The vascular system can be divided into two circuits: the pulmonary circulation, which circulates blood through the lungs, and the systemic circulation, which serves the needs of the body's tissues.
The path of blood through the lungs can be traced as follows: Blood from all regions of the body first collects in the right atrium and then passes into the right ventricle, which pumps it into the pulmonary trunk. The pulmonary trunk divides into the pulmonary arteries, which divide into the arterioles of the lungs. The arterioles then take blood to the pulmonary capillaries, where carbon dioxide and oxygen are exchanged. The blood then enters the pulmonary venules and flows through the pulmonary veins back to the left atrium. Since the blood in the pulmonarv arteries is oxygen-poor but the blood in the pulmonary veins is oxygen-rich, it is not correct to say that all arteries carry blood that is high in oxygen and that all veins carry blood that is low in oxygen. In fact, it is just the reverse in the pulmonary system.
The pulmonary arteries transport oxygen-poor blood to the lungs, and the pulmonary veins return oxygen-rich blood to the heart.
The systemic circulation includes all of the other arteries and veins of the body. The largest artery in the systemic circuit is the aorta, and the largest veins are the superior and inferior venae cavae. The superior vena cava collects blood from the head, chest, and arms, and the inferior vena cava collects blood from the lower body regions. Both venae cavae enter the right atrium. The aorta and venae cavae are the major pathways for blood in the systemic system.
The path of systemic blood to any organ in the body begins in the left ventricle, which pumps blood into the aorta. Branches from the aorta go to the major body regions and organs. Tracing the path of blood to any organ in the body requires only mentioning the aorta, the proper branch of the aorta, the organ, and the returning vein to the vena cava. In many instances, the artery and vein that serve the same organ have the same name. For example, the path of blood to the kidneys is: left ventricle; aorta; renal artery; arterioles, capillaries, venules; renal vein; inferior vena cava; right atrium. In the systemic circuit, unlike the pulmonary system, arteries contain oxygen-rich blood and appear bright red, while veins contain oxygen-poor blood and appear purplish.
The systemic circulation transports blood from the left ventricle of the heart to the arteries, arterioles, and capillaries, and then from the capillaries to the venules and veins to the right atrium of the heart. It serves the body proper.
The Major Systemic Arteries By examining the path of the aorta after it leaves the heart, one can see why it is divided into the ascending aorta, the aortic arch, and the descending aorta. The coronary arteries, which supply blood to the heart, branch off of the ascending aorta.
Three major arteries branch off the aortic arch: the brachiocephalic artery, the left common carotid artery, and the left subclavian artery. The brachiocephalic artery divides into the right common carotid and the right subclavian arteries. These blood vessels serve the head (right and left common carotids) and arms (right and left subclavians) .
The descending aorta is divided into the thoracic aorta, which branches off to the organs within the thoracic cavity, and the abdominal aorta, which branches off to the organs in the abdominal cavity.
The descending aorta ends when it divides into the common iliac arteries that branch into the internal iliac artery and the external iliac artery. The internal iliac artery serves the pelvic organs, and the external iliac artery serves the legs.
The Major Systemic Veins The external and internal jugular veins drain blood from the brain, head, and neck. The external jugular veins enter the subclavian veins that, along with the internal jugular veins, enter the brachia cephalic veins. These vessels merge, giving rise to the superior vena cava.
In the abdominal cavity, as discussed in more detail later, the hepatic portal vein receives blood from the abdominal viscera and enters the liver. Emerging from the liver, the hepatic veins enter the inferior vena cava.
In the pelvic region, veins from the various organs enter the internal iliac veins, while the veins from the legs enter the external iliac veins. The internal and external iliac veins become the common iliac veins that merge, forming the inferior vena cava.
All the arteries in the systemic circulation can be traced from the aorta. All the veins in the systemic circulation can be traced to the venae cavae.
January 18, 2011