The vertebrate heart, which is essentially a modified blood vessel half artery and half vein, consists of two kinds of chambers, a thin-walled venous receiving chamber, where the returning blood collects, and a thick-walled arterial muscular forwarding chamber, separated from the former by valves which prevent the retreat of the blood when the muscular walls contract (Fig. 289).
The walls of the heart are composed of the three histological layers common to blood vessels, with the exception that the involuntary muscle cells of the tunica media are of a peculiar striated branching type which is particularly effective for the enormous and unremitting work that the heart has to do.
Dr. Keen apostrophizes the heart as follows: “The heart is one of the most wonderful pieces of mechanism in the world, more powerful in proportion to its weight than any Baldwin locomotive, more delicately constructed than the finest watch, an organ which must do and - mirabile dictu! - does do its own repairs while, busy at its work. It knows no Fourth of July or Christmas or Easter holiday, never can even know the joy and relief of sleep, ‘tired nature’s sweet restorer.’ It begins its orderly reiterated contractions and relaxations long before birth, and they cease only at death. It must continue them in health and in sickness, when its function is often sadly disturbed. In mid career let it stop for but a few moments and death comes swiftly, almost instantly.”
As the ventral parts of the hypomeres approach one another to form the ventral mesentery in the region of the pharynx, they thicken. Between them mesenchymal cells establish two thin-walled endothelial tubes (Fig. 290). These vessels soon fuse into a single endocardial tube surrounded by thickened ventral mesentery the layers of which meet both above and below the tube as dorsal and ventral mesocardia, respectively. While the mesocardia disappear almost immediately, the rest of the ventral mesentery, adjacent to the endocardial tube, continues to thicken, especially in the region where the ventricle is to form. The wrall of the original endocardial tube becomes the endocardium, the lining layer of the adult heart, while the surrounding material develops into the myocardium, the muscular layer of the heart, and a thin covering layer, the visceral pericardium. With the disappearance of the mesocardia, the heart lies free in the pericardial cavity attached only posteriorly, at the transverse septum, and anteriorly at its exit where it passes over into the ventral aorta (Fig. 291).
Although the heart is at first a relatively simple tube, it soon enlarges and becomes modified. Included among the changes which occur are: constrictions into several chambers; differential thickening of the myocardium, resulting in the establishment of the thin-walled receiving part and the thick-walled forwarding part of the heart; and a kinking of the chambers, necessitated by rapid growth within crowded quarters, so that they no longer lie in a straight line.
In amphioxus, in which the circulatory apparatus is so primitive that red blood corpuscles are scarce, an accessory lymphatic system is not yet present and there is no heart at all. The ventral blood vessel which extends between the liver diverticulum and the gills is contractile enough to send the blood forward. It is this part of the ventral aorta that marks the location of the future vertebrate heart. From such a beginning the evolving vertebrate heart passes through a series of modifications of increasing complexity, until eventually there is developed the four-chambered mammalian heart.
The first step in the differentiation of the vertebrate heart is encountered in the larval ammocoetes stage of the lamprey eel in which the prophetic ventral aorta lying between the liver and the gills becomes somewhat enlarged and modified by constrictions, unequal thickening, and kinking. Although the ammocoetes stage of the lamprey shows the heart still in the common body cavity with the liver, intestine, and other visceral organs, among fishes generally a transverse septum forms by a proliferation of the peritoneal walls, and ever after the heart is housed within the privacy of an enveloping space of its own, the pericardial cavity.
In elasmobranch fishes the pericardio-peritoneal canal, a slitlike opening between the pericardial cavity and the peritoneal cavity, represents the last step before the establishment of pericardial independence.
The relatively small heart of fishes consists typically of a series of four chambers through which only non-aerated blood passes, since the spent blood sent forward from the heart to the gills for aeration must make the grand tour of the body and become again non-aerated before it is returned to its starting point in the heart.
Beginning posteriorly the four chambers in the heart of elasmobranch fishes are the sinus venosus, atrium, ventricle, and conus arteriosus (Fig. 291b). The first two belong to the receiving region of the heart and are reservoirs with elastic rather than thick muscular walls, while the ventricle is thick-walled and muscular, as befits the forwarding pump of the blood.
