A SERIES OF TUBES
Newman and his colleagues found evidence of atherosclerosis in the aorta and in the heart's coronary arteries. The aorta is the largest artery in the body. Blood leaves the heart through the aorta, which branches out into a series of smaller arteries that deliver oxygen-rich blood to the rest of the body. The coronary arteries that branch off the aorta supply the heart muscle with blood. It is these smaller coronary arteries that are the usual sites of blockages that lead to a heart attack.
If the heart is full of blood, why does it need its own blood supply from arteries leading from the aorta back into the heart muscle? The answer has to do with how gases like oxygen and carbon dioxide, and nutrients like sugar, are exchanged between blood vessels and tissues.
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Arteries and veins are like major highways for blood transport—their main responsibility is bringing blood to and from the body. Situated between these two large types of vessel, however, are capillaries. Capillaries are tiny blood vessels, located in tissues, where gas and nutrient exchange occurs. They are where the action is. Oxygen and nutrients diffuse out of capillaries into tissues, while carbon dioxide and wastes diffuse from tissues into capillaries. Capillaries are ideally suited to do their job by virtue of their thin walls and their slow rate of blood flow, which allows ample opportunity for gas, nutrient, and waste exchange between blood and tissues. Every tissue in the body, including the heart, is infiltrated with capillaries (INFOGRAPHIC 27.5).
How does blood actually move through this series of blood vessels? Arteries are high-pressure vessels; the blood that flows through them is propelled by the force exerted by the heart. As blood slows down and loses pressure in the capillaries, it enters the veins at low pressure. Therefore, veins are low-pressure vessels; they rely on muscular contractions of the skeletal muscles in which they are found, and on valves, to prevent the backflow of blood and return it to the heart.
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The function of this interconnected system of blood vessels is to bring blood in proximity to nearly every cell in the body. What makes blood so vital? Blood is a complex tissue, made up of cells and a liquid known as the plasma, which is composed of water and proteins. For cardiovascular purposes, the most important cells are the red blood cells (or erythrocytes), which are specialized for carrying oxygen; they lack a nucleus and have a distinctive concave shape that allows them to squeeze through capillaries in single file. Oxygen diffuses directly across the membranes of these cells into tissues.
Also present are white blood cells (or leukocytes), which are involved in many aspects of the immune response (see Chapter 31), and platelets, which are cell fragments that play a critical role in blood clotting. Blood also carries dissolved carbon dioxide, the waste product of cellular respiration (see Chapter 6). Finally, blood contains traces of whatever else we put into our bodies—from the food we eat to the drugs we take to the smoke we breathe. Some of the things that circulate in our blood will contribute, over time, to the development of atherosclerosis (INFOGRAPHIC 27.6).
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