Evolution of Heart in Vertebrates

Vertebrates possess a pulsating heart that pumps blood anteriorly through arteries and then forces it to the dorsal side. It receives blood from various parts of the body through veins. The heart lies ventrally in a special coelomic compartment known as the pericardium. The pericardium is lined with an outer peritoneal and an inner visceral layer. The visceral layer is closely attached over the heart and is known as the epicardium. Comparative Anatomy: A well-organized heart is absent in protochordates, but in all vertebrates, it presents a gradual series of complexity and has reached its maximum in mammals.   1) Single-chambered heart: Cephalochordates: In a primitive chordate such as Branchiostoma, a true heart is lacking. Instead, a part of the ventral aorta below the pharynx becomes muscular and contractile, which some zoologists consider as a single-chambered heart. 2) Two-chambered: Single Circuit Venous Heart: Cyclostomes: The cyclostome heart is two-chambered and consists of an auricle and a ventricle. The auricle lies on the left side, and the ventricle assumes an S-shaped structure. The auricle communicates with the ventricle by means of a small aperture guarded by the atrio-ventricular valve. The lining of the ventricle is irregular and provided with muscular ridges. Elasmobranchs: The heart of a cartilaginous dogfish is typical and generalized for most fishes. It is a muscular and dorso-ventrally bent "S"-shaped tube consisting of four chambers arranged in a linear sequence.   Only the auricle and ventricle are true chambers. The sinus venosus and conus arteriosus are accessory chambers. The sinus venosus receives venous blood from the body through large veins. The sinus venosus opens anteriorly into the atrium through the sinus atrioventricular aperture, guarded by a pair of valves. The atrium is a large, thin-walled, elastic, and muscular chamber lying dorsal to the ventricle. It opens ventrally into the ventricle through the atrio-ventricular aperture, guarded by a pair of valves. The ventricle has a very thick and muscular wall and opens into a muscular tube of narrow diameter, the conus arteriosus, having a series of semilunar valves. These valves prevent backflow of blood. The heart of fishes is enclosed in a small pericardial cavity. Teleosts: The heart of bony fish resembles that of elasmobranchs in all respects, but the reduction in the number of valves in the conus arteriosus is marked. The conus is reduced or even absent and retains a single pair of semilunar valves. In fishes, the heart is small, two-chambered, and has a single circuit of blood circulation. All blood passing through the heart is non-oxygenated and is pumped into gills for aeration before distribution to the body. Such a heart is termed a venous heart. Three-chambered transitional heart: Dipnoi: The first advanced form of a 2-3 chambered condition of the heart is seen in lungfish (dipnoi). In dipnoi, the atrium is divided by an incomplete inter-atrial septum into right and left chambers. This results in a mixing of oxygenated blood received from the lungs into the left atrium and deoxygenated blood from the rest of the body into the right atrium. The ventricle is also partially divided. The conus arteriosus is spirally twisted and divided into dorsal and ventral parts. This heart is described as a transitional heart due to the incomplete partition mixing of oxygenated and non-oxygenated blood. Amphibians: The heart of amphibians, such as frogs, is myogenic and three-chambered. The auricle is divided into left and right by an inter-auricular septum. The sinus venosus, consisting of 3 caval veins, present dorsal to the right auricle and conus arteriosus, restricts blood flow to the right auricle. As the inter-auricular septum has shifted to the more left side, the right auricle is larger than the left auricle. The sinus venosus opens into the right auricle through the sinus atrio-ventricular aperture, guarded by valves. The opening to the right auricle receives impure blood. The two atria communicate to the ventricles by a pair of valves called atrio-ventricular valves, guarded by columnar carneae muscles. The ventricle is muscular, and its walls give rise to columnar carneae, which hold the atrio-ventricular valves. The ventricle opens to the conus arteriosus. The ventricle is undivided, but its thick muscular wall raised by a bulbus arteriosus. In Urodela, the conus is reduced and prominent and divided by a spiral valve, which directs deoxygenated blood into pulmonary vessels and oxygenated blood into systemic vessels.   Reptiles are the first of chordates to become terrestrial. Hence, there are two distinct types of blood passing through the heart: one for respiration through lungs and one for general circulation. Although a large sinus venosus is present in some reptiles like turtles, in general, it has been greatly reduced and incorporated within the walls of the right auricle. A complete inter-auricular septum separates the oxygenated blood of the left auricle from the deoxygenated blood of the right auricle. All reptiles have a three-chambered heart, except in Crocodiles, which has a four-chambered heart. The ventricle is partially divided by an incomplete inter-ventricular septum, which reduces the mixing of oxygenated and deoxygenated blood.   However, complete separation of oxygenated and deoxygenated blood is not achieved. The right and left systematic aortae, carrying arterial and venous blood respectively, join to form the dorsal aorta in which oxygenated blood flows. In reptiles other than crocodiles, due to incomplete separation of the ventricle, pure blood from the left auricle and impure blood from the right auricle get mixed and mixed blood is circulated. In birds, a complete double circulation occurs, meaning there is no point for mixing of oxygenated and deoxygenated blood. The sinus venosus has disappeared, and three large veins (two precaval and one postcaval) directly enter the right auricle. These three openings are guarded by the Eustachian valve, which first appears in birds. The pulmonary veins convey oxygenated blood to the left auricle. The avian heart is large and completely four-chambered. A single valve separates the right auricle from the right ventricle. A bicuspid valve separates the left auricle from the left ventricle. The advance of the heart of birds over reptiles is the elimination of the left aorta. So pulmonary aorta from the right ventricle arises and the systematic aorta from the left ventricle arises. The opening of these two aortae are guarded by semilunar valves. In birds, pure blood from the left ventricle passes to all parts of the body and again collected to the right auricle in deoxygenated form. From here, it passes to the right ventricle and sends to lungs for aeration. After aeration, it is collected at the left auricle and again circulated through the left ventricle. Mammals: The mammalian heart is highly specialized and essentially similar to birds. Two auricles and two ventricles are separated by a septum. A thin area on the inter-auricular septum is present during fetal life and is known as the fossa ovalis. Three caval veins are guarded by the Eustachian valve. The pulmonary veins carry oxygenated blood from the lungs into the left auricle. The deoxygenated blood from the right auricle passes to the right ventricle through the auriculo-ventricular aperture guarded by the tricuspid valve. Similarly, oxygenated blood from the left auricle to the left ventricle through the auriculo-ventricular aperture guarded by the bicuspid or mitral valve. The left ventricle is thicker than the right. The pulmonary aorta arises from the right ventricle, while the systemic aorta arises from the left ventricle. During contraction, pure blood from the left ventricle passes to the body, and from the right ventricle to the lungs for purification. Besides the above structure of the heart, a pacemaker or S-A node is present in the wall of the sinus venosus, which regulates the rhythmic contraction and relaxation of the heart. Another mass of muscles known as the atrio-ventricular node (AV node) is present, which follows a bundle of His tissue, distributed in the inter-ventricular septum. This bundle finally sends Purkinje fibers, causing the beat of the heart. From above anatomical considerations, it is understood that although the architectural pattern of the heart in all vertebrates is the same, it has been reached to its climax in mammals. The sophisticated heart in mammals always keeps separate oxygenated and deoxygenated blood. This fully developed heart in mammals keeps the body efficiently and energetically active.