The vertebrate circulatory system

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.

Deoxygenated blood collects in capillaries and then drains into larger and larger veins, which take it from various parts of the body to the heart. Of these, the anterior and posterior cardinal veins, each with left and right components, take blood to the heart from the front and rear of the body, respectively. They lie dorsal to the alimentary canal, while the heart lies ventral to it. There is a common cardinal vein on each side, often called the duct of Cuvier, which carries blood ventrally into the sinus venosus. Various other veins join the cardinal veins from all over the body. The ventral jugular veins drain the lower part of the head and take blood directly into the common cardinal veins.

Lower vertebrates have two so-called portal systems, areas of the venous system that begin in capillaries in tissues and join to form veins, which divide to produce another capillary network en route to the heart. They are called the hepatic (liver) and renal (kidneys) portal systems. The hepatic system is important because it collects blood from the intestine and passes it to the liver, the centre for many chemical reactions concerned with the absorption of food into the body and the control of substances entering the general circulation. The function of the renal portal system is less clear, but it involves two veins that pass from the caudal vein to the kidneys, where they break up into capillaries.

The coronary circulation is that which supplies the heart muscle itself. It is of crucial importance, for the heart must never stop beating. Cardiac muscle needs oxygen from early in embryonic development until death. In mammals the coronary blood supply comes from the aorta, close to the heart. In evolutionary terms, this was not always so; many lower vertebrates, including agnathans and amphibians, have no specialized coronary arteries. The heart obtains its oxygen from blood passing through it. Fish have well-developed coronary vessels that arise from various sources, but ultimately from the efferent branchial system.

The introduction of lungs changed the site of oxygenation of the blood. In lungfishes coronary arteries arise from those anterior arterial arches receiving the most oxygenated blood from the heart. In reptiles coronary arteries branch from the systemic arch, but their position of origin varies. In some species they arise close to the heart, as in birds and mammals. Coronary veins generally run beside corresponding arteries but diverge from them to enter the main venous supply to the right atrium, or to the sinus venosus in fishes.