Circulatory System

CIRCULATORY SYSTEM, a complex of cavities and channels serving to distribute fluids, containing primarily nutrients and oxygen, throughout the entire organism and to extract from individual parts of the organism metabolic products subject to subsequent complete removal. These basic functions of the circulatory system determine both its position in the organism and the nature of its connections. The source of the origin of the circulatory system in lower animals is the mesodermal parenchyma located between the endodermal wall of the gut—the organ delivering food material—and the ectodermal outer wall of the body, which acts as a transmitter of oxygen absorbed from the external environment. In this parenchyma, irregular cavities arise, filled with interstitial fluid, through which substances move by means of diffusion. During the movement of the animal and the contraction of its muscles, the fluids move without a definite direction. We begin to speak of a circulatory system only when definite paths appear along which the fluid flows in a definite direction. Such paths, bounded by their own walls, bear the name of blood vessels. Both the walls of these vessels and their contents—blood—represent a product of mesodermal connective tissue. In the majority of invertebrates possessing a circulatory system, the vessels are partly interrupted by more or less extensive interstitial spaces (lacunae, sinuses). Such a circulatory system is called open, in contrast to the closed system found in annelid worms and chordate animals. A distinct heart is often entirely absent in invertebrates, and the blood is pumped partly thanks to peristaltic contractions of the walls of the vessels themselves. In other cases, a section of a vessel with particularly strongly developed musculature acts as the main organ moving the blood and then receives the name of heart. The blood itself represents a proteinaceous fluid, colorless or colored by pigments dissolved in it, with formed elements suspended in it. In vertebrates, the blood plasma is colorless, and the pigment—hemoglobin—is contained in special disc-shaped (usually oval, but in mammals, round) blood corpuscles—erythrocytes. Besides the latter, there are also mobile cells of various types (leukocytes, lymphocytes, thrombocytes) in the blood of vertebrates. The walls of the vessels consist of an inner thin "endothelial" lining, a middle elastic layer with smooth muscle cells, and an outer connective tissue layer. If there is a heart or at least a main pulsating vessel, then one distinguishes arteries—vessels carrying blood away from the heart, capable of withstanding higher pressure and therefore possessing a more significant muscular and elastic layer—and veins—vessels carrying blood to the heart, thinner-walled and often equipped with valves regulating the direction of blood flow. When a closed circulatory system exists, the arteries and veins are connected to each other by a continuous network of vessels of very small caliber—capillaries, the thin walls of which consist of endothelium covered by a layer of longitudinal muscle cells embracing the vessels with their branches. The entire exchange of substances between the blood and tissues occurs through the walls of the capillaries. The circulatory system of vertebrates consists fundamentally of two main vessels—the ventral vessel, along which blood flows forward, and the dorsal vessel, along which blood flows backward. Both vessels are connected to each other by branches forming a network of capillaries in the walls of the intestine. In such a highly schematized form, this system is still similar to the circulatory system of higher worms. In the lancelet (Amphioxus), the ventral vessel is represented by the subintestinal and portal vein posteriorly and the subbranchial artery (ventral aorta) anteriorly. The dorsal vessel is represented by the dorsal aorta. This system is complicated, however, by the existence of paired cardinal veins in the lateral walls of the body. The circulatory system of lower vertebrates is built on the same type, and what is essentially new here is only the central organ moving the blood, which is absent in the lancelet. The heart of vertebrates represents, by its origin, a specially differentiated section of the anterior part of the ventral vessel with a strongly developed muscular wall. In fish, it consists of a comparatively thin-walled atrium and a muscular ventricle, separated from each other by two valves. To these two sections, however, are added posteriorly a sac-like sinus, separated by two valves from the atrium, and anteriorly from the ventricle a muscular tube—the arterial cone, equipped with several transverse rows of pocket-like valves. These sections are equipped (like the main chambers) with peculiar striated musculature and must be considered as parts of the heart itself. Being by its origin a part of the ventral