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Introduction to Circulatory System
There are about 100 trillion cells in the human body. For each of these cells to survive and perform its function, each cell needs to receive nutrition, replenish oxygen and remove waste. The circulatory system performs these functions for the cells. The circulatory system is the major highway network in the body. The arteries, veins and capillaries are the conduits. The blood is the transportation medium. The entire system is powered by the heart, the main pump.
There are three main types of blood vessels: arteries, capillaries and veins.
Arteries are thick vessels that carry blood away from the heart. The arteries generally carry oxygenated blood except the pulmonary artery (discussed in Heart Structure). The arterial vessels branch out so oxygenated blood can be delivered to the whole body. The smaller arteries are called arterioles which eventually connect with capillaries. Arteries have three main layers. The innermost layer that is in contact with the blood is called the tunica intima. This layer is composed of a flat lining of endothelial cells. The middle layer is called the tunica media, which is made of flexible muscle. The outermost layer is called the tunica adventitia.
The main arterial trunk is the aorta which originates from the heart.
The aorta then branches off to other arteries such as the coronary arteries (which supply the cardiac muscle), carotid arteries (which supply the head), subclavian arteries (which supply the neck and arms) and abdominal aorta (which supplies the abdomen and the legs).