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Overview of the Circulatory Pathways The two pumps of the heart are in series, meaning that the blood flows in one direction from one pump to the other. Starting from the left heart, the blood flows through the peripheral circulatory system, which provides the body with the nutrients it needs, such as oxygen, and removes metabolic waste, such as carbon dioxide. As blood moves through this circulation, it follows a path of blood vessels that change in their anatomy and function. Upon leaving the heart blood enters the major arteries. Arteries are the vessels in the circulatory system that carries blood away from the heart. These arteries branch and divide into arterioles that are smaller in diameter. These arterioles continue to branch out to create even smaller capillaries. These capillaries, which are found within each organ, are the site of gas and nutrient exchange. Once blood has perfused the organ, the capillaries begin to coalesce into larger venules that combine to create larger veins, ultimately returning blood to the heart through either the superior or inferior vena cava. Circulatory Pathways Blood vessels are organized into circulatory routes that deliver blood to different organs and tissues of the body. The right ventricle pumps blood to the lungs via the pulmonary circuit. The left ventricle pumps blood to organs and tissues via the systemic circuit. The output of the systemic circuit becomes the input of the pulmonary circuit, and vice versa. The pump for the systemic circuit is the left side of the heart, which also collects the oxygen-rich blood returning from the lungs. The pump for the pulmonary circuit is the right side of the heart, which receives oxygen-poor- blood from the systemic circulation. There are two important differences between the systemic circulation and pulmonary circulation. The first is that blood in the systemic circulation must be pumped farther than blood in the pulmonary circulation. The second is that, in pulmonary arteries, the diameter is larger, the walls are thinner, and the tissue is more elastic than in systemic arteries. Because of the characteristics of pulmonary arteries, there is very low resistance to blood flow in the pulmonary circuit, and thus pressures do not need to be very high to move blood through the lungs. In the right ventricle, the peak systolic pressure is typically onefifth that in the left ventricle. The Pulmonary Circuit The sole function of the pulmonary circuit is to carry blood to the alveoli of the lungs so that gases can be exchanged. Flow is matched to ventilation so that oxygen can move from the alveoli into the bloodstream and carbon dioxide can move from the bloodstream into the alveoli. The pulmonary circulation begins at the right ventricle, which pumps oxygen-poor, carbon dioxide-rich blood into the large pulmonary trunk. From its origin at the right ventricle, the pulmonary trunk takes an upward diagonal course for approximately 8 cm (3.1 inches (in.)). It then divides into the right pulmonary artery (to the right lung) and the left pulmonary artery (to the left lung). With the exception of the fetal circulation, the pulmonary arteries are the only arteries that transport oxygen-poor blood. After entering the lungs, the pulmonary arteries subdivide into three lobar arteries in the right lung and two lobar arteries in the left lung. Each lobar artery supplies one lung lobe. Further subdivisions result in the formation of capillaries around the alveoli. Carbon dioxide from the blood is transferred into the alveoli and exhaled. Inhaled oxygen is transferred from air in the lungs into the blood. Pulmonary capillaries merge, forming venules that unite to become pulmonary veins. Two of these veins leave each lung and transport oxygenated blood to the left side of the heart. As the left ventricle contracts, it pumps out the oxygenated blood into the systemic circulation. Again, except for fetal circulation, pulmonary veins are the only veins that transport oxygen-rich blood. It is easy to remember which vessels belong to the pulmonary circulation, because they all have either the word pulmonary or lobarin their names. The Systemic Circuit The systemic circuit delivers oxygenated blood and nutrients to the entire body except the lungs, and removes the waste products of metabolism from the same tissues. We will review the circuit from previous sections: After blood is pumped from the left ventricle into the aorta, it flows into smaller and smaller systemic arteries to perfuse all of the body's organs and tissues (except for the alveoli of the lungs). Systemic arteries branch off into smaller arterioles that lead into extensive networks of systemic capillaries. Nutrients and gases are exchanged between blood and systemic tissues through the thin capillary walls. After the blood gets rid of oxygen and picks up carbon dioxide, it usually travels through a single capillary before entering a systemic venule. The venules then transport oxygen-poor (deoxygenated) blood from systemic tissues before merging to create the larger systemic veins. All blood in the systemic circulation eventually empties into the right atrium. The arteries and arterioles of the systemic circulation receive oxygenated blood from the left ventricle and transport it to the systemic capillaries. The veins and venules of the systemic circulation carry deoxygenated blood back to the right atrium. Blood in systemic arteries is bright red in color. Blood in capillaries is dark red, because it has lost some oxygen and gained carbon dioxide. The arteries and veins of the systemic circuit share a number of similarities, most notably, the routes and names that many of these vessels share. The blood from the heart is pumped into a single systemic artery, the aorta, which carries oxygenated blood away from the left ventricle. Oxygen-poor blood returns to the right atrium via three systemic veins: the superior vena cava, inferior vena cava, and coronary sinus. The majority of blood that returns to the heart flows through the superior and inferior venae cavae. The names of these veins are derived from the location of their tributaries. The superior vena cava collects blood from veins located superior to (above) the diaphragm, with the exception of the wall of the heart and the alveoli of the lungs. The superior vena cava, which delivers blood to the right atrium, is formed by the merger of the right and left brachiocephalic veins. Each of these veins, in turn, is formed by the union of the internal jugular vein and subclavian vein on the right or left side of the body. The inferior vena cava, which is the largest-diameter blood vessel in the body, collects blood from veins located inferior to (below) the diaphragm and delivers it to the right atrium. This vein is located directly to the right of the abdominal aorta. The merger of the paired common iliac veins creates its distal end. Blood that drains from the myocardium of the heart empties into cardiac veins and returns to the right atrium via the coronary sinus.