Comparative Anatomy of Hearts of Vertebrate
In order to carry on vital life processes, all animals, from the simplest protozoans to the most complex vertebrates, required that —
- Food absorbed through digestive tract and oxygen collected in respiratory organs, must be transported to all parts of the body, continually, for metabolism.
- Waste products of metabolism (C02 , nitrogenous wastes, etc.) must be transported from the sites of their productions to excretory organs for their quick elimination from body.
- Hormones from endocrine tissues, substances for maintaining homeostasis or constancy of internal environment, and providing immunity from diseases, must be conveyed to suitable sites for utilization.
For these and other reasons, an adequate internal system for circulating nutrients and other materials throughout the body becomes necessary, called circulatory system. In one-celled body of protozoans, distribution occurs through cyclosis or streaming movements of cytoplasm. In simple and less active multicellular animals (Porifera, Coelenterata, Helminthes, etc.) exchanges occur by simple diffusion between various adjacent parts of their bodies. But most higher invertebrates and vertebrates are large and active, with most body organs and tissues well removed from exterior or gut. For them, diffusion alone cannot suffice. Thus, they possess a well-developed circulatory system for rapid internal transport of gases, nutrients, wastes, etc.
Parts of Circulatory System
Parts of Circulatory System Chordates have a completely closed circulatory system , further distinguished into two systems, blood vascular and lymphatic, having parts as follows :-
- Blood vascular system :- It consists of heart, arteries, veins, capillaries and blood.
- Blood consists of fluid plasma and free cells or blood corpuscles
- Heart is a modified blood vessel with muscular walls. It contracts periodically to pump blood through body.
- Arteries are blood vessels that carry blood away from the heart.
- Capillaries are minute tubes with thin walls in tissues, that connect the smallest arteries (arterioles) with the smallest veins (venules).
- Veins carry blood towards heart from capillary networks.
When blood flows through capillaries connected by arteries and veins, the blood vascular system is said to be ‘closed’, as in annelids and vertebrates. On the other hand, mollusc’s and arthropods lack capillaries and have an ‘open’ or ‘lacunar’ system. The blood pumped by thfeir heart through blood vessels to various organs, passes through body spaces or sinuses comprising the haemocoel.
- Lymphatic system :- It occurs exclusively in chordates, except cyclostomes and cartilaginous fishes, and consists of lymph and lymph channels.
- Lymph is the tissue fluid, lying between and bathing body cells. It is similar to blood plasma but lacks the red blood corpuscles and some proteins.
- Lymph capillaries forming a network of minute, blind-ending channels, collect lymph.
- Lymph vessels are larger thin-walled vessels formed by the union of lymph capillaries, and finally emptying into veins.
- Lymph nodes, found only in mammals on lymph vessels, produce lymphocytes of blood and form an important link in body’s defense mechanism.
In cyclostomes and chondrichthyes no lymphatics are present but little sinusoids are found, representing the first stage of development. Bony fishes and all tetrapodes are provides with lymphatic system. But as far as circulation is concerned, it is active in amphibian due to development of lymph heart. Lymphatic glands first appeared in reptiles and found in birds and mammals as well. Blood vascular system has undergone some striking changes during the evolution of vertebrates. These are mostly correlated with shift from gills to lungs as the site for external respiration during transition from water to land, and with the development of an efficient, high pressure circulatory system necessary for an active terrestrial life.
Evolution of Heart in Vertebrates
In the embryo, two longitudinal endothelial tubes, formed by mesenchyme in ventral mesentery below archenteron, fuse together to give rise to the heart. The vertebrate heart is built in accordance with a basic architectural plan . It is a sac-like muscular organ comprising a series of chambers, that receives blood from veins and pumps it through arteries.
In a primitive chordate, such as Branchiostoma, a true heart is lacking. Instead a part of ventral, aorta below pharynx becomes muscular and contractile. Some zoologists consider it as a single-chambered heart. Progressive modifications of heart from primitive to higher chordates occurs on the following lines :-
- Cardiac tube forms chambers due to constrictions.
- Each chamber tends to divide into two separate chambers due to formations of partitions.
- Heart gradually shifts from just behind head (fishes, amphibians) near gills ‘ into thoracic cavity (amniotes) with elongation of neck and development of lungs.
2-Chambered, single circuit venous hearts
Simplest conditions of vertebrate heart is seen in cyclostomes (ammocoete larva, lamprey, hagfish). It shows a linear series of 4 chambers : sinus venosus, atrium, ventricle and a small conus arteriosus, through which blood flows in that sequence. It is present in the common body cavity along with other visceral organs.
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 4 chambers arranged in a linear sequence. Of these, sinus venosus and conus arteriosus are accessory chambers. Only auricle and ventricle are true chambers so that heart is considered two-chambered in fishes. Thin-walled sinus venosus receives venous blood of body through larger veins (common cardinal and hepatic), serves chiefly as a reservoir and opens anteriorly into atrium through the sino-atrial aperture guarded by a pair of valves. Atrium is large, thin-walled, elastic and muscular chamber lying dorsal to ventricle. It opens ventrally into ventricle through an atrio-ventricular aperture guarded by a pair of valves. Ventricle has very thick and muscular walls. It opens into a muscular tube of narrow diameter, the conus arteriosus, having a series of semilunar valves. All the valves of heart prevent backflow or regurgitation of blood. Heart of fishes is enclosed in a small pericardial cavity separated from general coelom by a transverse septum. In front of pericardial cavity, conus becomes continuous with the ventral aorta. In elasmobranchs, transverse septum is perforated by a pair of openings through which pericardial cavity communicates with coelom.
