The Cardiovascular System
The study of blood vascular and lymphatic systems officially is termed angiologia. Angiology strictly means the study of vessels, but its scope is conveniently enlarged to include the heart, spleen, and various lymphatic organs.
A circulatory system is essential to any organism that exceeds that relatively trivial size at which diffusion can deliver the metabolic fuel and take away the waste. That critical size is quickly realized in the rapidly growing embryo. Although the circulatory system is not the first to be laid down, it 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" are arranged as an endothelium whereas 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 one another and in relation to the appearance and growth of the regions and organs of the embryo.
The heart also appears early because pumping is required for proper circulation. 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 splanchnopleure. 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.
91). At this stage the heart consists of paired endothelial (endocardial) tubes placed ventral to the foregut, but these tubes shortly fuse to form a single median organ that gradually shifts caudally to the level of the thoracic somites (Fig. 7.1/5 and 7).At the outset, the heart is connected at one end with vessels that become the aorta and at the other with vessels that form the vitelline (omphalomesenteric) veins, the umbilical veins, and the cardinal veins, which drain the yolk sac, the chorioallantoic placement, and the body, respectively. 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 (Fig. 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). The developing circulatory system responds 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 and of the particularly dramatic changes that occur in the circulation at birth are found later in this chapter.