Anatomy and Physiology: Here, There, Everywhere
Here, There, Everywhere
In The Heart, I discussed the three simultaneous circuits in our cardiovascular system: systemic, pulmonary, and coronary. The Heart introduced you to the pulmonary and coronary circuits, and the pulmonary circuit will be explored further in The Respitory System, so in this section I concentrate on the systemic circuit. As you have seen from the blood flow in the pulmonary circuit, blood vessels tend to follow parallel tracks, to and fro. Later in this section I explore another parallel track?although not a complete circuit as it only goes one way?called the lymphatic system, which follows the pathway of the major arteries and veins.
Given the central location of so many structures, one thing you will find throughout the body are examples of collateral circulation (see Figure 12.3). Collateral circulation means that more than one artery feeds the capillary bed of an organ. When these arteries fuse it produces an arterial anastomosis; this is just another example of the body being clever, because a blockage on one side still means that the blood will get to the organ. As for the rest of the body, I will get you started with the major pathways here. Any good college textbook can get you the rest of the way. What follows will also help you to avoid some pitfalls.
When you look at the vessels of the upper body I want you to think about where the blood is coming from, and where it has to go. First of all, remember how blood leaves the heart in a single vessel? Well, since most of the body is below the heart, the aorta arches downward (in a part called the aortic arch, of course) almost immediately after it leaves the heart. At the top of the arch, however, the blood for the upper body leaves the aorta. The only problem is that there are only three vessels to go four places: the right arm, the right side of the head, the left arm, and the left side of the head. The names of the three branches provide a clue as to their destinations.
On each side of the neck there is a major artery?which you feel for when you take your pulse?called the carotid artery (left and right, of course). There are also branches that go to each arm; these branches are called subclavian arteries. (Guess what bone they go under? Yup! The clavicle!) Since the aortic arch is more on the left side of the body, the middle branch is the left carotid artery, and the left branch is the left subclavian artery (refer to Figure 12.1). The name of the right branch, which must go to both the right arm and the right side of the head is called the brachiocephalic (brachio = arm, cephalic = head) artery.
There are smaller branches from the subclavian that go up through foramina in the transverse processes of the cervicalvertebra, called the vertebral arteries. These vessels join in front of the brainstem (see The Central and Peripheral Nervous Systems) to become the basilar artery; the basilar later divides to become the circle of Willis, an example of collateral circulation around the pituitary gland.
Blood returning to the heart is mostly parallel to the arteries, with subclavian veins, vertebral veins, and (parallel to the carotids) internal and external jugular veins. The actual return to the superior vena cava, which is on the right side of the heart, is more symmetrical than the aorta, with both left and right brachiocephalic veins, both of which merge into the superior vena cava.
Following the abdominal aorta down, the blood flow varies based upon the organs' locations. Many organs are large enough, or long enough, to receive blood from both sides of the aorta (you will find a similar situation in the orientation of the spinal nerves; see The Senses). In some cases there is more collateral circulation, such as the left and right gastric arteries (to the stomach), or the connecting branches of the inferior mesenteric (within the mesentery; see The Respitory System).
The blood flow to a few organs deserves special mention. The spleen, which is on the left side of the upper abdomen, next to the stomach, has the splenic artery and splenic vein that travel to and from the spleen, directly over the pancreas. As long as the blood travels over the pancreas, there are, of course, little pancreatic arteries and pancreatic veins.
The kidneys differ from many of the abdominal organs, simply because they are actually behind the abdominal wall, directly on either side of the abdominal aorta and inferior vena cava. The renal arteries and renal veins (renal means kidney) are thus kind of short, although the left and right vessels for each kidney will be slightly different, as the right kidney is closer to the vena cava than it is to aorta, and vice versa (refer to Figure 12.1). The pelvic blood flow is discussed in the next section. As for the return from the rest of the abdominal organs, well, that deserves its own section.
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.