Heart of bony fishes resembles in all respects that of elasmobranchs. In some Chondrostei (Polypterus) and Holostei (Lepidosteus), conus is fairly long with numerous valves. In Amia, conus and number of its valves are reduced. While in Teleostei, conus is much reduced, or even absent, as it fuses with ventricle, and retains a single pair of semilunar valves. Instead, the part of ventral aorta in contact with conus becomes greatly enlarged with thick muscular walls, and called bulbus arteriosus. It is elastic and inflates like a balloon when the ventricle contracts. In fishes, heart is small, 2-chambered and with a single circuit of blood circulation. All blood passing only once through heart is non-oxygenated. It is pumped into gills for aeration before distribution to body. Such a heart is termed a branchial or venous heart.
3-Chambered transitional hearts
Correlated with the shift from aquatic (gills) to terrestrial respiration (lungs), heart and aortic arches also become modified. Parallel with the systemic circulation, a new shorter pulmonary circulation develops so that aerated blood from lungs (or swim bladder), returns directly to the heart without making a detour of the whole body. Atrium of lung fishes (and most urodele amphibians) is divided by an incomplete inter-auricular septum, perforated by the foramen ovale, into right and left chambers or auricles. This results in a mixing of oxygenated blood received from lungs into left auricle, and deoxygenated blood from rest of body into right auricle. A partial partition also divides the ventricle, while a horizontal septum divides the conus of lungfishes into a dorsal and a ventral part.
Amphibians heart (anurans) shows an advance over the piscine heart. A twisting or curving results in dorsal atrium shifting anteriorly to ventricle. Similarly, sinus venosus opens into the right atrium dorsally instead of posteriorly. The inter-auricular septum is complete, without foramen ovale. This keeps the oxygenated and deoxygenated bloods separate. Ventricle is undivided or single, but its thick muscular wall raised into trabeculae permitting only little mixing • of the two bloods. In urodeles, conus is reduced and replaced by a bulbus arteriosus. In anurans, from left ventricle, on its anterolateral margins conus (or truncus) arteriosus arises which is prominent and divided by a spiral valve which directs deoxygenated blood into pulmonary vessels and oxygenated blood into systemic vessels. The lumen of conus arteriosus is called, pylangium is occupied by spiral valve (= septum bulbf). This valve is very complicated in its disposition. It is attached to the walls of conus dorsally and free at other three faces. It divides the lumen of conus arteriosus into two chamber— cavum pulmocutaneum and cavum aorticum.
Heart of reptiles shows further improvement over that of amphibians. It becomes more strongly muscular. It shows two auricles and two ventricles. In most reptiles, ventricle is partially divided by an incomplete inter-ventricular septum, which reduces the mixing of oxygenated and deoxygenated blood. In crocodilians, this septum is complete thus making an effective 4-chambered heart, having two auricles and two ventricles. However, complete separation of oxygenated and deoxygenated blood is not achieved. The right and left systemic aortae, carrying arterial and venous bloods, respectively, join to form the dorsal aorta in which the two bloods get mixed before distribution. Besides, a small opening, called foramen of panizza, connecting the two aortae at their base, brings about some mixing of blood. In crocodiles foramen panizza becomes obliterated. A sinus venosus is present in all reptiles, large in turtles, small in snakes and lizards, and distinct internally in crocodiles. Conus and ventral aorta of embryo become split in the adult into three distinct trunks-pulmonary and right and left systemic. Amphibian heart with only 3 major chambers (2 auricles, 1 ventricle), and reptilian heart with partially 4 chambers (2 auricles, 2 incomplete ventricles), permit a partial mixing of arterial and venous bloods before distribution. Thus, they represent transitional hearts showing a midway condition between 2-chambered heart of fishes with a single circulation and 4-chambered hearts of birds and mammals with double circulation and complete separation of arterial and venous bloods.
4-Chambered, double circuit pulmonary hearts
Birds and mammals
Birds and mammals have a completely divided ventricle, so that their heart is completely 4-chambered (2-auricles, 2 ventricles) Left auricle receives aerated blood from lungs, pours into left ventricle which pumps it to entire body through systemic circulation. Right auricle receives deoxygenated blood returning from body, passes it to right ventricle which pumps it to lungs for reoxygenation. Thus there is double circulation in which there is no mixing of oxygenated and non-oxygenated blood at all. Such a heart is known as a pulmonary heart. Sinus venosus is absent being completely incorporated into right auricle which directly receives two precavals, postcaval. The union of sinus with right auricle in some cases is marked externally by a groove called sulcus terminalis and internally by a muscular ridge, crista terminalis which separates right auricular chamber (sinus venerum) from smaller ventral chamber (appendix auricular). Similarly, the left auricle receives blood directly through pulmonary veins. Primitive conus arteriosus is completely replaced by a pulmonary aorta leaving the right ventricle for lungs, and a single systemic aorta leaving the left ventricle for body. All major vessels have valves basally at the point of exit from or entry into heart. Blood from the walls of the heart is brought to the auricle by means of coronary sinus in right atrium. The opening of the sinus is guarded by valves called coronary valve (= Thebsian valve). The inner surface of right auricle wall is marked by small depressions of Thebesian foramina in which fine veins directly pass the blood from atrial walls to right atrium. Although, interauricular septum is complete in adults but a fine depression, fossa ovalis is present which marks the site of foramen ovale. The fossa ovalis is surrounded by a prominent ridge annulus ovalis.