The exchanged with the surrounding tissue 146. Blood with

The circulatory system of mammals is a closed network encompassing heart, blood vessels and blood. Heart is a major organ, which pumps the blood through the vasculature, which can be distinguished, as pulmonary (by which blood is oxygenated in the lungs) and systemic (by which blood flows through the whole body). Systemic circulation allows the transport of oxygen and nutrient to all tissues as well as eliminates the waste from the body 143, 144. Moreover, the cardiovascular system distributes hormones, signaling molecules and regulates body temperature 144, 145. There are different categories of blood vessels referred as arteries, arterioles, capillaries and veins. 

Muscular contraction of the left heart ventricle pumps oxygenized blood into the aorta and through branching arteries into capillaries, in which oxygen (O2), carbon dioxide (CO2) and nutrient are exchanged with the surrounding tissue 146. Blood with low oxygen content is then transported back to the heart through veins and pumped by the right ventricle into the pulmonary circulation, where CO2 diffuses from the blood into the lung alveoli and O2 diffuses into the blood. The oxygenized blood then enters the body circulation through the left ventricle again. The circulatory system of mammals is a closed network encompassing heart, blood vessels and blood. Heart is a major organ, which pumps the blood through the vasculature, which can be distinguished, as pulmonary (by which blood is oxygenated in the lungs) and systemic (by which blood flows through the whole body). Systemic circulation allows the transport of oxygen and nutrient to all tissues as well as eliminates the waste from the body 143, 144. Moreover, the cardiovascular system distributes hormones, signaling molecules and regulates body temperature 144, 145. There are different categories of blood vessels referred as arteries, arterioles, capillaries and veins. 

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Muscular contraction of the left heart ventricle pumps oxygenized blood into the aorta and through branching arteries into capillaries, in which oxygen (O2), carbon dioxide (CO2) and nutrient are exchanged with the surrounding tissue 146. Blood with low oxygen content is then transported back to the heart through veins and pumped by the right ventricle into the pulmonary circulation, where CO2 diffuses from the blood into the lung alveoli and O2 diffuses into the blood. The oxygenized blood then enters the body circulation through the left ventricle again. The most complex and large blood vessels of the human body are arteries; they consist of the tunica intima as the most inner and thinnest layer, which comprises a single layer of endothelial cells surrounded by subendothelial connective tissue interlaced with the internal elastic membrane and the basement membrane. The tunica media mainly consists of smooth muscle cells and elastic tissue and is the thickest layer in arteries. Smooth muscle cells control the diameter of the vessel and therefore play an important role in the regulation of blood pressure. The tunica adventitia consists of connective tissue and contains nerves that supply the vessel, as well as capillaries in larger arteries. Arteries branch into arterioles, which furthermore branch into capillaries. Arterioles are covered with one or two layers of smooth muscle cells and are the primary sites of vascular resistance 144, 145.

The nutrient exchange takes place in capillaries, which form a tight network in the whole body. Capillaries consist of a single layer of endothelial cells with a supporting basement membrane and covered by pericytes. Three different types of capillaries exist, such as continuous, fenestrated and sinusoid capillaries. Each type manages different tasks, from the supply of surrounding tissue with small molecules, water and lipid-soluble molecules (continuous), to additional supply with larger molecules and limited amounts of proteins (fenestrated) to exchange of blood cells (sinusoid).

Oxygen and nutrient deprived blood gathers in venules and from their flows into veins. Veins usually contain up to 75% of the blood of the body and consist of a single layer of endothelial cells, a supporting basement membrane, a thin layer of smooth muscle cells and the adventitia, which is the thickest layer of veins. In contrast to arteries, veins possess valves to prevent blood from flowing backwards.

            Nascent vessels initially consist only of a monolayer consisting of endothelial cells (EC). The EC tubes are stabilized by a sparse coverage of pericytes (mural cells). With increased luminal diameter, the mural cell coverage also increases. Arterioles and venules are covered by vascular smooth muscle cells (VSMCs), which surround the endothelial tube for stabilization and regulating the vascular tone. The VSMC layers differ in thickness comparing arteries and veins. In the context the arterial V SMC layer is thicker providing biomechanical stability opposing higher blood pressure, whereas in veins the number of VSMCs is reduced to allow these vessel the storage of higher amounts of blood (capacitance vessels) compared to the strong but rigid larger arteries. The vessel surrounding tunica adventitia contains connective tissue consisting of collagen fibers to ensure vascular stability by anchoring the vessel to the surrounding tissue. Blood contains red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes). Erythrocyte delivers oxygen to the tissues while leukocytes are important for the immune responses. Plasma controls the intravascular osmotic effects containing proteins, glucose, clottings factors, electrolytes, hormones and some other required ingredients. Platelets act with clotting factors in order to stop bleeding and clogging blood vessel injuries respectively.