Functional anatomy of the heart

Introduction

THE VASCULAR system is divided for descriptive purposes into (a) the blood vascular system, which comprises the heart and bloodvessels for the circulation of the blood; and (b) the lymph vascular system, consisting of lymph glands and lymphatic vessels, through which a colorless fluid, the lymph, circulates. It must be noted, however, that the two systems communicate with each other and are intimately associated developmentally.

The heart is the central organ of the blood vascular system, and consists of a hollow muscle; by its contraction the blood is pumped to all parts of the body through a complicated series of tubes, termed arteries. The arteries undergo enormous ramification in their course throughout the body, and end in minute vessels, called arterioles, which in their turn open into a close-meshed network of microscopic vessels, termed capillaries. After the blood has passed through the capillaries it is collected into a series of larger vessels, called veins, by which it is returned to the heart. The passage of the blood through the heart and blood-vessels constitutes what is termed the circulation of the blood, of which the following is an outline.

The human heart is divided by septa into right and left halves, and each half is further divided into two cavities, an upper termed the atrium and a lower the ventricle. The heart therefore consists of four chambers, two, the right atrium and right ventricle, forming the right half, and two, the left atrium and left ventricle the left half. The right half of the heart contains venous or impure blood; the left, arterial or pure blood. The atria are receiving chambers, and the ventricles distributing ones. From the cavity of the left ventricle the pure blood is carried into a large artery, the aorta, through the numerous branches of which it is distributed to all parts of the body, with the exception of the lungs. In its passage through the capillaries of the body the blood gives up to the tissues the materials necessary for their growth and nourishment, and at the same time receives from the tissues the waste products resulting from their metabolism. In doing so it is changed from arterial into venous blood, which is collected by the veins and through them returned to the right atrium of the heart. From this cavity the impure blood passes into the right ventricle, and is thence conveyed through the pulmonary arteries to the lungs. In the capillaries of the lungs it again becomes arterialized, and is then carried to the left atrium by the pulmonary veins. From the left atrium it passes into the left ventricle, from which the cycle once more begins.

The course of the blood from the left ventricle through the body generally to the right side of the heart constitutes the greater or systemic circulation, while its passage from the right ventricle through the lungs to the left side of the heart is termed the lesser or pulmonary circulation.

It is necessary, however, to state that the blood which circulates through the spleen, pancreas, stomach, small intestine, and the greater part of the large intestine is not returned directly from these organs to the heart, but is conveyed by the portal vein to the liver. In the liver this vein divides, like an artery, and ultimately ends in capillary-like vessels (sinusoids), from which the rootlets of a series of veins, called the hepatic veins, arise; these carry the blood into the inferior vena cava, whence it is conveyed to the right atrium. From this it will be seen that the blood contained in the portal vein passes through two sets of vessels: (1) the capillaries in the spleen, pancreas, stomach, etc., and (2) the sinusoids in the liver. The blood in the portal vein carries certain of the products of digestion: the carbohydrates, which are mostly taken up by the liver cells and stored as glycogen, and the protein products which remain in solution and are carried into the general circulation to the various tissues and organs of the body.

Speaking generally, the arteries may be said to contain pure and the veins impure blood. This is true of the systemic, but not of the pulmonary vessels, since it has been seen that the impure blood is conveyed from the heart to the lungs by the pulmonary arteries, and the pure blood returned from the lungs to the heart by the pulmonary veins. Arteries, therefore, must be defined as vessels which convey blood from the heart, and veins as vessels which return blood to the heart.

