with sparse, peripheral myofibrils and much glycogen.
TISSUES OF THE VASCULAR WALL
Walls of all blood vessels except capillaries contain smooth muscle and connective tissue in addition to the endothelial lining.
The amount and arrangement of these tissues in vessels are influenced by mechanical factors, primarily blood pressure, and metabolic factors reflecting the local needs of tissues.
The Endothelium is a specialized single layer of a squamous epithelium that acts as a semipermeable barrier between two internal compartments: the blood plasma and the interstitial tissue fluid.
Vascular endothelial cells are squamous, polygonal, and elongated with the long axis in the direction of blood flow.
Endothelium with its basal lamina is highly differentiated to mediate and actively monitor the bidirectional exchange of molecules by simple and active diffusion, receptor mediated endocytosis, transcytosis, and other mechanisms.

Smooth muscle fibers occur in the walls of all vessels larger than capillaries and are arranged helically in layers. In arterioles and small arteries, the smooth muscle cells are connected by many more gap junctions and permit vasoconstriction and vasodilation which are of key importance in regulating the overall blood pressure.

Connective tissue components are present in vascular walls in variable amounts and proportions based on local functional requirements. Collagen fibers are found in the subendothelial layer, between the smooth muscle layers, and in the outer covering. Elastic fibers provide the resiliency required for the vascular wall to expand under pressure.
Elastin is a major component in large arteries where it forms parallel lamellae, regularly distributed between the muscle layers. Variations in the amount and composition of ground substance components such as proteoglycans and hyaluronate also contribute to the physical and metabolic properties of the wall in different vessels, especially affecting their permeability.
The walls of all blood vessels larger than the microvasculature have many components in common and similar organization.

Branching of the vessels helps produce reductions in their size which are accompanied by gradual changes in the composition of the vascular wall. Transitions such as those from “small arteries” to “arterioles” are not clear-cut. However, all of these larger vessels have walls with three concentric layers, or tunics (L. tunica, coat).
■ The innermost tunica intima consists of the endothelium and a thin subendothelial layer of loose connective tissue sometimes containing smooth muscle fibers. In arteries and large veins, the intima includes a prominent limiting layer, the internal elastic lamina, composed of elastin, with holes allowing better diffusion of substances from blood deeper into the wall.
■ The tunica media, the middle layer, consists chiefly of concentric layers of helically arranged smooth muscle cells. Interposed among the muscle fibers are variable amounts of elastic fibers and elastic lamellae, reticular fibers, and proteoglycans, all of which are produced by the smooth muscle cells. In arteries, the media may have a thin external elastic lamina, separating it from the outermost tunic.
■ The outer adventitia, or tunica externa, consists principally of type I collagen and elastic fibers. The adventitia is continuous with and bound to the stromal connective tissue of the organ through which the blood vessel runs.
Just as the heart wall is supplied with its own coronary vasculature for nutrients and O2, large vessels usually have vasa vasorum (“vessels of the vessel”): arterioles, capillaries, and venules in the adventitia and outer part of the media. The vasa vasorum are required to provide metabolites to cells in those tunics in larger vessels because the wall is too thick to be nourished solely by diffusion from the blood in the lumen. Luminal blood alone does provide the needs of cells in the intima. Because they carry deoxygenated blood, large veins commonly have more vasa vasorum than arteries.
The adventitia of larger vessels also contains a network of unmyelinated autonomic nerve fibers, the vasomotor nerves, which release the vasoconstrictor norepinephrine. The density of this innervation is greater in arteries than in veins.


VASCULATURE
Large blood vessels and those of the microvasculature branch frequently and undergo gradual transitions into structures with different histologic features and functions.
Elastic Arteries
Elastic arteries are the aorta, the pulmonary artery, and their largest branches; these large vessels are also called conducting arteries because their major role is to carry blood to smaller arteries. the most prominent feature of elastic arteries is the thick media in which elastic lamellae, each about 10 μm thick, alternate with layers of smooth muscle fibers. The adult aorta has about 50 elastic lamellae (more if the individual is hypertensive).
The intima is well developed, with many smooth muscle cells in the subendothelial connective tissue, and often shows folds in cross section as a result of the loss of blood pressure and contraction of the vessel at death. The internal elastic lamina is not easily discerned because it is similar to the elastic laminae of the next layer. The adventitia is much thinner than the media.

Muscular Arteries

The muscular arteries distribute blood to the organs and help regulate blood pressure by contracting or relaxing the smooth muscle in the media. The intima has a very thin subendothelial layer and a prominent internal elastic lamina. The media may contain up to 40 layers of large smooth muscle cells interspersed with a variable number of elastic lamellae (depending on the size of the vessel). An external elastic lamina, the last component of the media, is present only in the larger muscular arteries. The adventitia consists of connective tissue. Lymphatic capillaries, vasa vasorum, and nerves are also found in the adventitia, and these structures may penetrate to the outer part of the media.

Arterioles

Muscular arteries branch repeatedly into smaller and smaller arteries, until reaching a size with three or four medial layers of smooth muscle. The smallest arteries branch as arterioles, which have only one or two smooth muscle layers; these indicate the beginning of an organ’s microvasculature ( where exchanges between blood and tissue fluid occur. Arterioles are generally less than 0.1 mm in diameter, with lumens approximately as wide as the wall is thick. The subendothelial layer is very thin, elastic laminae are absent, and the media consists of the circularly arranged smooth muscle cells. In both small arteries and arterioles, the adventitia is very thin and inconspicuous.
Arterioles almost always branch to form anastomosing networks or beds of capillaries that surround the parenchymal cells of the organ. Smooth muscle fibers act as sphincters closing arterioles and producing periodic blood flow into capillaries . Acting as “resistance vessels,” muscle tone usually keeps arterioles partially closed and makes these vessels the major determinants of systemic blood pressure.

