Circulation in agnathans

In the lamprey heart the atrium and ventricle are side by side, with the sinus venosus entering the atrium laterally. Nonmuscular valves prevent backflow of blood, and the conus arteriosus contains no cardiac muscle. There is no separate coronary blood supply, and the heart must obtain its oxygen from the blood as it goes through.

The arterial system in agnathans is most obviously modified because there are more than six sets of gills. Eight branches emerge from the ventral aorta, which splits into two, unlike the single vessel in most vertebrates with gill slits. Oxygenated blood from the gills is then collected into eight efferent vessels, which join to form a dorsal aorta, single for most of its length. Internal carotid arteries arise from the dorsal aorta, but the ventral part of the head is supplied from anterior efferent branchial (gill) vessels, not from the anterior part of the ventral aorta.

The venous system does not include a renal portal section, and there is asymmetry of the common cardinal veins, which take blood from the dorsal anterior and posterior cardinal veins down to the ventral heart. In embryos there are two of these, one on each side of the body; in lampreys, the left one disappears during development, while in hagfishes the right one disappears. Hagfishes also have accessory hearts in the venous system at several points. No other vertebrate has these structures.

It is not clear why the circulatory system of agnathans differs in these ways from the basic vertebrate plan. It is important to remember, however, that lampreys and hagfishes are specialized descendants of what was once a more diverse and widespread group of animals. Their circulatory systems may be less similar to the basic vertebrate plan than those of their ancestors because of their present mode of life.
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.

Land vertebrates use their lungs to exchange carbon dioxide for oxygen from the air. Lungs may have evolved from a structure in fishes called the swim bladder, a sac that grows out from the anterior part of the gut. Fishes use it for buoyancy control, but it is possible that it was originally useful as an accessory for respiration. The problem is that lungs are found at a different site in the circulatory system from that of the gills, where oxygenation occurs in fishes. Instead of circulating around the body, as it does in fishes, oxygenated blood from the lungs returns to the heart. Therefore, in evolutionary terms, if mixing of oxygenated and deoxygenated blood was to be avoided in the heart, alterations to its structure had to occur. Land vertebrates developed lungs, a new vein (the pulmonary vein) to take blood from them to the heart, and a double circulation, whereby the heart is effectively divided into two halves—one-half concerned with pumping incoming deoxygenated blood from the body to the lungs and the other with pumping oxygenated blood from the lungs around the body.

There are also modifications in the arterial and venous systems related to the appearance of lungs in the circulation. In the venous system, the paired posterior cardinal veins are replaced by a single posterior vena cava, and the renal portal system disappears. The main modifications to the basic arterial pattern involve what are the gill arteries of fishes. The anterior of these became responsible for carrying oxygenated blood to the head and to the brain; the intermediate arteries for carrying blood to the dorsal aorta, and so around the body; and the posterior arteries for carrying deoxygenated blood to the lungs.

In fishes the four chambers of the heart are all well developed. Blood passes in sequence through the sinus venosus, atrium, ventricle, and conus arteriosus. The ventricle is the main pumping chamber, as it is in the hearts of all land vertebrates. During the evolution of the heart, the ventricle and atrium came to predominate; the sinus venosus became part of the atrium, while the conus arteriosus was incorporated into the ventricle. The atrium itself became a double structure—the two auricles—as did the ventricle, but the conversion of the ventricle into two chambers occurred later in evolution than the division of the atrium.
Modifications among the vertebrate classes
Fishes

The hearts of fishes show little modification from the basic plan, except that lungfish hearts tend to become subdivided. In them, the oxygenated blood carried by the pulmonary vein does not enter the sinus venosus along with the deoxygenated blood from the body. Instead, the oxygenated blood remains separate and enters the left side of the atrium. The atrium is partially divided into two auricles, and the ventricle also has a partial septum. Lungfishes show further signs of circulatory developments in their venous system. As in land vertebrates, there is a median posterior vena cava, and the posterior cardinal veins are reduced.