The conus has a muscular wall of moderate thickness, which by its elasticity aids in regulating the back pressure of the blood as it is forced through the ventral aorta into the nearby capillaries of the gills.
In elasmobranchs the single atrium receives the blood that has been poured into the sinus venosus from the ducts of Cuvier and the hepatic veins (Fig. 292). A row of cup-like valves, with their concavities in front, guard the atrio-ventricular opening permitting the blood to go into the ventricle but filling with blood to block the opening when the ventricle contracts to force the blood through the conus (Fig. 293). Several rows of semilunar valves in the conus allow this blood to flow forward but, when the ventricle begins to relax, they block the conus by filling with blood as soon as it starts to flow backwards (Fig. 294A).
In bony fishes the conus and ventricle gradually telescope together until in most teleosts the conus is represented by a very short region including a single row of semilunar valves (Fig. 294). With this shortening, the posterior part of the ventral aorta is drawn back into the pericardial cavity where it develops a thick muscular wall and becomes known as the bulbus arteriosus.
With the introduction of land life and lungs a new secondary shorter circuit is initiated by means of which aerated blood from the lungs is returned directly to the heart before making the excursion around the body. The pulmonary blood is poured into the left side of the atrium, a partition having developed that divides the original receiving chamber longitudinally into two chambers, a right and a left auricle. This inter auricular septum develops to the left of the opening from the sinus which therefore sends all of its blood into the right auricle (Fig. 292c).
In dipnoans and amphibians, which accomplish the precarious transfer from gills to lungs, there is thus developed what may be regarded as a heart and a half, or a heart with one ventricle and two atria or auricles. The auricular partition is incomplete in dipnoans so that a mixture of aerated and non-aerated blood results within the auricles of the heart through the so-called foramen ovale. However, as this mixture is passed on through the ventricle, a twisted partition in the conus, which has not yet become incorporated in the ventricle, tends to shunt the mixed blood two ways, that is, to the lungs and over the body.
In amphibians while the auricular partition is complete and the foramen ovale is obliterated, there is a mixture of aerated and non-aerated blood in the common cavity of the ventricle. When sent over the body without having first been revivified by a trip to the source of oxygen in the lungs, there results a condition comparable with burning coal that is half burned-out ashes and “slag.” It burns poorly. This is one reason why these animals are “cold-blooded,” since the only pure blood in an amphibian is in the short pulmonary veins.
This handicap of mixed blood within the ventricle of the amphibian heart is partially avoided by the rapidity of the heart-beat which does not allow time for a thorough mixing of the two kinds of auricular blood that enter the ventricle from the two auricles, and by the spongy reticular structure of the ventricular chamber. Every time that the ventricle is filled, the mass of blood occupying the ventricular cavity may be thought of as momentarily of three kinds, arranged in a sort of temporary stratification, with non-aerated blood from the right auricle placed nearest the exit of the ventricle (Fig. 295), the aerated blood from the lungs farthest away from this exit, and the inevitable mixture of the two somewhere in between. As the ventricle expels its contents before these relations have time or opportunity to change, the result is that the non-aerated blood nearest the exit is directed by the septum of the conus into the first possible avenues of escape, which are the pulmonary arteries leading to the lungs. The intermediate mixed blood, unable to enter the already filled arteries to the lungs, is forced along into the next available blood vessels, which are the systemic aortae distributing blood over the body generally. The best aerated blood of all at the bottom of the ventricle, being the last to emerge and finding all other passage-ways crowded full, passes on to the carotid arteries that supply the brain. Thus the brain, that always needs the best available aerated blood, is in the way of obtaining it, even in such unintellectual ancestors as frogs and toads.
The transition from a single to a double heart is further shown in the reptiles, which have come to forsake entirely the gill method of respiration, but, with the exception of the Crocodilia, have not yet arrived at the estate of a complete double heart.
