How can reproduction and circulation in vertebrates taken as an increase in complexity of living organisms?
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ans.The VertebrateCirculatory System
The basic vertebrate pattern
The plan
All vertebrates have circulatory systems based on a common plan, and so vertebrate systems show much less variety than do those of invertebrates. Although it is impossible to trace the evolution of the circulatory system by using fossils (because blood vessels do not fossilize as do bones and teeth), it is possible to theorize on its evolution by studying different groups of vertebrates and their developing embryos. Many of the variations from the common plan are related to the different requirements of living in water and on land.
The heart
The vertebrate heart lies below the alimentary canal in the front and centre of the chest, housed in its own section of the body cavity. During the development of an embryo, the heart first appears below the pharynx, and although it may also be in this position in adult animals, the heart often moves posteriorly as the animal grows and matures.
The heart is basically a tube made of special muscle (cardiac muscle) that is not found anywhere else in the body. This cardiac muscle beats throughout life with its own automatic rhythm. Deoxygenated blood from the body is brought by veins into the most posterior part of the heart tube, the sinus venosus. From there it passes forward into the atrium, the ventricle, and the conus arteriosus (called the bulbus cordis in embryos), and eventually to the arterial system. The blood is pushed through the heart because the various parts of the tube contract in sequence. As the heart develops from embryo to adult, each part of the tube becomes a chamber, separated from the others by valves, so that blood can neither flow backward in the system nor reenter the heart from the arteries. As the heart grows, it bends into an “S” shape, so that the sinus venosus and atrium lie above the ventricle and conus arteriosus.
The blood vessels
Gill slits are a fundamental feature of all vertebrate embryos, including humans. With few exceptions, there are six gill slits on each side. Blood leaving the heart travels from the conus arteriosus into the ventral aorta, which branches to send six pairs of arteries between the gill slits. The arterial branches join the dorsal aorta above the alimentary canal. Anterior to the gill slits, the ventral aorta branches again, forming two external carotid arteries that supply the ventral part of the head. Two internal carotids, which are the anterior extensions of the dorsal aorta, supply the brain in the dorsal part of the head.
Circulation in jawed vertebrates
Although clearly related to its mode of life, the blood system of a species also reflects its evolutionary history. The most significant change that occurred during early vertebrate evolution was the appearance of animals that could live and breathe on land. The first of these were the amphibians. Reptiles became even more independent of water because of their waterproof skins and shelled eggs, and from them evolved the most sophisticated land vertebrates, the mammals and birds. Obtaining oxygen entirely from air, instead of from water, involved drastic changes in the circulatory system.
The basic vertebrate pattern
The plan
All vertebrates have circulatory systems based on a common plan, and so vertebrate systems show much less variety than do those of invertebrates. Although it is impossible to trace the evolution of the circulatory system by using fossils (because blood vessels do not fossilize as do bones and teeth), it is possible to theorize on its evolution by studying different groups of vertebrates and their developing embryos. Many of the variations from the common plan are related to the different requirements of living in water and on land.
The heart
The vertebrate heart lies below the alimentary canal in the front and centre of the chest, housed in its own section of the body cavity. During the development of an embryo, the heart first appears below the pharynx, and although it may also be in this position in adult animals, the heart often moves posteriorly as the animal grows and matures.
The heart is basically a tube made of special muscle (cardiac muscle) that is not found anywhere else in the body. This cardiac muscle beats throughout life with its own automatic rhythm. Deoxygenated blood from the body is brought by veins into the most posterior part of the heart tube, the sinus venosus. From there it passes forward into the atrium, the ventricle, and the conus arteriosus (called the bulbus cordis in embryos), and eventually to the arterial system. The blood is pushed through the heart because the various parts of the tube contract in sequence. As the heart develops from embryo to adult, each part of the tube becomes a chamber, separated from the others by valves, so that blood can neither flow backward in the system nor reenter the heart from the arteries. As the heart grows, it bends into an “S” shape, so that the sinus venosus and atrium lie above the ventricle and conus arteriosus.
The blood vessels
Gill slits are a fundamental feature of all vertebrate embryos, including humans. With few exceptions, there are six gill slits on each side. Blood leaving the heart travels from the conus arteriosus into the ventral aorta, which branches to send six pairs of arteries between the gill slits. The arterial branches join the dorsal aorta above the alimentary canal. Anterior to the gill slits, the ventral aorta branches again, forming two external carotid arteries that supply the ventral part of the head. Two internal carotids, which are the anterior extensions of the dorsal aorta, supply the brain in the dorsal part of the head.
Circulation in jawed vertebrates
Although clearly related to its mode of life, the blood system of a species also reflects its evolutionary history. The most significant change that occurred during early vertebrate evolution was the appearance of animals that could live and breathe on land. The first of these were the amphibians. Reptiles became even more independent of water because of their waterproof skins and shelled eggs, and from them evolved the most sophisticated land vertebrates, the mammals and birds. Obtaining oxygen entirely from air, instead of from water, involved drastic changes in the circulatory system.
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