The cardiovascular system
The cardiovascular or circulatory system is a transport system that carries both blood and lymph. Its principle function is the transport of oxygen and nutrients as well as carbon dioxide and metabolic waste products to and from the tissues of the body. It’s also involved in temperature regulation, hormone distribution, and immunity. The cardiovascular system is comprised of the heart, major arteries, arterioles, capillaries, venules, and veins.
The heart is a muscular pump that contracts rhythmically, propelling blood through the cardiovascular system. Its walls consist of three major layers or tunics: the inner endocardium, the middle myocardium, and the outer epicardium.
The endocardium consists of a single layer of squamous endothelium and thin subendothelial connective tissue containing elastic and collagen fibers, as well as smooth muscle cells. Deep to this, is the subendocardial layer of connective tissue, where small blood vessels and Purkinje fibers can be found.
Purkinje fibers are specialized conducting fibers that are rich in mitochondria and glycogen. They extend from the interventricular septum, to the papillary muscles, and up the lateral walls of the heart’s ventricles.
The myocardium is the thickest of all three layers and consists of cardiac muscle fibers. The epicardium is made up of simple squamous mesothelium, and an underlying subepicardial layer containing coronary blood vessels and adipose tissue.
Structural plan of vessels
The function of the arterial system is to distribute blood from the heart to capillary beds throughout the body. Like the heart, blood vessel walls are comprised of three layers: the tunica adventitia, the tunica media, and the tunica intima.
The tunica adventitia/externa is the outer layer, consisting mainly of type I collagen and elastic fibers. It’s continuous with the stromal connective tissue of the organ through which the blood vessel runs.
The tunica media is the middle layer, comprised of concentric layers of helically arranged smooth muscle fibers. Within these fibers are variable amounts of elastic fibers, collagen type III fibers, proteoglycans, and glycoproteins. The tunica media has a thin external elastic lamina which separates it from the tunica adventitia.
The tunica intima/interna is the innermost layer. It has a layer of endothelial cells supported by a thin subendothelial layer of loose connective tissue with occasional smooth muscle cells. The intima has a fenestrated internal elastic lamina (the most eternal component of the intima) separating it from the tunica media.
The blood vessels are distinguished by the amount of elastin and smooth muscle fibers present in their walls. In general, the amount of elastic tissue decreases as the vessel becomes smaller, and the smooth muscle fibers become relatively more prominent.
There are three main types of arteries in the arterial system: elastic arteries, muscular arteries, and arterioles.
Elastic arteries include the aorta, the brachiocephalic trunk, the common carotids, the subclavian arteries, and most of the large pulmonary arterial vessels. They have a broad and highly elastic tunica media that helps in stabilizing the blood flow.
Muscular arteries are the main distributing branches of the arterial tree, for example, the radial, femoral, and cerebral arteries. Their tunica media has numerous layers of smooth muscle cells, which help in controlling blood flow by relaxing and contracting.
Arterioles are the terminal branches of the arterial tree. They only have a few layers of smooth muscle cells, and are responsible for supplying the capillary beds.
Capillaries are sites of metabolic exchange between blood and the surrounding tissues. They are comprised of a single layer of elongated polygonal endothelial cells rolled up in the form of a tube. Capillaries can be grouped into three types: continuous, fenestrated, and sinusoidal. This classification depends on the continuity of the endothelial cells and the external lamina.
The structure of the venous system is similar to the general three layered arrangement found in the arteries, however, the elastic and muscular components are far less prominent. Capillaries gradually unite to form venules. Venules accompany arterioles, and have a structure similar to the capillaries, with contractile cells called pericytes. The venous blood initially flows into smaller postcapillary venules, which then converge into larger collecting venules. Muscular venules are venules with greater size and a more distinguishable tunica media.
Venules converge into veins of increasing size. Veins can be classified by size: small, medium, and large. They accompany arteries, and compared to them, have thinner walls, larger diameters, and greater structural variation.
Most veins have valves, which help the venous blood to remain directed towards the heart, preventing backflow. These valves are made up of semilunar folds from the tunica intima that project into the lumen. Valves are rich in elastic fibers, and are more prominent in larger veins. Large veins have a well-developed tunica intima and adventitia, but the tunica media is thin, with few smooth muscle cells and abundant connective tissue.
Large vessels, be it arteries or veins, have vasa vasorum: ‘vessels of vessels’. Vasa vasorum are comprised of arterioles, capillaries, and venules present in the tunica adventitia and the outer part of the tunica media. They provide metabolites to the cells located in these layers since the walls of larger vessels are too thick to be nourished by diffusion from luminal blood alone. Veins typically have more vasa vasorum than arteries.