All vertebrates have circulatory systems based on the same development structure, and therefore they show less variety as compared to that of invertebrates. It is not possible to trace their evolution based on fossils because blood vessels do not fossilize the same way as bones and teeth do. One way of tracing their circulatory system is by studying and comparing different groups of vertebrates and keenly following how their embryos develop. The variations depend on different requirements like living in water and living on land. This paper is going to give a description of the classification of vertebrates, a brief history of their evolution and compare in detail the circulatory system of different groups of vertebrates and how they came to be.
Classification of Vertebrates
Vertebrates are a classification of Chordates (phylum Chordata) with a spinal cord. Chordates have notochords, jointed appendages and are segmented. The distinct features of vertebrates from other chordates are found in their structure and development process which help trace their evolution. Vertebrate’s notochord is replaced by a bony vertebral column in the course of their embryonic development (Green, Stephen A., Marcos, and Marianne 477). Also all vertebrates except the earliest fishes have a well-differentiated head. They have skull and brain. Other features include the existence of a group of embryonic cells called neutral crest that contributes to the development of most of the vertebrates’ structures, an existence of internal organs like liver and kidneys and existence of an endoskeleton which is made of cartilage or bones in most vertebrates.
Brief History of Vertebrate’s Evolution
The first vertebrates to evolve were jawless fishes with one caudal fin over 470 million years ago in the oceans. Amphibians descended them and invaded the land. That amphibian gave rise to first reptiles that were better suited to living on land which were replaced by today’s dominant land vertebrates. Those reptiles evolved in sizes from very small animals to animals that are the size of a truck. Among them are the great terrestrial vertebrates, the birds, and the mammals. The history of vertebrates has been evolution advances that have allowed vertebrates to first invade the sea then the land. Their evolution regarding the development of structures including that of the circulatory system involves invasions in the sea, land, and air.
Vertebrates’ Circulatory Systems and how they came to be
The circulatory system varies from a simple circulatory system in invertebrates to a complex circulatory system in vertebrates. Vertebrates are characterized by a closed circulatory system. The variation in the structure of the heart and the circulation of blood between different groups of vertebrates is due to adaptation during their evolution process. When the structures are compared, they show clear records of the evolutionary process. These circulatory systems in different groups of vertebrates include:
• Fish circulatory system
• Amphibians circulatory system
• Reptiles circulatory system
• Mammal and bird circulatory systems
Fish Circulatory Systems. The fish circulatory system has a single circuit flow with a two-chambered heart, a single atrium and a single vertical. The atrium receives blood from the body to the heart while the ventricle pumps blood from the earth to the gills where oxygenation of blood takes place in a process called gill circulation and the blood proceeds to the rest of the body before arriving back to the atrium.
The fishes’ heart shows little modification from the basic plan of vertebrates. The arterial system of the fishes has afferent and efferent parts of the gill blood vessels. These blood vessels are called arterial arches. When the embryos were developing, the arterials arches were interrupted by the capillaries in the gills (Kardong 187). This structured them such that each arch consisted of a ventral afferent section collecting blood from the gills capillaries and taking it to the dorsal aorta. The fish circulatory system is a one-way arrangement within the heart pumping deoxygenated blood to the gills for oxygenation and the distributing it to the body. The embryo of a fish has six-gill slits which are retained by some few adult fish, but most adult fish have four of the six-gill slits after one slit became the spiracle, (the first branchial arch) while the other gill is the variable second branchial arch. Another modification is that the external carotid is connected with the efferent portion of the second branchial arch instead of arising from ventral aorta anterior part. This modification ensures that oxygenated blood reaches the head despite the modifications of different anterior and arterial arches. It may have happened that poorly oxygenated water in certain habitats led to the evolution of lungs to get oxygen from the air hence the land vertebrates. Fish also have structures for obtaining oxygen from the air. These are the modified gill chambers with the dense network of capillaries. Lungs in fish may have evolved from the swim bladder found in most fishes. The core function of the swim bladder is obtaining balance in fish, but it can also act as oxygen reserve for the highly oxygenated blood obtained from its blood supply. The veins would be expected to be here if they were bringing oxygenated blood from the body to the heart.
The lung fishes have been modified further to having lungs although not all species breathe the same extend. Some of their modifications can be related to the changes that have taken place in amphibians. Their lungs can be seen working side by side with the gills. Their circulatory systems are strictly the same as those of amphibians, and although the lung fishes do not seem to have descended from amphibians, they relate to fishes that gave rise to amphibians (Cieri, Robert L., et al. 17219).