vessel, the heart develops initially in the form of a straight tube. In lamprey larvae, the sections of the heart are still arranged in a single row, and the ventricle lies in front of the atrium. In all other vertebrates, the heart tube bends in the shape of the letter S while still in the embryo, so that the venous sinus and atrium, moving forward, are placed dorsally above the ventricle and the arterial cone. The wall of the heart consists essentially of the same layers as the wall of the vessels. Its main thickness is represented by muscular mass (myocardium). From the inside, it is lined with endothelium (endocardium) and from the outside covered by a thin layer of connective tissue (epicardium). In fish, the heart is placed in the posterior part of the head immediately behind the gill apparatus. By means of rhythmic sequential contraction of all sections of the heart and with the participation of a whole system of valves, blood is sucked from the venous trunks flowing into the venous sinus and is pumped forward through the main arterial trunk—the ventral aorta—into the gill apparatus for oxidation. Thus, the heart of fish carries exclusively deoxygenated ("venous") blood. The ventral aorta, into which this blood enters, divides into a series (usually 4-5) of paired branchial arteries passing along the gill arches and breaking up in the gill filaments into the smallest branches. The latter unite into efferent branchial arteries, also running along the gill arches in the direction of their dorsal end, where they flow on the right and left into the paired longitudinal trunk—the roots of the aorta. The latter continue anteriorly into the head in the form of carotid arteries and unite posteriorly to form the unpaired dorsal aorta, passing under the vertebral column along the entire trunk. The dorsal aorta gives off branches both to the muscular walls of the body and to the limbs, as well as to the internal organs in the following order: to the forelimbs (aa. subclaviae), to the intestine (aa. coeliaco-mesentericae), to the reproductive organs (aa. genitales), to the kidneys (aa. renales), to the hindlimbs (aa. iliacae), and continues posteriorly in the form of the caudal artery (a. caudalis). Deoxygenated blood from the walls of the intestine is collected into capillaries, forming the portal system of the liver (developing in connection with the subintestinal vein of the embryo). From the liver, blood flows along the hepatic veins into the venous sinus of the heart. Into the latter also flow the paired Cuvierian ducts, formed by the union of the paired (anterior and posterior) cardinal veins. The former carry blood from the head, the latter from the walls of the trunk and a number of its organs (especially the kidneys and gonads). From the tail, blood returns along the caudal vein to the kidneys, where this vein breaks up into capillaries, forming the portal system of the kidneys. The renal veins constitute the beginning of the posterior cardinal veins. From the hindlimbs, deoxygenated blood returns along the lateral veins (vv. laterales), lying in the ventral wall of the body, into the Cuvierian ducts. Into the lateral veins also flow the veins coming from the forelimbs. With the acquisition of pulmonary respiration by terrestrial vertebrates, very major transformations in the circulatory system are connected, and primarily in the structure of the heart. Into the heart now enters, besides deoxygenated blood, also oxygenated blood from the lungs. The evolution of the arterial system of vertebrates is accompanied by a gradually more and more perfect separation of both blood currents, forming finally in birds and mammals completely independent and closed "greater" and "lesser" circles of blood circulation. In connection with this, septa develop in the heart itself, first in the atrium, and then in the ventricle. The heart becomes "four-chambered" from "two-chambered." These transformations are outlined already in lungfish, possessing lungs as an auxiliary organ of respiration. The lung of lungfish, as well as of amphibian larvae, is supplied by an artery departing from the fourth pair of efferent branchial arteries. The pulmonary vein flows directly into the atrium on its left side, and the venous sinus turns out to be displaced to the right side. Inside the heart, there are incomplete septa dividing it (mainly the atrium) into a right half, carrying blood coming mainly from the venous sinus, and a left half, carrying blood mainly from the pulmonary artery. This adaptation is supplemented further by the existence in lungfish and amphibians of a longitudinal valve in the arterial cone and longitudinal septa in the arterial trunk, so that deoxygenated blood from the right half of the heart enters mainly into the pulmonary arteries, and oxygenated blood from the left half—primarily into the carotid arteries. Into the dorsal aorta enters mixed blood.

