Although the heart plays one of the most important roles in the circulatory system, the rest of the system is also very important as it is what transports blood to and from the heart to keep the body alive . Blood is transported throughout the body by blood vessels. Blood vessels are complex structures but can be simplified quite significantly. The two main components of blood vessels are arteries and veins. Arteries carry oxygenated blood away from the heart, while veins carry deoxygenated blood back to the heart. Other blood vessels present in the circulatory system are arterioles and venules. Arterioles are blood vessels that branch from arteries and lead to capillaries. Venules are similar to arterioles but branch off from veins and also lead to capillaries. Finally, capillaries are the smallest of the blood vessels that connect arterioles and venules and play an important role in the exchange of gases such as oxygen. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essayThe endothelium can be found throughout the circulatory system. It is the thin, permeable layer of cells found on the inner walls of all blood vessels. Permeability is the ability of a membrane to allow certain molecules to be transported across it. The permeability of the endothelium is important because oxygen, nutrients and white blood cells must be transported through blood vessels to reach other organs. The three main functions of blood include transportation, protection, and regulation. Blood carries many different nutrients throughout the body, plays a key role in protection as it decreases inflammation, carries antibodies that destroy bacteria, and carries platelets that initiate clotting, and finally, blood is helpful in regulating the body. For example, the blood, together with the nervous system, is responsible for regulating pH and water balance. Although blood vessels are responsible for many different tasks in the human body, they are not involved in regulating blood transport. Blood transport is controlled by the nervous system. The nervous system is responsible for telling blood vessels when to narrow and when to dilate. When the body is hot, vasodilation occurs, which is when the diameter of the blood vessels increases allowing more blood to reach the surface of the skin to cool the body. When the body is cold, vasoconstriction occurs, which is when the diameter of the blood vessels decreases, keeping heat inside the body. Blood pressure is the amount of force exerted against the walls of blood vessels by circulating blood. Blood pressure decreases as blood moves from arteries to arterioles, capillaries, and then veins. Blood pressure generally refers to the pressure in the arteries as they are under more strain. Blood pressure can be measured in two stages; systolic and diastolic pressure. Systolic pressure is the pressure measured at the highest point, at the beginning of the cardiac cycle, while diastolic pressure is the pressure measured at the lowest point, the resting phase of the heart. Although the circulatory system is considered a large system, there are many subsystems within it. The pulmonary circuit is the part of the circulatory system that transports blood to and from the heart. The pulmonary circuit is made up of the pulmonary vein which transports oxygenated blood from the lungs to the heart, while the pulmonary artery which transports deoxygenated blood from the heart to the lungs. The second circuit is the coronary circulation, i.e. the vessels that transport oxygen to the.