The blood vascular and lymphatic systems are com­bined under a single heading, angiologia, in the official terminology.

Angiology strictly means the study of vessels, but its scope is conveniently enlarged to include the heart, spleen, and various lymphatic organs in addition to the arteries, veins, and other vessels.

A circulatory system is essential to any organism that exceeds that relatively trivial size in which diffusion can deliver the metabolic fuel and other substances required by the tissues and convey away their products, whether waste for excretion or materials that are utilized else­where. Obviously, the critical mass must vary with the level of metabolic activity. It is soon reached in the rapidly growing mammalian embryo, in which the cir­culatory system, although not the first to be laid down, is the first body system to reach a “working state.”

The circulatory organs and the blood cells have a common origin in clusters of mesenchymal cells that first appear in the wall of the yolk sac. The outermost cells of these “blood islets” flatten and become arranged as an endothelium that lines spaces in which the remain­ing cells, hemocytoblasts or stem blood cells, float within a fluid plasma. The islets first formed are soon supplemented by others that appear in the mesoderm of the chorioallantois and within the body of the embryo; as the various patches spread and link up they form a diffuse system of connecting vessels that is then extended further by branching from existing channels. The principal vessels thus form independently of each other and in relation to the appearance and growth of the regions and organs of the embryo.

Because no proper circulation through this system can occur until a means of pumping blood is created, the heart necessarily makes a very early appearance. It is formed by differentiation of channels within a part of the mesoderm appropriately known as the cardio­genic area. This area lies in front of the oral membrane of the discoidal embryo, and the heart rudiments are related from the outset to the most rostral of the tissue spaces that later coalesce to form the celomic cavity, which divides the somatopleure from the splanchno- pleure. The cardiogenic area, including both heart and pericardial rudiments, becomes folded ventrally and carried caudally in the process that converts the embryonic disk into a cylindrical body (p.

From the beginning the heart is connected at one extremity with the vessels that become the aorta and at the other with those that form three sets of veins: the vitelline (omphalomesenteric) veins that drain the yolk sac, the umbilical veins that drain the chorioallantoic placenta, and the cardinal veins that drain the body. The ventral aorta, continuous with the heart, is soon joined to an independently formed dorsal aorta by a system of aortic loops contained within the pharyngeal (bran­chial) arches lateral to the pharynx (Figure 7-2). It is possible to trace the origin of certain arteries of adult anatomy from the six pairs of aortic arches that develop (although not all persist), but the reader must refer to textbooks of embryology for details of this process and for a description of the even more complicated evolu­tion of the veins. The reader is reminded that a hallmark of the developing circulatory system is its ability to respond to changing functional requirements by refash­ioning the pattern of vessels, always retaining obsoles­cent parts until their replacements have become operative.

Descriptions of the development of the heart itself (p. 234) and of the particularly dramatic changes that occur in the circulation at birth (p. 256) are found later in this chapter.