The HEART is a hollow muscular organ, which is situated in thoracic cavity in middle mediastinum. It has a heart apex, which is directed down to the left and heart base. Heart has a sternocostal (anterior) surface, diaphragmatic (posterior) surface, right/left pulmonary surfaces. Coronal sulcus passes on diaphragmatic and partially on sternоcostal surfaces, which marks the border between ventricles and atriums. Anterior interventricular sulcus and posterior interventricular sulcus pass from coronal sulcus downward and project borders between right and left ventricles. On heart base right and left auricles are situated, which envelop the great vessels. On heart base at the anterior from right ventricle pulmonary trunk passes, which subdivides into two pulmonary arteries. Aorta passes behind pulmonary trunk; behind from aorta from right side superior vena cava and inferior vena cava, and to the left four pulmonary veins. Heart cavity subdivides on right and left atriums and right and left ventricles. Left chambers of heart are arterial and in adult do not communicate with right venous half of heart. Exist two blood circles. Big circle or systemic circulation of the blood starts in left ventricle by aorta and terminates in right atrium by vena cava superior and inferior. Systemic circulation of the blood provides by arterial blood all of organs and tissues. The small circle or pulmonary circulation of the blood begins by pulmonary trunk from right ventricle and terminates in left atrium by 4 pulmonary veins. Venous blood flows in arteries of pulmonary circulation of which and arterial (oxygenated) blood – in veins.

Right atrium consists of own atrium and right auricle.

Internal wall is smooth, but in auricle pectinate muscles are situated. Right atrium receives the superior and inferior venae cavae, which open by foramen of inferior vena cava and foramen of superior vena cava. Intervensus tubercle is situated between these foramens. Broadened posterior area, where two venae cavae fall is called as sinus venae cavae. Right atrium is separated from left by interatrial septum, where oval fossa is situated. It is limited by limbus of ovalfossa. Atrium communicates by right ventricle through the right atrioventricular ostium. Foramen of coronal sinus situated between last and foramen of inferior vena cava. Alongside are contained foramens of venarum minimarum.

Right ventricle consists of own ventricle and conus arteriosus – superior part, which continues through the ostium of pulmonary trunk into pulmonary trunk. The right and left ventricles are separated by interventricular septum, which has muscular part (greater) and membranous part (lesser). On internal surface of right ventricle are situated the trabeculi carneae, which carry cone-shaped anterior, posterior and septal pappillar muscles. From top of these muscles chordae tendineae start and terminate at cusps of right atrioventricular valve.

Right atrioventricular ostium closes by right atrioventricular (tricuspidal) valve, which consists of anterior cusp, posterior cusp and septal cusp edges of which attach to chordae tendineae. During contraction of atria blood stream presses the cusps to the wall of ventricle. During contraction of ventricles free edges of cusps close up but do not pull out because they are kept by chordae tendineae from ventricle. Ostium of pulmonary trunk closes by valve of pulmonary trunk, which consists of right, left and anterior semilunar valvulae, which have on superior margin the nodules of semilunar valvulae. Nodules assist to compact closing up. Between each semilunar valvula and pulmonary trunk wall sinuses of pulmonary trunk are situated.

Left atrium has an irregular cube shape; anterior wall forms a left auricle. Internal wall surfaces of left atrium is smooth and only in auricle area pectinate muscles are situated. The ostia of 4 pulmonary veins open into left atrium. Left atrium communicates with left ventricle by the means of left atrioventricular ostium. Oval fossa makes a mark poorly on interatrial septum.

Left ventricle is the largest heart chamber, its wall forms larger part of diaphragmatic surface. Internal surface containes the trabeculi carneae, which attach anterior papillary muscle and posterior papillary muscle. The tops of these muscles by means of cordae tendineae hold the cusps of mitral valve.

Left atrioventricular ostium closes by left atrioventricular (bicuspidal) valve [valve mitralis], which consists of anterior cusp and posterior cusp edges of which attach to chordae tendineae. From left ventricle aorta starts. Aortic ostium closes by aortic valve, which consists of right, left and posterior semilunar valvulae, which have on superior margin the nodules of semilunar valvulae. Between each semilunar valvula and aorta walls are situated aortic sinuses.

THE DISTRIBUTION of the systematic arteries is like a highly ramified tree, the common trunk of which, formed by the aorta, commences at the left ventricle, while the smallest ramifications extend to the peripheral parts of the body and the contained organs. Arteries are found in all parts of the body, except in the hairs, nails, epidermis, cartilages, and cornea; the larger trunks usually occupy the most protected situations, running, in the limbs, along the flexor surface, where they are less exposed to injury.

