HEART

HEART

Heart in simple terms is a highly muscular pumping organ that pumps blood into arteries and sucks it back through the veins. In vertebrates it has undergone transformation by twisting from a straight tube to a complex multichambered organ, enclosed in membranous pericardium.

 In protochordates, a definite heart does not exist and major blood vessels contract rhythmically to maintain circulation. Amphioxus also has small bulbuli to pump blood into gills.

FISH HEART

             In fishes heart has three chambers, namely, sinus venosus, atrium and ventricle. Ventricle is connected with conus arteriosus that has a series of semilunar valves to prevent backflow of blood. In cyclostomes there is a single set of semilunar valves in conus arteriosus. Because of twisting of fish heart atrium is placed dorsally to the ventricle. Sinus venosus receives deoxygenated blood from all parts of body and ventricle pumps it into ventral aorta to be taken to gills for absorption of oxygen. In hagfishes there are no nerve fibres supplying heart muscles but in all other fishes there is a node of neural tissue in sinus venosus and another AV node at the junction of atrium and ventricle.

            Outer layer of heart wall, called epicardium, is made of connective tissue and elastic fibres. Endocardium consists of flattened epithelial cells. Myocardium is a muscular layer. Between this layer and endocardium occur a layer of purkinje fibres which are made of nerve cells whose peripheral parts transform into striated muscle fibres.

            In fishes atrium and ventricle are single chambered but in lungfishes both atrium and ventricle possess an incomplete septum dividing the lumen into two.

In bony fishes conus arteriosus is reduced with one or two sets of valves only and instead a thin walled bulbus arteriosus occurs at the base of ventral aorta which inflated when blood from ventricle is forced into it.

            In lung fishes owing to the appearance of lungs a pulmonary artery is formed from the 6^th aortic arch and a pulmonary vein from lungs opens into the right atrium. Therefore, a septum, albeit incomplete, is formed in atrium and also in ventricle to keep oxygen rich and oxygen deficient blood from mixing. Lungfishes also possess a spiral valve in the bulbus cordis.

AMPHIBIAN HEART

            Urodeles possess an incomplete septum in atrium and no septum in ventricle. Also there is no spiral valve in the truncus. But urodeles have a pulmonary artery and vein and a single set of semilunar valves in the truncus.

            Anurans have a complete septum in atrium but no septum in ventricle. The inner lining of ventricle possesses spongy trabeculae. Truncus arteriosus displays aspiral valve or septum bulbi, which divides its lumen into a dorsal and a ventral channel and also serves to shunt the oxygenated blood into systemic arches and deoxygenated blood to pulmonary arteries for carrying it to lungs.

REPTILIAN HEART

            There is a complete septum in atrium and an incomplete one in ventricle, with a foramen of Panizza, through which two kinds of blood mixes before it is pumped to the body organs. Also, ventral aorta in reptiles divides into two so that the right systemic arch emerges from the left ventricle and the left one emerges from the right ventricle. At the point of their crossing a foramen of Panizza allows the blood to flow from one arch to another. SA node in reptiles migrates near the junction of sinus venosus with atrium and the former is reduced in size.

            In crocodiles, both atrium and ventricle are completely partitioned so that there is no mixing of blood in heart but the two systemic arched still have a foramen of Panizza connecting them and hence blood mixes, rendering crocodiles the status of cold blooded animals.

AVIAN HEART

            Birds are endothermic animals for which a complete separation of oxygenated and deoxygenated blood is necessary. So, there is a complete partition in atrium as well as in ventricle and left systemic arch disappears, allowing only oxygenated blood to be pumped to head region and to the rest of body organs. In birds there is single valve between right atrium and ventricle. Sinus venosus has disappeared in birds.

MAMMALIAN HEART

            Mammals also have a four chambered heart and only one systemic arch on the left side, allowing no mixing of blood anywhere. There is no sinus venosus in both birds and mammals and pulmonary veins directly enter the right atrium. The SA node is also housed in the atrium now. In mammals sometimes there are extensions of atrial lumen to increase its area. These are called auricles. In monotremes the left atrio-ventricular valve is tricuspid.

ARTERIAL SYSTEM

            Arteries carry blood from heart to different organs. In fishes blood from ventral aorta is propelled into gills through the afferent branchial arteries and then collected by efferent branchial arteries and taken to the paired dorsal aorta and eventually to body organs. The arteries of pharyngeal region are called aortic arches. In basic pattern there are six aortic arches supplying gills and on the anterior side external and internal carotids take the blood to head and neck region.

CYCLOSTOMES      

            In Petromyzon there are 8 pairs of aortic arches, in Bdellostoma 15 pairs and in Myxine 6 pairs which correspond to the number of gills present in these fishes.

ELASMOBRANCHS

            Except for the primitive sharks, Hexanchus and Heptanchus, all other elasmobrachs possess 5 pairs of aortic arches because the first pair transforms into pseudobranchial artery.

TELEOSTS

            Bony fishes have 4 pairs of gills and hence four pairs of aortic arches are present and the first two disappear.