Among reptiles an interventricular septum forms which tends to keep separate the aerated blood, returning from the lungs by way of the left auricle to the left side of the ventricle, and the non-aerated blood of the body entering the right side from the right auricle (Fig. 292d). This partition is incomplete in most reptiles, so that there still exists some degree of mixture between the right and left ventricles through the foramen Panizzae (Fig. 296), which represents the last gap in the uncompleted ventricular septum. Non-aerated blood from the right ventricle goes out not only to the lungs but in part also to the dorsal aorta, the main distributing trunk of the body. The result is that in reptiles, although the blood is kept unmixed as far as the dorsal aorta, from that point on it is mixed blood, being distributed over the body with a corresponding inevitable sluggishness of behavior.
In the Crocodilia (Fig. 292E) the foramen Panizzae finally becomes obliterated and two complete hearts, superficially incorporated into one, are established. One of these hearts, made up of the left auricle and powerful left ventricle, constitutes the pump for the major circuit over the body, while the other, the right auricle and right ventricle, takes care of the blood coming back from the general body tissues and being sent to the lungs.
Aerated and non-aerated blood, which are mixed within the single ventricle of the heart in amphibians and in the dorsal aorta of reptiles, are kept completely separate among birds and mammals. As in Crocodilia the blood of birds and mammals passes alternately through two circuits, the pulmonary and the systemic. Blood entering the right auricle is sent into the right ventricle, then out to the lungs, back to the left auricle, to left ventricle, out to the general body tissues, and back once more to the right auricle. In birds and mammals the telescoping of the receiving chambers reaches the point where the sinus venosus is virtually eliminated, the chief systemic veins emptying directly into the right auricle.
Size and Position
The heart of a bird is proportionately larger than that of any other vertebrate, for the reason that an especially efficient pumping apparatus is required to keep the machinery of strenuous aerial locomotion going. Among mammals small species have relatively larger hearts than large forms. The proportionate size of the heart also decreases with the relative decrease of the heat-dispensing body surface that accompanies growth. For example, the weight of a newly-born rabbit’s heart has been found to be 5.9 per cent of the total weight, while that of an adult rabbit is 2.8 per cent.
The position of the vertebrate heart is always ventral to the digestive tube, and in gill-breathing vertebrates, far anterior. When the head of a fish or a salamander is cut squarely off, the heart is usually included with it. With the development of a neck there is a backward migration of the heart in higher vertebrates, until in such long-necked forms as swans and giraffes, it comes to lie a long distance from its original location, being much more centralized with reference to the bodv. To have the “heart in the mouth” is, therefore, a sort of ancestral sensation that should in no way disquiet a comparative anatomist.
The adult human heart weighs not far from ten ounces, and is approximately the size of the clenched fist. It is median in position between the lungs (Fig. 297), and not on the left side where it is popularly located by tragic actors and sentimental lovers. The reason it seems to be on the left side is because the throbbing tip of the cone-shaped ventricular part normally projects from behind the sternum towards the left side, where its kick is most readily felt.
There are many misconceptions centering around the human heart. For instance, it is never “heart-shaped” according to the conventional Saint Valentine’s outline, but instead is a flattened cone.
The most constant valves of the heart in the vertebrate series are the auriculo-ventricular valves, which separate the receiving auricle from the forwarding ventricle. They are present in all vertebrates and, in higher forms, are kept from reversing under the pressure of the contracting ventricle by tendon-like guys, chordae tendineae, that are anchored in the muscular walls of the ventricle (Fig. 298). There are only two such valves in the heart of fishes but in the double heart of mammals there are five present, two between the auricle and ventricle of the left side (bicuspid valves) and three (tricuspid valves) on the right side. The bicuspid valves are commonly known as mitral valves from a fancied resemblance to a bishop’s miter. It was Huxley who once humorously said that he could always easily remember the location of the mitral valves on the left side of the heart because he “never knew a bishop to be on the right side.”
The semilunar valves of the conus region are best seen in elasmobranchs and ganoids, where as many as eight rows may appear in some species (Fig. 294A and b). They are cuplike pockets, lying flat against the inner wall as the blood passes out over them, but filling immediately to block the passage-way when the blood attempts to retreat. Similar valves guard the exits from the heart to the pulmonary arteries and to the systemic aorta in the higher forms.