  There is considerable variation in the mode of division of the arteries: occasionally a short trunk subdivides into several branches at the same point, as may be observed in the celiac artery and the thyrocervical trunk: the vessel may give off several branches in succession, and still continue as the main trunk, as is seen in the arteries of the limbs; or the division may be dichotomous, as, for instance, when the aorta divides into the two common iliacs.

  A branch of an artery is smaller than the trunk from which it arises; but if an artery divides into two branches, the combined sectional area of the two vessels is, in nearly every instance, somewhat greater than that of the trunk; and the combined sectional area of all the arterial branches greatly exceeds that of the aorta; so that the arteries collectively may be regarded as a cone, the apex of which corresponds to the aorta, and the base to the capillary system.

  The arteries, in their distribution, communicate with one another, forming what are called anastomoses, and these communications are very free between the large as well as between the smaller branches. The anastomosis between trunks of equal size is found where great activity of the circulation is requisite, as in the brain; here the two vertebral arteries unite to form the basilar, and the two anterior cerebral arteries are connected by a short communicating trunk; it is also found in the abdomen, where the intestinal arteries have very ample anastomoses between their larger branches. In the limbs the anastomoses are most numerous and of largest size around the joints, the branches of an artery above uniting with branches from the vessels below. These anastomoses are of considerable interest to the surgeon, as it is by their enlargement that a collateral circulation is established after the application of a ligature to an artery. The smaller branches of arteries anastomose more frequently than the larger; and between the smallest twigs these anastomoses become so numerous as to constitute a close network that pervades nearly every tissue of the body.

  Throughout the body generally the larger arterial branches pursue a fairly straight course, but in certain situations they are tortuous. Thus the external maxillary artery in its course over the face, and the arteries of the lips, are extremely tortuous to accommodate themselves to the movements of the parts. The uterine arteries are also tortuous, to accommodate themselves to the increase of size which the uterus undergoes during pregnancy.

The Pulmonary Artery (A. Pulmonalis)

  The pulmonary artery conveys the venous blood from the right ventricle of the heart to the lungs. It is a short, wide vessel, about 5 cm. in length and 3 cm. in diameter, arising from the conus arteriosus of the right ventricle. It extends obliquely upward and backward, passing at first in front and then to the left of the ascending aorta, as far as the under surface of the aortic arch, where it divides, about the level of the fibrocartilage between the fifth and sixth thoracic vertebræ, into right and left branches of nearly equal size.

Relations.—The whole of this vessel is contained within the pericardium. It is enclosed with the ascending aorta in a single tube of the visceral layer of the serous pericardium, which is continued upward upon them from the base of the heart. The fibrous layer of the pericardium is gradually lost upon the external coats of the two branches of the artery. In front, the pulmonary artery is separated from the anterior end of the second left intercostal space by the pleura and left lung, in addition to the pericardium; it rests at first upon the ascending aorta, and higher up lies in front of the left atrium on a plane posterior to the ascending aorta. On either side of its origin is the auricula of the corresponding atrium and a coronary artery, the left coronary artery passing, in the first part of its course, behind the vessel. The superficial part of the cardiac plexus lies above its bifurcation, between it and the arch of the aorta.

  The right branch of the pulmonary artery (ramus dexter a. pulmonalis), longer and larger than the left, runs horizontally to the right, behind the ascending aorta and superior vena cava and in front of the right bronchus, to the root of the right lung, where it divides into two branches. The lower and larger of these goes to the middle and lower lobes of the lung; the upper and smaller is distributed to the upper lobe.

  The left branch of the pulmonary artery (ramus sinister a. pulmonalis), shorter and somewhat smaller than the right, passes horizontally in front of the descending aorta and left bronchus to the root of the left lung, where it divides into two branches, one for each lobe of the lung. Above, it is connected to the concavity of the aortic arch by the ligamentum arteriosum, on the left of which is the left recurrent nerve, and on the right the superficial part of the cardiac plexus. Below, it is joined to the upper left pulmonary vein by the ligament of the left vena cava.

  The terminal branches of the pulmonary arteries will be described with the anatomy of the lungs.