            Lung fishes have three pairs of gills but they also have lungs. The first arch disappears and the 2^nd, 5^th and 6^th supply the gills. The 3^rd and 4^th arches serve as gill bypass, allowing the blood to flow directly to body organs. Pulmonary artery that takes the blood to lungs emerges from the junction of the 6^th arch with the paired dorsal aorta.

URODELA

            Urodeles possess only three arches, namely the 3^rd, 4^th and 6^th. The connection of paired dorsal aorta between the 3^rd and 4^th which is called ductuscaroticus disappears in tailed amphibians, separating the blood supply to head region from rest of the body. Also due to the presence of tretrapod limbs subclavian arteries emerge from the systemic arch.

ANURA

            In frogs and toads the aortic arches are similar to urodeles, except that the pulmonary artery severs its connection with the paired dorsal aorta. This connection is known as ductus arteriosus in urodeles.

REPTILES

            In reptiles ventral aorta splits into two so that the left systemic arch emerges from the right ventricle and the right one emerges from the left. There is aforamen of panizza where both systemic arches cross each other. This foramen allows blood to pass from one systemic arch to another.

AVES

            Since the left systemic arch carries deoxygenated blood from the right side of ventricle, it disappears in birds, so that only oxygen rich blood from the left ventricle now flows in all body parts.

MAMMALS

            Mammals also possess only one systemic arch, which is the left one, but it emerges from the left ventricle and not from the right one as in reptiles. Therefore, it carries oxygen rich blood to all body organs.

VENOUS SYSTEM

            Veins bring blood from all body organs back to sinus venosus in basic pattern. Anterior cardinals collect blood from the head and neck region and post cardinals from kidneys and tail. They both join to form common cardinals that enter sinus venosus. Subclavians collect blood from the fore limbs and lateral abdominals from hind limbs. In embryonic system of veins, a pair of vitelline veins collect blood from the yolk sac.

FISHES

            In all fishes portal systems are formed. Hepatic portal system is formed by subintestinal vein that severs connection with caudal. Renal portal system is formed by postcardinal disconnecting before kidneys and a new postcardinal is formed on the inner side of kidney. Now blood from the tail region must pass through kidneys before it can go to sinus venosus. In bony fishes sometimes there are bypasses between the renal portal and new postcardinal vein to permit faster circulation of blood.

AMPHIBIA

            In amphibians a postcava develops that connects kidneys with sinus venosus but the postcardinal persists in urodeles and connects with renal portal vein. Both the lateral abdominals are fused in the middle in the abdominal region but remain separated in the hind limb region, where they are called pelvic veins. An iliac connection joins renal portal with pelvic veins.

            In frogs and toads owing to the absence of tail, the caudal splits to form two sciatic veins entering in each leg. Pelvic is called femoral in anurans. Hepatic portal is present in all vertebrates.

REPTILES

            Reptilian venous system is similar to the amphibian system except that the two lateral abdominal veins remain separated and there is only one pelvic vein collecting blood from the hind limbs. There is a caudal vein because reptiles sport a heavy tail.

AVES

            In birds there is no sinus venosus and hence precava enter directly into the right atrium and so does the postcava. Birds being warm blooded, there is a tendency to do away with renal portal system, so there develops a bypass between the renal portal vein and the postcava. Abdominal veins are absent or sometimes present in the form of a thin epigastric vein that connects caudal with the liver.

MAMMALS

            Mammals also have no sinus venosus and therefore right atrium receives both the precava and a postcava. Postcava does not end in kidneys but runs up to the tail and collects blood from all organs on the way including from kidneys. In mammals postcardinal veins persist in the form of a pair of azygos veins that collect blood from the backbone and costal muscles. In mammals, including man, left side of precava disappears requiring a brachiocephalic connection between the two subclavians to carry out uninterrupted flow of blood.

LYMPHATIC SYSTEM

            Lymphatic system is not an independent circulatory system but all major lymph channels eventually open into larger veins. Lymph channels collect materials that cannot be collected by blood capillaries as the walls of lymph capillaries are highly permeable and allow even tissue debris to pass through. Lymph channels collect tissue debris, intercellular fluid, pathogens, fats etc. and drain them into main veins after removing harmful materials.

            A true lymphatic system has not developed in cyclostomes. In fishes there are lymph channels that ultimately open into a pair of lateral channels. There is a paired lateral sinus on the anterior side of body and a ventral sinus, which serve to stores lymph in them. There are no lymph hearts or nodes in fishes. Amphibians too have a lymphatic system similar to fishes but they possess several pulsating lymph hearts to maintain flow of lymph. Frogs and toads have a pair each of cranial and inguinal lymph hearts. Frog also possesses extensive subcutaneous lymph sacs which prevent water loss from the body. Reptiles possess only posterior lymph hearts.

            Birds and mammals do not have lymph hearts but lymph nodes instead, which serve as filter beds for pathogens. Most mammals have a large lymph reservoir in the abdominal region called cysterna chyli which receives lipid carrying lymphatics from the intestine. The larger median thoracic lymph channel takes all the lymph collected in the cysterna chyli into precava.