The primitive heart of fishes in which the sinus venosus still persists as a distinct chamber has a pair of sinu-auricular valves between the sinus venosus and the atrium that, like swinging doors, allow the blood to pass to either way. This does no harm since both sinus and atrium have the common function of acting as reservoirs for returning blood.
Between the atrium, or auricle, and the ventricle, on the other hand, one-way traffic must be maintained when the muscular ventricle forces out the blood, consequently mitral and tricuspid valves with their chordae tendineae, swing only so far and no farther.
The Work of the Heart
While compressing muscular movements of the body are largely responsible for the propulsion of lymph through the lymphatic channels, the heart is the indispensable pump by means of which the circulation of the blood is accomplished.
The blood must be kept in constant motion. That this is done is shown by the familiar fact that from any wound, however slight, which makes a break in the circulatory channels, blood immediately flows out.
In amphioxus and certain annelid worms a constant circulation is brought about simply by the contraction of arterial blood vessels, but in vertebrates generally, owing to the enormous expanse of the capillaries developed, contraction of the arterial walls is not sufficient to keep the blood in motion, and a heart becomes necessary. As has been indicated, the heart acts both as a force pump (Fig. 289), filling the arteries from the ventricles, and as a suction pump, drawing venous blood into the auricles.
The rate of flow of the blood is faster of course when an animal is active than when quiet. The contractions of the heart of a hibernating fish, for example, may fall from over 100 a minute to two or three, while that of a mouse, whose normal heart-beat is about 175 per minute, may go up to 600 per minute under the sudden stimulus of fright.
When a person is sitting quietly, about five pints of blood per minute are forced into the aorta, an amount which upon violent exercise may rise to an output of thirty-five pints per minute. Since the total amount of blood in a human adult is only ten to fourteen pints, it is evident that, while undergoing moderate exercise, all of the blood of the body passes through the heart at least twice every minute. Thus, by the most conservative estimates, the strenuous red blood corpuscles in their brief lifetime travel many miles, while accompanying leucocytes that detour constantly from the main path, like an active exploring dog on a country ramble with his master, have still more extensive locomotor adventures. Though ranging wider, they do much of it more slowly and therefore do not maintain the average rate of the red corpuscles.
Another way of reckoning the marvelous work performed normally and continuously by the human heart is to recall that with 72 beats per minute two ounces of blood are squeezed out at each beat, making the total daily output approximate 13,000 pounds. Even the heart of delicate Juliet, sighing in her balcony, did that. When a husky stevedore is forced to handle 13,000 pounds of freight in his day’s work, he is honestly weary at nightfall and quite in the frame of mind to strike for shorter hours.
The constancy of the flow is aided not only by frictional resistance of the moving blood against the inside walls of blood vessels, but also by adjustable variations in pressure upon the blood stream exerted by the contractile walls of the blood vessels under the regulatory stimulus of involuntary vasomotor nerve endings, that act as “stopcocks”. “As these terminal arteries number tens of thousands, and each of them is regulated and controlled, one can conceive how complex the stopcock system of the human machine is” (Keith). If the varying work of the heart were not regulated by some kind of automatic device for adjusting the blood pressure and controlling the flow, disaster would inevitably follow whenever in the countless exigencies of life, a sudden extra load is thrown upon this faithful pump.
The tireless beat of the heart itself is initiated and regulated at the sinu-auricular node (Fig. 299). This “pace setter of the vertebrate heart” is a narrow zone of tissue that marks the transitional region between the sinus venosus and the atrium in the fish heart, and which becomes incorporated as a part of the auricle in higher vertebrates. Another indispensable part of the mechanism of the throbbing heart is the auriculo-ventricular node, a dense network of cardiac muscle fibers connecting the auricular and ventricular walls, and acting somewhat like the “timer” in an automobile. Across this bridge the initiatory stimulus, originating in the sinu-auricular node, is transmitted to the ventricle completing the heart-beat. The auriculo-ventricular node was discovered in the human heart by His in 1893, and is consequently known as the bundle of His.
Although the heart beats in successive throbs, a constant flow of blood is maintained because the elastic arteries, stretched by the pressure generated by the ventricular contraction, gradually contract until they are suddenly distended a train by the next “beat” of the ventricle. There is thus a constant flow of blood from arteries into capillaries.