Fig 1: Fish circulatory system
Fig 2: Lung fish Circulatory system
Amphibian’s Circulatory System. In amphibians, reptiles, birds and mammals there is a two circuit blood circulation. One circulation is blood through the lungs and then back to the hearts while the other flows throughout the body including the brain. The two circulations are called pulmonary circulation and systematic circulation respectively. Like the fish, amphibians have two chambered heart but with two atria and one ventricle. The atria receive blood from the two circulations. Blood mixes in the heart reducing the efficiency of oxygen. The arrangement ensures the highly pressured blood in the vessels is pushed to the lungs. The mixing of blood is because of the diversion of oxygenated blood to the systematic circulation and the deoxygenated blood to the pulmonary circuit. This is the reason amphibian are said to have double circulation.
Fig 3: Circulatory system of amphibians
The skin of amphibians is moistened by mucous secretions where carbon dioxide may pass out of the body through the skin. Blood traveling to the atria comprise of that from the skin and lungs. Comparing these to lungless salamanders, respiration occurs entirely through the skin, but for salamanders with lungs, carbon dioxide can pass out of the body through the skin. The lungless salamanders do not have an atrial septum, but one small group called caecilians have a septum in the ventricle. The absence of septum in some modern form may explain how these animals were evolving into adapting the skin or the lungs or both for respiration. There is another relative similarity between amphibians and fish circulatory system. In amphibians, there is a spiral valve contained in the conus arteriosus which maintains separation ensuring blood is directed to the correct atrium. This is similar in lungfishes. Blood from the lungs and skin is directed by the spiral valve where though some mixing occurs, blood is directed into the arterial arch to the body as the other is directed to the left atrium to the head.
The venous system of amphibians shows features of land vertebrates. An example is the posterior cardinal veins which are replaced by posterior vena cava which are still visible in salamanders. Another related feature is the larvae of amphibians which has gills. There are also four arterial arches in salamanders. These features show that vertebrates have similar origin and the variation occurred as the species adapted to living in their habitats.
Reptile Circulatory Systems. The heart of most reptiles is divided into three chambers similar to that of amphibians. Its heart directs blood to both pulmonary and systematic circuits. Unlike the lungfishes and amphibians, the reptiles entirely use their lungs for respiration and gill and skin are not an additional source of oxygen as with the case of lungfishes and amphibians. Only the crocodiles almost near mammals and birds in having an almost complete double circulation. This can be explained by their elongated neck region in which its development may have displaced the heart and altered the arrangement of arteries and veins. However, the general circulation resembles that of a frog.
There are various changes seen in the hearts of a reptile. The left atrium is smaller and completely separate from the right atrium. These additional modifications have reduced the mixing of blood even further. The ventricle is subdivided into three arterial trunks namely the right and left systematic arches and the pulmonary arch. The two atria separate the oxygenated blood to the carotid arteries and direct the deoxygenated blood to the lungs.
The crocodile are slightly distinct because they are the only living representative of the group from which the birds evolved that included dinosaurs. The group is called the archosaurian reptiles. Crocodile’s heart has a complete ventricular septum and two equally sized chambers. Blood does not mix between the chambers, and this serves them good when diving such that blood flow to the lungs is decreased.
Fig 4: Circulatory system of a reptile
When comparing the amphibian and reptile respiratory system, it is evident that the lungs involved a major change in the pattern of circulation resulting in two circulations. If no changes had occurred in the structure of the heart, then it means the oxygenated and deoxygenated blood would mix in the heart. The result would be pumping of this mixture instead of only the oxygenated blood.
Mammal and Bird Circulatory System. The heart of bird and mammals is used to deliver oxygen and nutrients in the body. Mammals evolved from reptiles though not from the same group where reptiles evolved from. Studies show that mammals must have developed their circulation system independently from the early reptiles. Nevertheless, the heart of mammals, birds, and crocodiles developed into four chambers each with two separate atria and two separate ventricles. The purpose of the four chambers is to separate the oxygenated and deoxygenated blood. Other animals have two chambers or no chambers and therefore all blood mixes up. Birds and mammals’ four chambered hearts are the most efficient in separating oxygenated and deoxygenated blood.As Jensen, Bjarke, et al. (786) states, the atrias function exchange the deoxygenated blood from the body through the right atrium which directs it to the right ventricle. The right ventricle pumps it to, lungs for oxygenation and directs it to the left ventricle which then circulates this oxygenated blood to other body parts. The heart has been modified into an efficient working mechanism. The two atrias contract simultaneously pumping blood to the two ventricles which in turn contract simultaneously pushing blood to the two circuits namely the pulmonary and systematic circuit. As Jensen, Bjarke, et al. (787) states the increased efficiency of the double circulatory system in mammals and birds is said to have been important in the warm-blooded species since they needed it to adopt to the high rate of metabolism.