Capillary Beds

Capillaries permit and regulate metabolic exchange between blood and surrounding tissues. These smallest blood vessels always function in groups called capillary beds, whose size and overall shape conforms to that of the structure supplied. The richness of the capillary network is related to the metabolic activity of the tissues. Tissues with high metabolic rates, such as the kidney, liver, and cardiac and skeletal muscle, have an abundant capillary network; the opposite is true of tissues with low metabolic rates, such as smooth muscle and dense connective tissue.
Capillary beds are supplied preferentially by one or more terminal arteriole branches called metarterioles, which are continuous with thoroughfare channels connected with the postcapillary venules. True capillaries branch from the metarterioles, which are encircled by scattered smooth muscle cells, and converge into the thoroughfare channels, which lack muscle. At the beginning of each true capillary, muscle fibers act as precapillary sphincters that contract or relax to control the entry of blood. These sphincters contract and relax cyclically, with 5 to 10 cycles per minute, causing blood to pass through capillaries in a pulsatile manner. When the sphincters are closed, blood flows directly from the metarterioles and thoroughfare channels into postcapillary venules.
Capillaries are composed of a single layer of endothelial cells rolled up as a tube . The average diameter of capillaries varies from 4 to 10 μm, which allows transit of blood cells only one at a time, and their individual length is usually not more than 50 μm. These minute vessels make up over 90% of the body’s vasculature, with a total length of more than 100,000 km and a total surface area of approximately 5000 m2. Because of the cyclical opening and closing of the sphincters, most capillaries are essentially empty at any given time, with only about 5% (~300 mL in an adult) of the total blood volume moving through these structures. Their thin walls, extensive surface area, and slow, pulsatile blood flow optimize capillaries for the exchange of water and solutes between blood and tissues.

Capillaries are generally grouped into three histologic types, depending on the continuity of the endothelial cells and the external lamina .
■ Continuous capillaries: have many tight, well-developed occluding junctions between slightly overlapping endothelial cells, which provide for continuity along the endothelium and well-regulated metabolic exchange across the cells. This is the most common type of capillary and is found in muscle, connective tissue, lungs, exocrine glands, and nervous tissue. Ultrastructural studies show numerous vesicles indicating transcytosis of macromolecules in both directions across the endothelial cell cytoplasm.
■ Fenestrated capillaries: have a sieve like structure that allows more extensive molecular exchange across the endothelium. The endothelial cells are penetrated by numerous small circular openings or fenestrations (L. fenestra, perforation), approximately 80 nm in diameter. Some fenestrations are covered by very thin diaphragms of proteoglycans; others may represent membrane invaginations during transcytosis that temporarily involve both sides of the very thin cells. The basal lamina is continuous and covers the fenestrations. Fenestrated capillaries are found in organs with rapid interchange of substances between tissues and the blood, such as the kidneys, intestine, choroid plexus, and endocrine glands.
■ Discontinuous capillaries, commonly called sinusoids, permit maximal exchange of macromolecules as well as allow easier movement of cells between tissues and blood. Individual endothelial cells here have large perforations without diaphragms; collectively they form a discontinuous layer, with wide, irregular spaces between the cells. Sinusoids also differ from other capillaries by having highly discontinuous basal laminae and much larger diameters, often 30 to 40 μm, which slows blood flow. Sinusoidal capillaries are found in the liver, spleen, some endocrine organs, and bone marrow.
At various locations along continuous capillaries and postcapillary venules are mesenchymal cells called pericytes (Gr. peri, around + kytos, cell), with long cytoplasmic processes partly surrounding the endothelial layer. Pericytes produce their own basal lamina, which may fuse with that of the endothelial cells. Well-developed networks of myosin, actin, and tropomyosin in pericytes indicate these cells’ primary contractile function to facilitate flow of blood cells. After tissue injuries, pericytes proliferate and differentiate to form smooth muscle and other cells in new vessels as the microvasculature is reestablished.


Venules
The transition from capillaries to venules occurs gradually. The immediate postcapillary venules are similar structurally to capillaries with pericytes, but range in diameter from 15 to 20 μm.
postcapillary venules are the primary site at which white blood cells adhere to endothelium and leave the circulation at sites of infection or tissue damage.
Postcapillary venules converge into larger collecting venules that have more contractile cells. With even greater size, the venules become surrounded by a recognizable tunica media with two or three smooth muscle layers and are called muscular venules. A characteristic feature of all venules is the large diameter of the lumen compared to the overall thinness of the wall.

Veins

Veins carry blood back to the heart from microvasculature all over the body. Blood entering veins is under very low pressure and moves toward the heart by contraction of smooth muscle fibers in the media and by external compressions from surrounding muscles and other organs. Valves project from the tunica intima to prevent backflow of blood. Most veins are small or medium veins, with diameters of 10 mm or less. Such veins are usually located close and parallel to corresponding muscular arteries. The intima usually has a thin subendothelial layer, and the media consists of small bundles of smooth muscle cells intermixed with reticular fibers and a delicate network of elastic fibers.
The collagenous adventitial layer is well developed. The big venous trunks, paired with elastic arteries close to the heart, are the large veins. Large veins have a well-developed intima, but the media is relatively thin, with alternating layers of smooth muscle and connective tissue. The adventitial layer is thicker than the media in large veins and frequently contains longitudinal bundles of smooth muscle. Both the media and adventitia contain elastic fibers, but internal and external elastic laminae like those of arteries are not present.
Medium and large veins have valves consisting of paired folds of the intima projecting across the lumen. They are rich in elastic fibers and are lined on both sides by endothelium. The valves, which are especially numerous in veins of the legs, help keep the flow of venous blood directed toward the heart.