The aorta, the largest artery in the body (Figs. 3, 5, and 7), leaves the heart and takes blood ejected from the heart to the rest of the body. The blood from the right ventricle is ejected through the pulmonary valve into the pulmonary artery, which traverses the lung (Figs. 3, 4, 6,and 7). Blood is returned from the lungs to the left atrium via the thin-walled pulmonary veins.
The heart valves open and close to allow the heart chambers to fill with blood and to allow blood to be ejected during systole or contraction of the heart. The leaflets of the valves are smooth but may become thickened by diseases such as rheumatic fever, which may lead to leaking or tight (stenotic) valves.

D. Coronary Arteries
There are two main coronary arteries, left and right, which originate from the root of the aorta as it leaves the heart (see Figs. 5 and 8). Arteries are tubes which carry blood from the heart. The channel within the tube is the lumen of the artery. The branches of the two main arteries carry oxygen and nutrients to the heart muscle and cells.

As shown in Fig. 8, the coronary arteries run along the outer surface of the heart. The left main coronary artery is very short, 0.2–4 cm, and divides almost immediately into two branches. The first branch, called the anterior descending artery, runs down the front surface of the heart near the undersurface of the left margin of the breastbone. It supplies blood to a major portion of the left ventricle. The second branch, called the circumflex artery, circles around and feeds the back of the heart. The right coronary artery leaves the aorta, veers sharply left, then is directed toward the breastbone and curves downward to run along the border of the right ventricle. The right coronary artery supplies branches to the electrical system, which involves special cells that cause the heart to beat (the sinus node or pacemaker), and to the conducting bridge for electrical transmission between the atrium and ventricle (atrioven-tricular (AV) node). The branches subdivide several times and perforate the heart muscle at different points to bring nutrients to the muscle. These arteries have nothing to do with the blood flow inside the heart, which is pumped around the body. In addition, their internal diameter is only about the size of a soda straw, thus, they are easily obstructed. When the coronary arteries are completely obstructed, a heart attack (myocardial infarction) results. Consider the heart as being supplied by four arteries, the left main coronary artery, the left anterior descending, the circumflex, and the right coronary artery (see Fig. 8). It is easy to visualize blockage of the left main coronary artery as the most dangerous, as it would block off two major arteries. Fortunately, this occurrence is rare. Most heart attacks block the right coronary artery,the anterior descending, and less common, the circum¬flex or smaller branches of all three arteries (see the chapters Heart Attacks and also Atherosclerosis/ Atherothrombosis).

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