There are slight differences between birds and mammals’ hearts which are based on the adaptation of their lifestyle. Birds need a body that enables them to fly other than mammals who entirely live on land. These slight differences include the size of their hearts where birds have proportionately larger hearts as compared to mammals because flight requires a lot of energy. Hamming birds, for example, have bigger hearts as compared to their size and fast heart rate to enable them to hover in flight. Birds’ circulatory systems also have extra blood capillaries to transport nutrients and oxygen during aviation. This special structure of their heart makes them be said to have aviation hearts. These aviation hearts pumps more blood than the mammals’ hearts. The only major difference between birds and human hearts is that birds have one aortic arch on the right side of their body while humans have one on the lefts side.
Fig 5: Circulatory system of a mammal
Throughout the history of evolution, the modifications of the vertebrate’s circulatory system can be traced from the fish circulatory system to the mammalian and bird’s circulatory system with the sinus venosus serving as the pacemaker (Jensen, Bjarke, et al. 22). Sinus venosus is the site where hearts impulses are initiated. Sinus venosus forms a major chamber in the heart of a fish. It is then reduced in size in amphibians and even further in reptiles. This reduction continues in mammals and birds such that it almost disappears. Traces of sinus venosus can be seen as tissue remains in walls of the right atrium in mammals and birds. The tissue is referred to as ‘sinoatrial node,’ and this is still the site where heartbeats begin.
If we analyze the embryonic development of human heart and its great vessels, the different stages in the development process resemble that of fish, amphibians, and reptiles thus making sense of the evolutionary process. The first stage of human heart development is the forming of elongated tubes with thick walls. This occurs as the fourth week begins. At this point, there is only one atrium that received blood from the body, and there is also a single ventricle which takes blood ways from the heart. As days go by, this tube heart takes a compact shape, and by the end of week four the sinus venosus which forms the end of one structure get absorbed into the atrium while the bulbus cordis (the other end of the structure) gets absorbed into the ventricles (Kardong 286).By week six the single atrium and the single ventricle gets partitioned into the four-chambered heart, we know today. This process resembles the process of development of the rest of the vertebrates. The tube hearts resembles to the early structure of the heart as the fish develops. The adult fish ends up with only a single atrium and a single ventricle. In amphibians, the same process unfolds the same way as in fish but further the single atrium gets divided into two, but the ventricle remains single. The same sequence occurs in reptiles, but additionally, the atrias gets divided into two. The ventricle division varies in different types of reptiles, but it is complete in crocodiles. Thus this evidence shows that if evolution is true then, each vertebrate’s group evolved by being modified further from the pre-existing group. Fish were the first vertebrates to evolve followed by amphibians, then reptiles and eventually mammals and birds.
In summary, the evolution changes started from the development of the heart. The atrium was the first to separate into right and left atrium then the ventricles divided into right and left ventricle. Deoxygenated blood was separated from the oxygenated blood in the right side of the heart and the left side of the heart respectively. The separation is not yet complete, but the trabeculae help to maintain the separation. Aortic arches changes followed where arches 1, 2 and 3 were lost. Arches 4 and 5 formed the pulmonary circulation while both sides of arch 6 were maintained by reptiles. Birds lost the left side of arch 6 while mammals lost the right side of arch 6.Separation of oxygenated and deoxygenated blood resulted in changes from gill to lung respiration as species adapted the terrestrial life from aquatic life. These changes explain the development of lungfish from a single circulatory system to a double circulatory system. Vertebrates have a closed circulatory system where blood stays within the vessels as it travels away and back from the heart. The heart of the fish has two chambers, amphibians and reptiles have three chambers while mammals have four chambers. If evolution exits, then we should expect more modified organism after us as the process continues. The structural characteristics of the vertebrate’s heart and its great arterial vessel have a sole role of constructing a modified circulatory system in different species. The fact that all vertebrates exhibit six arch arteries in their development is evidence that they all inherited the pattern from a common ancestor which was used in developing the respiratory structures.
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