In reptiles, there is also not yet a complete separation of blood currents in the heart and arteries. The ventricular septum, extending from the ventral wall, does not reach (with the exception of crocodiles) the dorsal one. Although during systole its free edge obviously closely adjoins the dorsal wall and practically the septum functions as a complete one, nevertheless, mixing of blood still occurs in the region of the arterial arches (see below). The venous sinus as an independent part of the heart disappears, becoming part of the right atrium, where in reptiles its rudiments are present. The arterial cone, starting from reptiles, is also reduced. In higher vertebrates, the only remnant of this part of the heart is merely the first row of its pocket-shaped valves at the base of the aorta and at the base of the pulmonary artery (valvulae semilunares). With the formation of a septum in the atrium, and then in the ventricle, the opening connecting these parts is also partitioned, along with the two membranous valves present here. In crocodiles, birds, and also in monotreme mammals, a special muscular fold develops in the right atrioventricular opening on the outer wall, playing the role of a valve (muscular valve). In placental mammals, the right atrioventricular valve is divided into three independent flaps (valvula tricuspidalis), and the left into two (valvula bicuspidalis). The heart of adult mammals has complete septa, and oxygenated blood is completely separated from deoxygenated blood not only in the heart itself but also in the arteries. With the transition to pulmonary respiration, the system of branchial arteries naturally underwent the most profound changes. The history of these changes is perfectly clear and can be traced both comparatively-anatomically (when comparing lungfish and amphibians with higher vertebrates) and embryologically (when studying the individual development of the latter). In all vertebrates, a main arterial trunk—the ventral aorta—develops in front of the heart, giving off paired arterial arches that pass along the visceral arches to the dorsal side, where they unite into paired roots of the aorta. Such arches usually develop in 6 pairs corresponding to the mandibular, hyoid, and four branchial arches. The first two pairs disappear already in the embryo, and the last four are divided in fish into afferent and efferent branchial arteries with a network of anastomoses connecting them in the gill filaments. Already in some lungfish, the gills disappear on the first two branchial arches; the small branching of vessels also disappears, and the branchial arteries appear in the form of arches directly connecting the ventral aorta with the dorsal one. In terrestrial vertebrates, these changes go further. Here, too, all six pairs of arterial arches are laid down. The first two pairs disappear. The third pair (passing along the first branchial arch) retains its connection with the anterior ends of the roots of the aorta, i.e., with the carotid arteries, but separates from the parts of the roots of the aorta lying behind. Remnants of the original connection are preserved in the adult state in the form of the carotid duct (ductus caroticus) in some amphibians and lizards and in the most primitive of modern reptiles—the New Zealand tuatara. The fourth pair (going along the second pair of branchial arches) acquires great importance in all terrestrial vertebrates as the actual aortic arches. These arches are almost symmetrically developed in adult amphibians and reptiles. In birds, the left arch atrophies, and only the right is preserved, while in mammals, conversely, only the left aortic arch is preserved. The fifth arch disappears and is only sometimes preserved in the adult state in tailed amphibians in the form of an insignificant duct. The sixth arch (the fourth branchial) gives off the pulmonary arteries and then loses its connection with the dorsal aorta. This embryonic connection of the pulmonary arteries with the dorsal aorta is called Botallo's duct and is preserved in the adult state in tailed amphibians, turtles, and the aforementioned tuatara; in other cases, only a ligament remains of it. Arterial system. The single main arterial trunk, the ventral aorta of fish, already begins to subdivide in amphibians down to the very base into the right and left arch and, in addition, is divided by a septum into a dorsal part leading to the pulmonary arteries and a ventral part leading to the aortic arches and carotid arteries. In higher vertebrates, this separation is even more complete, and the pulmonary arteries, which developed as branches of the fourth pair of branchial arteries, depart independently as a single trunk from the right half of the ventricle. The aortic arches, which originated from the second pair of branchial arteries, depart in reptiles and birds as separate trunks: the right from the left half of the ventricle, and the left in the region of the septum. In amphibians, the arterial system is still built almost symmetrically, and both carotid arteries depart as separate trunks from the right and left arches. In reptiles, a fairly noticeable asymmetry is already emerging, expressed primarily in the fact that the carotid arteries depart as a single trunk from the right aortic arch. In lizards, both subclavian arteries also depart from the latter as a single trunk. Since the right aortic arch departs from the left ventricle, it receives almost pure oxygenated blood, which is directed primarily to the head. The left aortic arch departs in the region of the septum and receives mixed blood. Most of the latter is directed to the intestine along the celiac artery (a. coeliaca) departing from it. Thus, already in reptiles, the predominance of the right aortic arch is emerging, which in birds becomes the only one, receiving, moreover, exclusively oxygenated blood. In mammals, the separation of both aortic arches down to the base has not occurred, and they depart as a common trunk from the left ventricle. The right arch, however, is reduced and preserved only in its proximal part as the site of origin of the right subclavian artery. Thus, only the left aortic arch serves to connect the ventral aorta with the dorsal one. - The origin of the main arteries is different in various mammals. Sometimes the subclavian and carotid arteries depart independently, or the carotid arteries depart as a single common trunk, or some of them or both together are connected