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 aorticarches. 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 2nd, 5th and 6th supply the gills. The 3rd and 4th 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 6th arch with the paired dorsal aorta.
URODELA
Urodeles possess only three arches, namely the 3rd, 4th and 6th. The connection of paired dorsal aorta between the 3rd and 4th 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 ductusarteriosus 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 a foramenofpanizza 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 lateralchannels. There is a paired lateralsinus on the anterior side of body and a ventralsinus, 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 lymphhearts 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 lymphnodes instead, which serve as filter beds for pathogens. Most mammals have a large lymph reservoir in the abdominal region called cysternachyli which receives lipid carrying lymphatics from the intestine. The larger median thoracic lymph channel takes all the lymph collected in the cysternachyli into precava.