Structure of Arteries—The arteries are composed of three coats: an internal or endothelial coat (tunica intima of Kölliker); a middle or muscular coat (tunica media); and an external or connective-tissue coat (tunica adventitia). The two inner coats together are very easily separated from the external, as by the ordinary operation of tying a ligature around an artery. If a fine string be tied forcibly upon an artery and then taken off, the external coat will be found undivided, but the two inner coats are divided in the track of the ligature and can easily be further dissected from the outer coat.

The arteries, in their distribution throughout the body, are included in thin fibro-areolar investments, which form their sheaths. The vessel is loosely connected with its sheath by delicate areolar tissue; and the sheath usually encloses the accompanying veins, and sometimes a nerve. Some arteries, as those in the cranium, are not included in sheaths.

All the larger arteries, like the other organs of the body, are supplied with bloodvessels. These nutrient vessels, called the vasa vasorum, arise from a branch of the artery, or from a neighboring vessel, at some considerable distance from the point at which they are distributed; they ramify in the loose areolar tissue connecting the artery with its sheath, and are distributed to the external coat, but do not, in man, penetrate the other coats; in some of the larger mammals a few vessels have been traced into the middle coat. Minute veins return the blood from these vessels; they empty themselves into the vein or veins accompanying the artery. Lymphatic vessels are also present in the outer coat.

Arteries are also supplied with nerves, which are derived from the sympathetic, but may pass through the cerebrospinal nerves. They form intricate plexuses upon the surfaces of the larger trunks, and run along the smaller arteries as single filaments, or bundles of filaments which twist around the vessel and unite with each other in a plexiform manner. The branches derived from these plexuses penetrate the external coat and are distributed principally to the muscular tissue of the middle coat, and thus regulate, by causing the contraction and relaxation of this tissue the amount of blood sent to any part.

The Capillaries.—The smaller arterial branches (excepting those of the cavernous structure of the sexual organs, of the splenic pulp, and of the placenta) terminate in net-works of vessels which pervade nearly every tissue of the body. These vessels, from their minute size, are termed capillaries. They are interposed between the smallest branches of the arteries and the commencing veins, constituting a net-work, the branches of which maintain the same diameter throughout; the meshes of the net-work are more uniform in shape and size than those formed by the anastomoses of the small arteries and veins.

The diameters of the capillaries vary in the different tissues of the body, the usual size being about 8μ. The smallest are those of the brain and the mucous membrane of the intestines; and the largest those of the skin and the marrow of bone, where they are stated to be as large as 20μ in diameter. The form of the capillary net varies in the different tissues, the meshes being generally rounded or elongated.

The rounded form of mesh is most common, and prevails where there is a dense network, as in the lungs, in most glands and mucous membranes, and in the cutis; the meshes are not of an absolutely circular outline, but more or less angular, sometimes nearly quadrangular, or polygonal, or more often irregular.

Elongated meshes are observed in the muscles and nerves, the meshes resembling parallelograms in form, the long axis of the mesh running parallel with the long axis of the nerve or muscle. Sometimes the capillaries have a looped arrangement; a single vessel projecting from the common net-work and returning after forming one or more loops, as in the papillæ of the tongue and skin.

The number of the capillaries and the size of the meshes determine the degree of vascularity of a part. The closest network and the smallest interspaces are found in the lungs and in the choroid coat of the eye. In these situations the interspaces are smaller than the capillary vessels themselves. In the intertubular plexus of the kidney, in the conjunctiva, and in the cutis, the interspaces are from three to four times as large as the capillaries which form them; and in the brain from eight to ten times as large as the capillaries in their long diameters, and from four to six times as large in their transverse diameters. In the adventitia of arteries the width of the meshes is ten times that of the capillary vessels. As a general rule, the more active the function of the organ, the closer is its capillary net and the larger its supply of blood; the meshes of the network are very narrow in all growing parts, in the glands, and in the mucous membranes, wider in bones and ligaments which are comparatively inactive; bloodvessels are nearly altogether absent in tendons, in which very little organic change occurs after their formation. In the liver the capillaries take a more or less radial course toward the intralobular vein, and their walls are incomplete, so that the blood comes into direct contact with the liver cells. These vessels in the liver are not true capillaries but “sinusoids;” they are developed by the growth of columns of liver cells into the blood spaces of the embryonic organ.

