The cardiovascular system is composed
of the heart and of blood vessels. The heart consists of two
ventricles ?? right and left ?? that pump the blood out of the
heart through arteries, and two atria, functioning as reservoirs for
the ventricles, where the blood is brought back to the heart, by
veins, from the body (deoxygenated) or lungs (oxygenated). Blood
passes from the atria to the ventricles through a one-way valve.
Blood circulation has two ??branches? that have a simultaneous and continuous blood flow. The right ventricle pumps blood (deoxygenated, previously gathered from the body into the right atrium) to the lungs through the pulmonary artery, where it is oxygenated; the oxygenated blood is then carried back to the heart by the pulmonary veins, into the left atrium (this is the pulmonary circulation). At the same time, the left ventricle, having received the oxygenated blood from the left atrium, pumps it into the aorta artery, which then distributes it throughout the entire body, by means of small arteries, arterioles and capillaries. After the oxygen and metabolites pass through the microscopic capillaries into the cells by diffusion, the venous (deoxygenated) blood is collected, from capillaries, by the venous system (venules, small veins) and enters the right atrium via the two venae cavae. This is called systemic circulation.
The aorta and the pulmonary artery divide into smaller arteries, which branch to form tiny arteries of 0.1-0.5 mm, called arterioles. From arterioles, the blood passes to the tiny capillaries (4-7?m). Capillaries are also responsible for taking up the venous blood and conducting it to venules, which connect to small veins, these latter ones forming the larger veins of the body.
The structure of all blood vessels, except capillaries, comprises three layers: the innermost layer is called tunica intima, the middle one - tunica media and the outer layer - tunica adventitia. The intima consists of a sheet of flat endothelial cells resting on a layer of connective tissue; it has the role of secreting many vasoactive compounds, but is mechanically weak. It is the media that supplies the mechanical strength, as it is composed of smooth muscle cells disposed circularly and embedded in a matrix of collagen and elastin fibres; this layer also has contractile power. The adventitia is the outer protective layer composed of connective tissue. In larger arteries, the adventitia contains tiny blood vessels (??vasa vasorum?), which have the role of nourishing the media.
Each type of vessel is particularly structured so as to adapt to one extra function, besides conducting blood. From the functional point of view, we can divide the blood vessels into the following categories:
Elastic arteries, such as the aorta, pulmonary artery, and other major arteries (the iliac artery), that have a diameter of 1-2 cm in men and owe their characteristic to the elastin found in the tunica media; the elastin fibers allow the large arteries to expand when they receive the stroke volume pumped out by the ventricles, as well as to relax in the diastolic period. This helps the flow of blood to smaller vessels.
Conduit arteries (muscular arteries), which are the medium to small arteries and have a diameter of 0.1-1 cm in man (e.g. the cerebral, coronary, popliteal arteries); their tunica media contains more smooth muscle, and is thicker relative to the vessel lumen which makes them very resistant, thus preventing collapse at joints (the knee joint, for example). They also have the ability to contract, which can prevent severe bleeding in critical situations.
Resistance vessels - the smallest arteries (100-500?m), which have a high resistance to blood flow, due to their narrow lumen and limited number; they act as ??taps? for the local circulation, turning the flow up or down, to match the needs (when they dilate, the resistance to flow falls, so blood flow increases, and when they contract, they allow lesser blood flow).
Exchange vessels ?? the most numerous and the smallest of all vessels: the capillaries and the post-capillary venules. The capillaries are situated at 10-20?m from the cells, and their thin walls (a single layer of endothelial cells) facilitate the gas and metabolite transfer.
Capacitance vessels ?? the veins and venules, which differ in size and number rather than in structure. Their tunica media is thin and composed of smooth muscle and collagen. The intima of the limb veins has pairs of semilunar valves that prevent the backflow of blood. All of them offer weak resistance to blood flow, so a small blood pressure is sufficient to ensure flow from venules to the inferior and superior vena cava. Their name comes from the fact that, being much more numerous than the arterioles and arteries, approximately 2/3 of the circulating blood is found in the venous system at any time.
The arterio-venous anastomosis (anastomosis=interconnection) occurs particularly in the skin tissues and nasal mucosa, where shunt vessels having a diameter of 20-135 ?m connect arterioles to venules directly. They have an important role in temperature regulation (skin) and warm up the air that comes into contact with the nasal mucosa.
The distribution of blood to tissues and organs is actively controlled, so as each of them receives blood in proportion to their activity.
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