with one or both subclavian arteries. The vertebral arteries usually depart from the subclavian artery. Further, the intestinal arteries and several mesenteric ones depart from the dorsal aorta; then the genital, renal, and iliac arteries with their branches, of which the femoral artery gains predominant importance in mammals. Conversely, the sciatic artery—the main artery of the limbs of terrestrial vertebrates—is noticeably weaker here. In the embryo, there are also vitelline arteries (aa. omphalo-mesentericae) as branches of the intestinal ones, and in higher vertebrates (reptiles, birds and mammals)—umbilical arteries (aa. umbilicales), which are branches of the iliac artery and supply blood to the allantois. In mammals, in connection with the establishment of placental respiration and fetal nutrition, the umbilical artery gains predominant importance. The venous system of lower vertebrates consists, as we have seen, of a transformed unpaired subintestinal vein, of paired anterior and posterior cardinal veins, and a paired lateral vein, all of which pour deoxygenated blood into the venous sinus of the heart (the paired veins through the intermediary of the paired Cuvierian ducts). Along the way, there are two portal systems: in the kidneys and in the liver. In general, the entire venous system of lower vertebrates has a symmetrical structure (see figure, vol. XII, p. 301). However, already in higher fish, an asymmetry in the development of the posterior cardinal veins is emerging. In lungfish and amphibians, a new large unpaired posterior vena cava develops partly at the expense of the posterior cardinal veins, carrying blood mainly from the kidneys directly into the venous sinus. The cardinal veins now play a subordinate role: they collect blood only from the muscles of the lateral walls of the body and, due to asymmetric development in higher vertebrates, receive the name of azygos veins (vv. azygeae). Both lateral veins usually merge into an unpaired abdominal vein and, in terrestrial vertebrates, flow into the portal vein of the liver. The anterior cardinal veins in terrestrial vertebrates are partly transformed and receive the name of the internal jugular vein (v. jugularis interna). In lower terrestrial vertebrates, both portal systems are still present: blood from the caudal vein and from the iliac veins is directed partly into the portal veins of the kidneys, partly along the abdominal vein into the portal vein of the liver. In birds, a direct connection is established between the portal vein of the kidneys and the posterior vena cava, and thus this portal system is reduced, and in mammals, it disappears altogether. The abdominal vein is laid down as a paired vessel flowing into the Cuvierian ducts, but in mammals, it is a purely embryonic formation, at the expense of which the umbilical veins of the embryo develop. In general, the embryonic venous system of mammals almost completely repeats that of lower vertebrates with its symmetrical arrangement of the main vessels, including the anterior and posterior cardinal veins. In connection with adaptations for the nutrition and respiration of the embryo, special embryonic vessels also develop. In connection with the subintestinal vein, vitelline veins develop, branching on the surface of the yolk sac and having particularly great importance in lower vertebrates (respiration and nutrition), as well as in reptiles and birds (nutrition), and receding into the background in mammals.

At the expense of the lateral veins, the umbilical veins (vv. umbilicales) develop in reptiles, birds, and mammals, which carry oxygenated blood from the surface of the allantois directly into the Cuvierian ducts and acquire special significance in mammals, where they give rise to the placental vessels of the embryo in the chorionic villi. The symmetrical arrangement of the vessels, however, is lost quite early in mammals, already in the embryo. From the left vitelline vein develops a duct—ductus venosus Arantii—which heads to the right into the venous sinus. The left umbilical vein forms an anastomosis to this duct, loses its original connection with the Cuvierian duct, and consequently drains through the Arantius duct into the venous sinus. The right umbilical vein disappears. Partly at the expense of the right vitelline vein, the posterior (in humans, inferior) vena cava develops, growing backward to the kidneys. From the iliac veins, the provisional portal system of the kidneys also develops. Soon, however, a direct communication is established between the portal veins of the kidneys and the posterior vena cava. The posterior parts of the cardinal veins are reduced, and the iliac veins drain directly into the posterior part of the posterior vena cava. In various mammals, one or the other of the cardinal veins disappears, but in the majority, as in humans, both are connected to each other by a transverse anastomosis, after which the left azygos vein (v. hemiazygea) loses its connection with the left Cuvierian duct. The subclavian veins, which initially empty into the posterior cardinal veins, with the reduction of the latter, connect directly with the anterior cardinal veins, i.e., with the internal jugular veins. The boundary of the Cuvierian duct, marked by the junction of both cardinal veins, loses its significance, and the main venous trunk, formed by the fusion of the jugular and subclavian veins, receives the name of the anterior vena cava. In some mammals, both anterior venae cavae are developed symmetrically even in the adult state; however, usually, the vessels of the right side gain predominance here as well. Both anterior venae cavae are connected to each other by a transverse anastomosis, which receives the name of the left innominate vein. After this, the left vena cava disappears, and all blood from the head, limbs, and azygos veins drains into the right anterior (in humans, superior) vena cava. In mammals, as indicated, the flows of oxygenated and reduced blood in the heart and the main arterial trunks are completely separated. However, in mammalian embryos, the heart carries mixed blood. Blood oxygenated in the placenta enters through the umbilical vein partly into the portal vein of the liver, but mainly through the Arantius duct into the venous sinus of the heart. The latter thus receives mixed blood, which is directed through the right half of the heart into the pulmonary artery. However, the lungs do not function, and the blood...