Introduction to the Circulatory System

All organisms, except for the very small, must have an internal transport system (1). Circulatory system is a type of a transport system that allows the transport gasses, nutrients, metabolic wastes, hormones and antibodies. Small organisms are structured in a way that allows their cells to be in close proximity to an aqueous environment eliminating the need for the transport system. Large organisms such as vertebrates, have the most complex and highly evolved circulatory system. The circulatory system of such animals functions in conjunction with the kidneys, liver, and many other organs and responds to a variety of changing external conditions helping the organism maintain constant internal environment (homeostasis).

The Lymphatic System in Relation to a Cardiovascular System

Figure 1. Lymphatic vessels pick up fluid in the tissues and return it to the blood in vessels near the heart.

Image Credit: Encyclopedia.bg.com 

Circulatory system is basically a set of connective tubes that transport fluid (2). The vertebrate circulatory system has two components: blood and lymph vascular systems. The lymphatic system consists of lymphatic vessels and the lymph, the fluid they carry. The blood-vascular system of vertebrates  unlike that of invertebrates (except for annelids), is a continuum of ducts and thus it is a closed system (1). Yet, fluid constituents of the blood leak out of the capillaries into the tissue fluids.This fluid does not accumulate in tissue, however, because it is drained away by the second component of the circulatory system - the lymphatic system. Tissue fluids enter net-like lymphatic capillaries where they constitute lymph. Lymph, then travels through the progressively larger lymphatic vessels until it is drained into the venous system at several points in the body. The heart provides no pressure to drive the lymphatic fluid circulation, therefore it relies exclusively upon the pressures generated by the respiratory and other body motions, as well as on their own intrinsic musculature. Like the veins of the cardiovascular system, the lymphatic vessel have one-way valves to prevent retrograde flow of  lymph. Many lower vertebrates also have lymph sinuses, some of which beat weakly as lymphatic hearts.

Ontogeny and Phylogeny

Ontogeny and phylogeny are more strikingly related in the circulatory system compared to any other organ system (1).  Embryonic hearts, arteries and veins of higher vertebrates closely resemble the corresponding organs of remote ancestors. For example, the pattern of circulation changes in embryonic development in much the same way as it must have changed in evolution. These marked similarities are useful in systematics. This is particularly true of the heart and major circuits at levels of the class and subclass, and patterns of arteries, in the limbs and basicranial area at levels of  the order and family. While there are obvious fundamental differences between fish and humans, there is a high level of genetic conservation, and, although morphological differences might suggest distinct developmental processes, it is now clear that evolution has utilized the same basic building blocks to create even the most diverse structures and animals (6).

The heart as an organ originated in the distant past. It is thought that the first organ that resembled a heart is have originated over 500 million years ago in an ancestral bilaterian. It is likely that it was a simple tubular vessel-like organs of tunicates and amphioxus, which contain a myoepithelial layer of cells and lacked chambers or valves.  The diversification of muscle cells led to the origin of skeletal, cardiac, and smooth muscle cells, and additional specialization of cardiac muscle cells ultimately yielded atrial and ventricular myocytes, as well as the cells of the mammalian cardiac conduction system (9). These cells are some of the the first to form and function during embryogenesis (7).

The Developing "Heart" in Amphioxus

Fig. 6. (A) Living 3-day larva with subenteric vessel (arrow) beginning to form ventral to hindgut; scale line, 50 μm. (B) Section through level of arrowhead in (A); the arrow indicates the heart (subenteric vessel); scale line 25 μm. Image Credit: ScienceDirect

Cardiac Muscle

Cardiac muscle or myocardium exhibits many structural and functional characteristics intermediate between those of skeletal and visceral muscle. Like the former, its contractions are strong and utilise a great deal of energy, and like the latter the contractions are continuous and initiated by inherent mechanisms, although they are modulated by external autonomic and hormonal stimuli.

Cardiac muscle fibres are essentially long cylindrical cells with one or at most two nuclei, centrally located within the cell. The ends of the fibres are split longitudinally into a small number of branches, the ends of which abut onto similar branches of adjacent cells giving the impression of a continuous three-dimensional cytoplasmic network; this was formerly described as a syncytium before the discrete intercellular boundaries were recognised.

Between the muscle fibres, delicate collagenous tissue analogous to the endomysium of skeletal muscle supports the extremely rich capillary network necessary to meet the high metabolic demands of strong continuous activity.