Structure.—The wall of a capillary consists of a fine transparent endothelial layer, composed of cells joined edge to edge by an interstitial cement substance, and continuous with the endothelial cells which line the arteries and veins. When stained with nitrate of silver the edges which bound the epithelial cells are brought into view. These cells are of large size and of an irregular polygonal or lanceolate shape, each containing an oval nucleus which may be displayed by carmine or hematoxylin. Between their edges, at various points of their meeting, roundish dark spots are sometimes seen, which have been described as stomata, though they are closed by intercellular substance. They have been believed to be the situations through which the colorless corpuscles of the blood, when migrating from the bloodvessels, emerge; but this view, though probable, is not universally accepted.

Kolossow describes these cells as having a rather more complex structure. He states that each consists of two parts: of hyaline ground plates, and of a protoplasmic granular part, in which is imbedded the nucleus, on the outside of the ground plates. The hyaline internal coat of the capillaries does not form a complete membrane, but consists of “plates” which are inelastic, and though in contact with each other are not continuous; when therefore the capillaries are subjected to intravascular pressure, the plates become separated from each other; the protoplasmic portions of the cells, on the other hand, are united together. In some organs, e. g., the glomeruli of the kidneys, intercellular cement cannot be demonstrated in the capillary wall and the cells are believed to form a syncytium.

In many situations a delicate sheath or envelope of branched nucleated connective tissue cells is found around the simple capillary tube, particularly in the larger ones; and in other places, especially in the glands, the capillaries are invested with retiform connective tissue.

Sinusoids.—In certain organs, viz., the heart, the liver, the suprarenal and parathyroid glands, the glomus caroticum and glomus coccygeum, the smallest bloodvessels present various differences from true capillaries. They are wider, with an irregular lumen, and have no connective tissue covering, their endothelial cells being in direct contact with the cells of the organ. Moreover, they are either arterial or venous and not intermediate as are the true capillaries. These vessels have been called sinusoids by Minot. They are formed by columns of cells or trabeculæ pushing their way into a large bloodvessel or blood space and carrying its endothelium before them; at the same time the wall of the vessel or space grows out between the cell columns.

Most veins are provided with valves which serve to prevent the reflux of the blood. Each valve is formed by a reduplication of the inner coat, strengthened by connective tissue and elastic fibers, and is covered on both surfaces with endothelium, the arrangement of which differs on the two surfaces. On the surface of the valve next the wall of the vein the cells are arranged transversely; while on the other surface, over which the current of blood flows, the cells are arranged longitudinally in the direction of the current. Most commonly two such valves are found placed opposite one another, more especially in the smaller veins or in the larger trunks at the point where they are joined by smaller branches; occasionally there are three and sometimes only one. The valves are semilunar. They are attached by their convex edges to the wall of the vein; the concave margins are free, directed in the course of the venous current, and lie in close apposition with the wall of the vein as long as the current of blood takes its natural course; if, however, any regurgitation takes place, the valves become distended, their opposed edges are brought into contact, and the current is interrupted. The wall of the vein on the cardiac side of the point of attachment of each valve is expanded into a pouch or sinus, which gives to the vessel, when injected or distended with blood, a knotted appearance. The valves are very numerous in the veins of the extremities, especially of the lower extremities, these vessels having to conduct the blood against the force of gravity. They are absent in the very small veins, i. e., those less than 2 mm. in diameter, also in the venæ cavæ, hepatic, renal, uterine, and ovarian veins. A few valves are found in each spermatic vein, and one also at its point of junction with the renal vein or inferior vena cava respectively. The cerebral and spinal veins, the veins of the cancellated tissue of bone, the pulmonary veins, and the umbilical vein and its branches, are also destitute of valves. A few valves are occasionally found in the azygos and intercostal veins. Rudimentary valves are found in the tributaries of the portal venous system.

The veins, like the arteries, are supplied with nutrient vessels, vasa vasorum. Nerves also are distributed to them in the same manner as to the arteries, but in much less abundance.